CA2061909A1 - Nrz clock and data recovery system employing phase lock loop - Google Patents
Nrz clock and data recovery system employing phase lock loopInfo
- Publication number
- CA2061909A1 CA2061909A1 CA002061909A CA2061909A CA2061909A1 CA 2061909 A1 CA2061909 A1 CA 2061909A1 CA 002061909 A CA002061909 A CA 002061909A CA 2061909 A CA2061909 A CA 2061909A CA 2061909 A1 CA2061909 A1 CA 2061909A1
- Authority
- CA
- Canada
- Prior art keywords
- signal
- nrz
- blivet
- frequency
- clock
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION, OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
- H03L7/00—Automatic control of frequency or phase; Synchronisation
- H03L7/06—Automatic control of frequency or phase; Synchronisation using a reference signal applied to a frequency- or phase-locked loop
- H03L7/08—Details of the phase-locked loop
- H03L7/085—Details of the phase-locked loop concerning mainly the frequency- or phase-detection arrangement including the filtering or amplification of its output signal
- H03L7/089—Details of the phase-locked loop concerning mainly the frequency- or phase-detection arrangement including the filtering or amplification of its output signal the phase or frequency detector generating up-down pulses
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B20/00—Signal processing not specific to the method of recording or reproducing; Circuits therefor
- G11B20/10—Digital recording or reproducing
- G11B20/14—Digital recording or reproducing using self-clocking codes
- G11B20/1403—Digital recording or reproducing using self-clocking codes characterised by the use of two levels
- G11B20/1407—Digital recording or reproducing using self-clocking codes characterised by the use of two levels code representation depending on a single bit, i.e. where a one is always represented by a first code symbol while a zero is always represented by a second code symbol
- G11B20/1419—Digital recording or reproducing using self-clocking codes characterised by the use of two levels code representation depending on a single bit, i.e. where a one is always represented by a first code symbol while a zero is always represented by a second code symbol to or from biphase level coding, i.e. to or from codes where a one is coded as a transition from a high to a low level during the middle of a bit cell and a zero is encoded as a transition from a low to a high level during the middle of a bit cell or vice versa, e.g. split phase code, Manchester code conversion to or from biphase space or mark coding, i.e. to or from codes where there is a transition at the beginning of every bit cell and a one has no second transition and a zero has a second transition one half of a bit period later or vice versa, e.g. double frequency code, FM code
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION, OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
- H03L7/00—Automatic control of frequency or phase; Synchronisation
- H03L7/06—Automatic control of frequency or phase; Synchronisation using a reference signal applied to a frequency- or phase-locked loop
- H03L7/08—Details of the phase-locked loop
- H03L7/085—Details of the phase-locked loop concerning mainly the frequency- or phase-detection arrangement including the filtering or amplification of its output signal
- H03L7/087—Details of the phase-locked loop concerning mainly the frequency- or phase-detection arrangement including the filtering or amplification of its output signal using at least two phase detectors or a frequency and phase detector in the loop
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M5/00—Conversion of the form of the representation of individual digits
- H03M5/02—Conversion to or from representation by pulses
- H03M5/04—Conversion to or from representation by pulses the pulses having two levels
- H03M5/14—Code representation, e.g. transition, for a given bit cell depending on the information in one or more adjacent bit cells, e.g. delay modulation code, double density code
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L7/00—Arrangements for synchronising receiver with transmitter
- H04L7/0079—Receiver details
- H04L7/0087—Preprocessing of received signal for synchronisation, e.g. by code conversion, pulse generation or edge detection
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L7/00—Arrangements for synchronising receiver with transmitter
- H04L7/02—Speed or phase control by the received code signals, the signals containing no special synchronisation information
- H04L7/033—Speed or phase control by the received code signals, the signals containing no special synchronisation information using the transitions of the received signal to control the phase of the synchronising-signal-generating means, e.g. using a phase-locked loop
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10S331/00—Oscillators
- Y10S331/02—Phase locked loop having lock indicating or detecting means
Abstract
Abstract of the Disclosure An improved NRZ clock and data recovery system lends itself to integration includes a NRZ phase detector, an NRZ frequency detector and a lock detector, and provides automatic centering of the clock edge within the bit interval in 2 manner that is independent of analog relays and process and temperature variations. NRZ data is applied to one side of an exclusive-OR sate (6) and a twice delayed (2 4) version of the NRZ data is applied to the other side. The output of the XOR gate (6) a "blivet" signal is applied to a NRZ phase detector (10) comprising two AND sates (12,14), one of which (12) has as its other input a recovered clock signal output of a VCO (20) and the other of which has as its other input an inverted (16) version of the recovered clock signal. The "up" and "down"
outputs of the AND sates (10,12) indicate which direction a frequency control signal (26) should change the VCO (20) frequency. A data holding flip-flop (28) whose input is a once delayed (2) version of the NRZ data is clocked with the recovered clock signal. The NRZ frequency detector (50) monitors the state of the recovered clock signal on opposite edges (52, 53) of the blivet to detect too-high (56) too low (55) and good (54) conditions. The results of the detection can be ignored if a lock signal indicates that the phase lock loop is locked. The lock detector (30) consists of a saturating up/down counter (42) that is incremented (38) by one when good blivets (32) occur and is decremented (40) by four when not-good blivets (36) occur.
outputs of the AND sates (10,12) indicate which direction a frequency control signal (26) should change the VCO (20) frequency. A data holding flip-flop (28) whose input is a once delayed (2) version of the NRZ data is clocked with the recovered clock signal. The NRZ frequency detector (50) monitors the state of the recovered clock signal on opposite edges (52, 53) of the blivet to detect too-high (56) too low (55) and good (54) conditions. The results of the detection can be ignored if a lock signal indicates that the phase lock loop is locked. The lock detector (30) consists of a saturating up/down counter (42) that is incremented (38) by one when good blivets (32) occur and is decremented (40) by four when not-good blivets (36) occur.
Description
~Z CLOCK A~ DA.A RECOV~R"
S"SmE~ E~IPLOYI.~!G P~.~SE LOCK LOO~
Backgrcund c' the Inventior This invention relates to digital data trars~issicr, and more particularly tc ron-return to zero (MRZ) cloc~ and data rccovery.
NP~Z data does nct contain a spectral component at the cloc'~ frequency. Therefore, some way must be 'oun~ to ascertain the clcck sisr.al 're~uency and phase. FIG. 1 shows one prior art cpproach to this problem. In this approach a non-linearity, in the form of a delay line and an exclusive-OR gate, is introduce~ to produce a frequency CGmponent at the ~RZ clock fre~uency. mhe celay element is ty2ical1y one half of the e~pected bit interval or less. A bandpass filter then detects the introduced frequency co~ponent at the -loc~
freq~ency and cither produces a clock directly or produces a clock indi ectly with the assistar.ce of a phase lock loop.
The difficulty with the approach shown in FIG. 1 and described above is that a bandpass filter does not lenc itcelf tc integration into an integrated circuit (TC), and the phase accuracy of the recovered clock depends on the tuning of the band~ass filter.
Another prior art approach is shown in FIG. ~.
This approach does not reauire a tuned circuit and is therefore more am~nar,le tc IC implementation.
However, the accuracy of the placement of the clock edge in the center of the hit interval depends ~rimarily on the length of the ~elay line. In all oE the 'ypical TC implementationc of a delay li~e, the amount of delay that results is subject to n pro-ess an~ tem?erature variations. mhus, thQ
c1Oc'~ edge is nct wel' centered within the bit interval and, in the presence of significant ~itter, the bit error ra_e suffers accordingly.
~1hat is desired is an i~proved ~1P~Z clock and data recovery sycte~ tha' lends itself tc integra-tion, that includes a ?hase and M~Z frequ~ency detectcr and a lock detector, and that ~rovides automatic centerins of the clock edge within the bit interval in a manner that is independent cf analog delays and proces- and temperature variatiors.
Summar~ o. the 'nvention ~lRZ data is applied to one side of an exclusive-OR gate and a twice dela~ed version of the ~RZ data is applied to the other side. ~he output of the XOR gatQ, a "blivet" signal, is applied to a NRZ phase detector comprising two A~D
sates, one of which has 25 its other input a recovered clock signal output of a VCO and the other of which has as its other input an inverted version of the recovered clock signal. mhe outputs of the two AMD sates are an "up" signal ana a "down' signal that indicate which direction a frequency control sign~7 should change the VCO
frequency. A data holding flip-flop whose input is a once delayed version o' the M~Z data is cloc~.ed with the recovered cloc~ signal.
In accordance with the invention, there is provided an NRZ frequencv detector that includes two flip-flops 'or ~ttQnitorins the stafe o' ~he recovered cloc!~ signal on opposite edges of the blivet signa1. Three gates monif^r ~he output of the two flip-flo~_ to aetec. '~oo-hl~h, .oo-l~w ~na .. v `~ ? ~
"goo~` conaitiors. Addi'ional flip-flops hold the results of this detection, and feedback from these flip-'lops is used to inhibit detecticn of error stztes that are the op~osite of an immediately precedinq error state. The fiip-flops holding the results of the detection can be forced to ignore thei~ input- and hold a "good" instead if a lock signal indicates that the phase lock loop is locke~.
0 Tn accordance with the invention there is also provided a lock detector consisting of a satur2ting up~cown counter that i~ inc emented by one when blivets occur and the ~RZ f-equency detector is sisnalin~ a "good" conditior and is decre~ented by a numbcr larger than one when blivets occur and the ~P~Z fre~uency detector is signaling 'not-good". ~ lock signal output is produced whenever the count in the counter is above a predetermined threshold, e.s. half full.
The sub,ect matter of the present invention is ?articularly pointed out and distinctly claimed in the concluding portion of this specification.
However, both the organization and metho~ of opera-ticn, tosether with further advantages and objects thereof, may best be understood by refererce to the following detailed ~escription an~ accompanying drawings.
Brief ~escriptior of the Draw~ncs F G. l is a block diasram of a prior art circuit for ~RZ clock recovery;
FIG. ~ is a block diagram of another prior art circuit for ~RZ clock ard data recove y;
FIG. 3 is a block diagram of a ?ortion of the ~IP~Z c'^ck and data recovery circuit accor~in~ to the present invention, wi.h schematic details of the ?hase detection block;
~IG. ~ is a block diagram of the ~R7 clock an~
data recovery system according to the present invention;
FTG. - is a schematic diagram of the NRZ
fre~uency detec.or according to the present invention;
FTG. 6 is a timins diasram il'ustratins the operation of the ~3RZ frequency detector acc^rding tc the present invention; and FIG. ? is G bloc!~ diagram of the lock detector accordins t- .he present invention.
lS Detailed Description FTG. 3 is a bloc'.~ diasram o~ a portion of the NRZ clock and data recovery circuit accordins to the present invention, with an inset schematic detail of the phase detection circ~itry 10. This desisn lends itself to in'esration anc provides automatic centering of the clock edge within the bit interval in a manner that is independent of analog delays and process and tem?era.ure variations.
The ~3RZ data input i_ apolied tc a uni~ue "blivet" senerator 8. ~ "blivet" is a pulse indi-cative of a transition, either low-goins or hish-going, in the ~3RZ data. Internal to the blivet qenerator 8, the NRZ data input is applied to one side of exclusive-OR ~OR) sate 6 and also to two matching delay elements ~ an~ 4 in series, a~ter which the doubly delayed output of those ~elay elements is applied to the other side oE the XOR
gate 5. In addition ~ generatins blivets, blivet generator 8 also provides an NRZ output that is delayed by one half of the blivet width.
mhe blivet output of the blivet seneratcr is a pulse whose duration is he sum of the delay im2osed by the two matching delay elements ~ and ~.
As will be further explained below, the sum of thcse delays should be less than one half of the bit interval of the ex?ected data, but as long as possible within that cons~raint.
~he blivet output of the blivet generator 8, from XOF~ sate 6, is a~lied to NRZ phase detector 0, which also receives a cloc'~ ou.put from a VCO
~0. mhe NRZ phase detector l~ comQrises AN~ gates 12 and 14 and inverter 16. The NRZ phase detector 10 generates two sisnals, an "up" sisnal that goes to the "+" input of loop filter ~6, and a "down"
signal that goeC tc ~he "-" input of the loop filter 26. The "up" and "down" signals are used by the loop filter 2fi to prcduce the control voltage for VCO 20. The loop filter ~, which is suitably a charge pum2, receives the "up" and "down" pulses from the NRZ phase detector 10 and integrates their difference into an analos VCO control signal with only low frequency components.
As will be further exptained ~elow, thc effec-tiveness of this invention relies on the centering of the low-going edge of the ~CO C recovere~ clock output within this blivet.
If the VCC output low-going edgc is already centered within the blivet ?roduced by the XOR
gate 6, the duration of the out?u. from ~ND gate l is the same as the duration of the output from A~ID
gate 14, and the inputs to the loop filtee ~6 cancel, indicating that no fre~uencv ad~ustment is necessary and the loop filter ~6 ccntinue_ ~o produce an unchansed ~'CO con rol signal.
I~- the ~cwnward trarsition of the ~CC ~0 clock signal output is late within the blivet, the output o ~il5 sate 1- has a longer d~ration than the output of A~ gate 14, and the "up" input to the 1GOP filter prevails over the "dcwn" input and the VC0 control voltase out?ut of the loop filter ~6 increases the ~IC0 frequency to cause the next low-going edse to occur sooner wi~hir. the blivet interval.
Conversely, if the ~ownward transition of the VC0 0 cloc~ si~nal is early within the blivet, the output of ~.ND sate 1~ has a longer duration than the cutput of AMD gate 12, and the "down" input to the 'oop filter ~6 prevails over the "up" input and lS the ~.'C0 control voltage output of the low?ass filter 6 decreases the VC0 fre~uency to cause the next low-going edge to arrive later in the blivet interval.
It will there'ore be seen that regardless of the pc,sition of the cloc~ signal relative to the center of the blivet, the ~IRZ phase de'ector 1~ and loop filter 26 operate over time to for^e the downward edge o~ the cloc~ signal ~rom ~C0 30 tc the center of the blivet interval.
Because the delay elements ' and a are identical, they produce equal delays des?ite process and temperature variations. ~herefore, changes in process or temperature af~ect the blivet wid~h, which affects ~he ~I~Z ?hase detec~or's sensitivity, but not its ability ~o properly detect phase differences. Sensitivity increases with increasing blivet width, because the dif'erence between the widths of the "up" an~ "down" "ulses can be greater.
~he c1oc'~ output ~,f the ~'C^ ~ is a'so a~?? ied r~
to ~h~ cloc~ i~put of 'li2-flo? ~. mh~ D i~pu' o' 'lir-flop 2~ is connec~ed tc reseive the once de~a~fe~ ~r~z data output cf the bli~et cenera,cr ~3, from the out?ut of delay ele~ent 2. In .he approach u'ilized in ~his invention, the abili'y to ?osition the clock edse 'hat cloc~s the dat2 into flip-flop 28 in the center of the "eye" of the bit interval depe~ds on the centeri.,s of the opoosite clcck edge within the blivet, as described a~ove.
The VCO is balanced to have a near perfect 50%
duty cycle. So, with the ~CC 2C clock output low-soins transitions centered in the blivets from XOR
gate ~, the high going tr2nsition.s are glaranteed to occur uhile the data ?resent at the junction of the two delay elements 2 and 4 is rish' in the middle of the bit interval. This is because the falling clock edge occurred half of blivet after the end of the las. bi' interval, and data at this junction is also de~ayed by one half blivet at the time of the rising clock edge. The D inpu. of flip-flop ~8 is conr.ec~ed to this junction between the delay elements ~ and ~ and is cloc'~ed by the rising ed~e of the VCO 20 output cloc'~ sigr,als.
mhus, the circuit shown in FIG. 3 reliably recovers NRZ clock and data signals an~ lends i~sel' tc integration into an TC.
mhe circuit shown in FTG. ' only performs phase detection, not frequency detection. Vet frequency detection permits much more efficient frequency acquisition by ~ ph2se lock loo?. ~onse-quently, referring now tc FTG. ~, a complete system for NRZ clock and data recovery includes 211 of the circuitry shown in FTG. ', plus an ~PZ frequency detec'or 50, a loc~ detec'or 30 ar.~ 0~ ga'es ^4 an~
22 for combining the "U?" ~nd "dcwn" out?uts of the r~
phace and NP~Z frequency detectcrs hefore they are presented to the locp filter 6.
In the fu!l system shown in F~G. 4, the blivet signal is supplie~ to the NRZ frequency detector SQ
an~ the lock detector 3Q, as well as the NRZ phase detector 10. The NRZ frequency detector 50 also receives the clock output of the VCO 2C an~ a "lock" signal from the loc~ detec.or 30. Li~e the NRZ phase detector 1?, the ~IP~Z frequency detectcr 50 produces "up" and "down" signals to steer the VCO 20 via the loop filter 26. These "up" and "down" signals are combined with the "up" and "down" signals from the ?IRZ phase dete-tor 10 by up OR gate 24 and down OR gate 22. mhe ~IRZ frequency detectcr 5Q also produces "not-good" sisnals that inform the lock detector 30 when the present blivet is soing to cause an "up" or "down" sisnal.
Referring now to ~IG. ', the W~Z frequency detector 50 contains four flip-flops 52, 57, 58 and 59 that are clocked by the risins edge of the blivet signal and one flip-f10p 53 that is clocked by the falling edge of the blivet signal, due to the operation of inverter 51. Flip-f1OpS 5~ and 53 monitor the clock output of the VCO 2~ on opposite edges of the blivet signal. A "good" blivet is a blivet whose rising edge occurs while the ~CO O
cloc~ out is hish and whose falling edge occurs while it is low. A good blivet therefore sets fli~-flop 52 an~ resets flip-f10F 53. NA~ sate 54 monitors the Q output of f lip-Çlop 52 and the /Q
output of flip-Flop 5~, anc pro~uces a low output whenever a good blivet has been detected by these two flip-flops.
The Following table summari~es the information inferred by the NRZ frequency detector 5~ circuitry from the relationship be.ween the clock output level and the blivet edges:
CLCCK/BLIVET IMFE~ENCES
High-going Low-ccing Fre-uency Edae (F~5~) Edge ~FF5~ Inference 1 ~ good 1 1 Too Low 0 0 Too High 1 Unclear (?) NAMr sate 55 mcnitors the states of fli?-flops 52 and 53 for the "too-low" condition, i.e., both flip-flops having a high ou'pu.. The ~hird input to this NAND gate 55 is fro~ the D-C fiip-flop created by r1AND sates 62 an~ 63, which produccs a high out?ut from NAND ~ate 62 and a low output from NAND gate 63 when the f equency is toc high. ~he reason for this thied input to NAr~D sate 55 is to prevent going directly from a "too-high" ccnditio~.
to a "too-low" condition without soing through a good state in-between. Thus, when the last state out of the ~RZ frequency detec.or was "Down ~too-high)", NAND gate 55 is prevented from producing an active-low output indicating that the next state should be "Up (toc-low)" by the low out?ut of NAND
gate 63.
Similarly, NAr!~ gate 56 monit^rs the states of flip-flops 5~ and 53 for the "too-high" condition, i.e., bo'h flip-flcps having a low outpu,. An~, i the same way as before, a high output of ~AND
gate 61 in the other D-C flip-flcp, in~.icating that the last state was not "'oo-low", is reqltired befoee an active "~co-hish" sicnzl can bc produced.
~he C-C f'ip-flops 6~ and 6~, co~p~ising ~!AND
gates 60,61 and 5~,63, hold the last '~no~n frec~enc~ direc~ion, ~Go-low, ~oc-hi~,h ^r neither, until a ~sood blivet oc-urs. An active low ~good sisnal a'tempts tc clear bcth of the D-C flip-flo?s, and will do so in the absence of an active low ,~tco-low or ~too-hi~h sisn21.
The states of the ~t~.?~D sates 54, 55 and 56 are cloc~ed intc flip-flo?s 5 " 5~ and ~9, respec-tively, by the rising edse of the next blivet, if the signal lock is inactive low, indic tins that the ?hase lock 1oo? is not loc~ed. If the sisnal lock is active high, indicatin~ that a r,hase lock loop loc~ has beer. detected, the signal lock ~orces the /good flip-flop 57 to its cleared state, producins an inactive hish output from that flip-flip, which indicates that the state is "good'.
The active high lock signal also forces the "too-70W" an~ "too-hish" flip-flops tc isncre their inputs from NA~D sates 55 and 56, respectively, and produce inactive hish outputs. Thus, uhile the loop is locked, the NRZ frequency detector cannot produce any out?ut to disrupt that lock, leavins control entirely up to the ~RZ phase detector 10.
Note, however, that even while the loop is locked and the up and down outputs of the ~RZ
frequency detector 50 are disabled, it can still produce /sood (blivet) signal outputs from NAND
gate 5~. Thus, as will be further explained below, if enough ~good blivets are detected, the lock detector 3~ can respon~ by decre~entins down ~o below its one half full level and soing ~o an unlocked condition.
Referrins now to FTG. ~, a series o' blivets prc~uces appro?riate results in the p.ese~ce of 3S three differen~ cloc~. ~requencies, one correc~, one t too-hish and onc too-low. In FIG. 6, socd rela-ticnships ~etween blive~s an~ clcck levels are indicated by "O"s abcve clock sisnal, while too low re7ationshi?s are indica.ed with ar. "L" and too-highs ~ith an "II". Unc7ear resu'ts, where the clock is low fcr the risins blivet edse and high for fallin~ blivet ed~e, are indicated ~ith a "?".
Everythin~ is simple for the "Corrcct Clock Freq" line, with each hlivet producins a "O". The "Clock Low" line is much more interesting, producing the sequence of relationshipc:
"OCL~??'IOC?'-. The unclear, ?, indications are totally unrecognized by the circuitry shown in FIG. 5, so the "H" at blivet 7 is ignored because the last indication was a ~lr " and so the output of NA~7D aate 61 is low disa~ling MAMD gate 56 which would otherwise detect this "H". Even though the "H" is not detected, the .ailure tc satisfy ~A~D
gate 54 because the Q out?ut of flip-flop 52 is low causes the output of MAND sate 54 tc co high, and :~ this hi~h is clocked into flip-f7o? 5' by the next blivet (blivet 8~. The continui~g hish output cf flip-flop 57 has no effect on the D-C f'ip-flops comprising NAND gates 60,61 and 6~,6'.
The sood blivet at blivet ~ is detectcd by NA~D sate 54, whose output goes low in response.
The sood state of flip-10ps 52 and 53 also means that neither of the other MAMD gates 55 or 56 are satisfied. The low out?ut of M~ND gate 54 is clccked into flip-flop 5' on the rising edae of blivet 9. The same rising clock edge of blivet 9 also cloc!~s the high levels that are present on the output of MA;~D qates 55 and 56 into flip-flo?s '3 and 59, respective~y. The low out?ut of ~'ip-flQp 57 immediately clears the two D-C flip-flo~s 6~,61 r! ~
and 6~ , so that their outputs on N~!D gates 6C
and 62 become lo~, since both of both of their inputs are now high. ~hQ cleare~ states of the D-C
lip-flops 60,61 and 62,63 enable the third inputs tc ~A~D gateC '5 and 56, allo-~ing them tc detect the next high or low s~ate. lf the frequency were now correct, ~ostly gocd blivets would be detected from now on and the N~Z frecuency detector wouLd behave as it does on the line labelQd "Correct Cloc~ Freq".
In the absence of ,itter, an incorrect frequency will produce indications like those shown in F G. 6, in which the desired indica.ion always follows a good blivet, and any erroneous indica-tions always follow a desired indication anC are locke~ out by it. In the presence of jitter, however, erroneous indications sometimes follow a good blivet and reach the out2ut. Even in the presence of heavy jitter, though, the desired indi-cations always heavily out-weish the erroneous ones and the system successfully locks onto the correct frequency.
For the NRZ frequency detector ~0 to work correctly, the blivet length has to be less than one half of the bit interval. And, since resistGrs implemented in silicon and metal capacitors both have tolerances of up to +/- 2Q~, and the delay elements 2 and 4 are based on an RC product of such devices, the worst case delay tolerance could be as bad as +/- 40~. Thus, if the delay lines are designed to produce a ccmbined delay of 35~ of the bit interval, the worst case deviation still leaves the resulting tota' delay un~er half of the bit interval, thereby permittins the N~ fre~uency detector ~ to function properly.
ReCerrins now tc F_G. ', the lock detectcr ~C
receives as its inputs the blivet sisnal and the ~goo~ signal. Counter 1~ is a saturating up/down counter. A saturatins counter is a counter that does not wrap aroun~; when it reaches a ful1 coun', it s.ays there as additional in~re~ent sisnals are received. The saturatins up~dcwn ccunt_r 42 is incremented b~ incrementing means 38 whene~er a "sood" blivet is detecte~ by ~ sate ' , and decremented by ~ecrementing means 40 whenever a "bac" blivet is detected by A~D sate 36. lnver'er 34 chanses the /sood signal into a sood signal for ~ND sate 32. When the count in the counter reaches a threshold, for instance, more than half full, a "lock" sisnal is generate~.
The ratio of the increment step size (typically one) tc the ecrement step size (typicall~ some multiple of increment size) deter-mines the tolerance of the lock detector 30 to jitter. A 1:4 ratio has been found to provide a suita~le jitter tolerance. The size of the saturating u~down counter 42 (in bits) determines the integration time of the lock detec~or ~0, ~nd hence its response time to a locked oz unlocked concition. ~here is a trade-ofc between the speed with which a lock is identified and the certainty of that identiCication. Tn the present applica-tion, 12 bits of binary, for a total count of 40a6, has been found to be a suitable length for this counter, so that a lock indication appears after a count of 2048. If jitter is minimal, a loc~ is achie~ed in a~out l~ microseconds at ~?0 Megabits per second.
ThiC lock detectcr 30 implemertation is completely digital and therefore does not require an an31Os integ!ation capacitcr an~ associate~ IC
?in. This is a significant benefit in some appli-caticns where 2 shortage of IC pin- is an important ~esisn constraint.
~h~s, i. can be seen that the present inven-tion provides an improved ~RZ clock and data recover~ syctem that len~s itself to in~ecra~icn, that includes a NRZ phase ~etector, an `i~Z
frequency de'ector anc a lcck detec'cr, and that provides au~omatic centering of the clock edge withiq the bit interval in a manner tha~ is inde?endent of analog delays and process and temperat~re variations.
While a preferred embodiment cf the present invention has been shown and described, it will be a?~arent to those skilled in the art that many changes and modifications may be made without departing frcm the invention in its broader aspects. The claims that follow are therefore intended to cover all such changes and modifica-tions as fall within the true s?irit and scope of the invention.
S"SmE~ E~IPLOYI.~!G P~.~SE LOCK LOO~
Backgrcund c' the Inventior This invention relates to digital data trars~issicr, and more particularly tc ron-return to zero (MRZ) cloc~ and data rccovery.
NP~Z data does nct contain a spectral component at the cloc'~ frequency. Therefore, some way must be 'oun~ to ascertain the clcck sisr.al 're~uency and phase. FIG. 1 shows one prior art cpproach to this problem. In this approach a non-linearity, in the form of a delay line and an exclusive-OR gate, is introduce~ to produce a frequency CGmponent at the ~RZ clock fre~uency. mhe celay element is ty2ical1y one half of the e~pected bit interval or less. A bandpass filter then detects the introduced frequency co~ponent at the -loc~
freq~ency and cither produces a clock directly or produces a clock indi ectly with the assistar.ce of a phase lock loop.
The difficulty with the approach shown in FIG. 1 and described above is that a bandpass filter does not lenc itcelf tc integration into an integrated circuit (TC), and the phase accuracy of the recovered clock depends on the tuning of the band~ass filter.
Another prior art approach is shown in FIG. ~.
This approach does not reauire a tuned circuit and is therefore more am~nar,le tc IC implementation.
However, the accuracy of the placement of the clock edge in the center of the hit interval depends ~rimarily on the length of the ~elay line. In all oE the 'ypical TC implementationc of a delay li~e, the amount of delay that results is subject to n pro-ess an~ tem?erature variations. mhus, thQ
c1Oc'~ edge is nct wel' centered within the bit interval and, in the presence of significant ~itter, the bit error ra_e suffers accordingly.
~1hat is desired is an i~proved ~1P~Z clock and data recovery sycte~ tha' lends itself tc integra-tion, that includes a ?hase and M~Z frequ~ency detectcr and a lock detector, and that ~rovides automatic centerins of the clock edge within the bit interval in a manner that is independent cf analog delays and proces- and temperature variatiors.
Summar~ o. the 'nvention ~lRZ data is applied to one side of an exclusive-OR gate and a twice dela~ed version of the ~RZ data is applied to the other side. ~he output of the XOR gatQ, a "blivet" signal, is applied to a NRZ phase detector comprising two A~D
sates, one of which has 25 its other input a recovered clock signal output of a VCO and the other of which has as its other input an inverted version of the recovered clock signal. mhe outputs of the two AMD sates are an "up" signal ana a "down' signal that indicate which direction a frequency control sign~7 should change the VCO
frequency. A data holding flip-flop whose input is a once delayed version o' the M~Z data is cloc~.ed with the recovered cloc~ signal.
In accordance with the invention, there is provided an NRZ frequencv detector that includes two flip-flops 'or ~ttQnitorins the stafe o' ~he recovered cloc!~ signal on opposite edges of the blivet signa1. Three gates monif^r ~he output of the two flip-flo~_ to aetec. '~oo-hl~h, .oo-l~w ~na .. v `~ ? ~
"goo~` conaitiors. Addi'ional flip-flops hold the results of this detection, and feedback from these flip-'lops is used to inhibit detecticn of error stztes that are the op~osite of an immediately precedinq error state. The fiip-flops holding the results of the detection can be forced to ignore thei~ input- and hold a "good" instead if a lock signal indicates that the phase lock loop is locke~.
0 Tn accordance with the invention there is also provided a lock detector consisting of a satur2ting up~cown counter that i~ inc emented by one when blivets occur and the ~RZ f-equency detector is sisnalin~ a "good" conditior and is decre~ented by a numbcr larger than one when blivets occur and the ~P~Z fre~uency detector is signaling 'not-good". ~ lock signal output is produced whenever the count in the counter is above a predetermined threshold, e.s. half full.
The sub,ect matter of the present invention is ?articularly pointed out and distinctly claimed in the concluding portion of this specification.
However, both the organization and metho~ of opera-ticn, tosether with further advantages and objects thereof, may best be understood by refererce to the following detailed ~escription an~ accompanying drawings.
Brief ~escriptior of the Draw~ncs F G. l is a block diasram of a prior art circuit for ~RZ clock recovery;
FIG. ~ is a block diagram of another prior art circuit for ~RZ clock ard data recove y;
FIG. 3 is a block diagram of a ?ortion of the ~IP~Z c'^ck and data recovery circuit accor~in~ to the present invention, wi.h schematic details of the ?hase detection block;
~IG. ~ is a block diagram of the ~R7 clock an~
data recovery system according to the present invention;
FTG. - is a schematic diagram of the NRZ
fre~uency detec.or according to the present invention;
FTG. 6 is a timins diasram il'ustratins the operation of the ~3RZ frequency detector acc^rding tc the present invention; and FIG. ? is G bloc!~ diagram of the lock detector accordins t- .he present invention.
lS Detailed Description FTG. 3 is a bloc'.~ diasram o~ a portion of the NRZ clock and data recovery circuit accordins to the present invention, with an inset schematic detail of the phase detection circ~itry 10. This desisn lends itself to in'esration anc provides automatic centering of the clock edge within the bit interval in a manner that is independent of analog delays and process and tem?era.ure variations.
The ~3RZ data input i_ apolied tc a uni~ue "blivet" senerator 8. ~ "blivet" is a pulse indi-cative of a transition, either low-goins or hish-going, in the ~3RZ data. Internal to the blivet qenerator 8, the NRZ data input is applied to one side of exclusive-OR ~OR) sate 6 and also to two matching delay elements ~ an~ 4 in series, a~ter which the doubly delayed output of those ~elay elements is applied to the other side oE the XOR
gate 5. In addition ~ generatins blivets, blivet generator 8 also provides an NRZ output that is delayed by one half of the blivet width.
mhe blivet output of the blivet seneratcr is a pulse whose duration is he sum of the delay im2osed by the two matching delay elements ~ and ~.
As will be further explained below, the sum of thcse delays should be less than one half of the bit interval of the ex?ected data, but as long as possible within that cons~raint.
~he blivet output of the blivet generator 8, from XOF~ sate 6, is a~lied to NRZ phase detector 0, which also receives a cloc'~ ou.put from a VCO
~0. mhe NRZ phase detector l~ comQrises AN~ gates 12 and 14 and inverter 16. The NRZ phase detector 10 generates two sisnals, an "up" sisnal that goes to the "+" input of loop filter ~6, and a "down"
signal that goeC tc ~he "-" input of the loop filter 26. The "up" and "down" signals are used by the loop filter 2fi to prcduce the control voltage for VCO 20. The loop filter ~, which is suitably a charge pum2, receives the "up" and "down" pulses from the NRZ phase detector 10 and integrates their difference into an analos VCO control signal with only low frequency components.
As will be further exptained ~elow, thc effec-tiveness of this invention relies on the centering of the low-going edge of the ~CO C recovere~ clock output within this blivet.
If the VCC output low-going edgc is already centered within the blivet ?roduced by the XOR
gate 6, the duration of the out?u. from ~ND gate l is the same as the duration of the output from A~ID
gate 14, and the inputs to the loop filtee ~6 cancel, indicating that no fre~uencv ad~ustment is necessary and the loop filter ~6 ccntinue_ ~o produce an unchansed ~'CO con rol signal.
I~- the ~cwnward trarsition of the ~CC ~0 clock signal output is late within the blivet, the output o ~il5 sate 1- has a longer d~ration than the output of A~ gate 14, and the "up" input to the 1GOP filter prevails over the "dcwn" input and the VC0 control voltase out?ut of the loop filter ~6 increases the ~IC0 frequency to cause the next low-going edse to occur sooner wi~hir. the blivet interval.
Conversely, if the ~ownward transition of the VC0 0 cloc~ si~nal is early within the blivet, the output of ~.ND sate 1~ has a longer duration than the cutput of AMD gate 12, and the "down" input to the 'oop filter ~6 prevails over the "up" input and lS the ~.'C0 control voltage output of the low?ass filter 6 decreases the VC0 fre~uency to cause the next low-going edge to arrive later in the blivet interval.
It will there'ore be seen that regardless of the pc,sition of the cloc~ signal relative to the center of the blivet, the ~IRZ phase de'ector 1~ and loop filter 26 operate over time to for^e the downward edge o~ the cloc~ signal ~rom ~C0 30 tc the center of the blivet interval.
Because the delay elements ' and a are identical, they produce equal delays des?ite process and temperature variations. ~herefore, changes in process or temperature af~ect the blivet wid~h, which affects ~he ~I~Z ?hase detec~or's sensitivity, but not its ability ~o properly detect phase differences. Sensitivity increases with increasing blivet width, because the dif'erence between the widths of the "up" an~ "down" "ulses can be greater.
~he c1oc'~ output ~,f the ~'C^ ~ is a'so a~?? ied r~
to ~h~ cloc~ i~put of 'li2-flo? ~. mh~ D i~pu' o' 'lir-flop 2~ is connec~ed tc reseive the once de~a~fe~ ~r~z data output cf the bli~et cenera,cr ~3, from the out?ut of delay ele~ent 2. In .he approach u'ilized in ~his invention, the abili'y to ?osition the clock edse 'hat cloc~s the dat2 into flip-flop 28 in the center of the "eye" of the bit interval depe~ds on the centeri.,s of the opoosite clcck edge within the blivet, as described a~ove.
The VCO is balanced to have a near perfect 50%
duty cycle. So, with the ~CC 2C clock output low-soins transitions centered in the blivets from XOR
gate ~, the high going tr2nsition.s are glaranteed to occur uhile the data ?resent at the junction of the two delay elements 2 and 4 is rish' in the middle of the bit interval. This is because the falling clock edge occurred half of blivet after the end of the las. bi' interval, and data at this junction is also de~ayed by one half blivet at the time of the rising clock edge. The D inpu. of flip-flop ~8 is conr.ec~ed to this junction between the delay elements ~ and ~ and is cloc'~ed by the rising ed~e of the VCO 20 output cloc'~ sigr,als.
mhus, the circuit shown in FIG. 3 reliably recovers NRZ clock and data signals an~ lends i~sel' tc integration into an TC.
mhe circuit shown in FTG. ' only performs phase detection, not frequency detection. Vet frequency detection permits much more efficient frequency acquisition by ~ ph2se lock loo?. ~onse-quently, referring now tc FTG. ~, a complete system for NRZ clock and data recovery includes 211 of the circuitry shown in FTG. ', plus an ~PZ frequency detec'or 50, a loc~ detec'or 30 ar.~ 0~ ga'es ^4 an~
22 for combining the "U?" ~nd "dcwn" out?uts of the r~
phace and NP~Z frequency detectcrs hefore they are presented to the locp filter 6.
In the fu!l system shown in F~G. 4, the blivet signal is supplie~ to the NRZ frequency detector SQ
an~ the lock detector 3Q, as well as the NRZ phase detector 10. The NRZ frequency detector 50 also receives the clock output of the VCO 2C an~ a "lock" signal from the loc~ detec.or 30. Li~e the NRZ phase detector 1?, the ~IP~Z frequency detectcr 50 produces "up" and "down" signals to steer the VCO 20 via the loop filter 26. These "up" and "down" signals are combined with the "up" and "down" signals from the ?IRZ phase dete-tor 10 by up OR gate 24 and down OR gate 22. mhe ~IRZ frequency detectcr 5Q also produces "not-good" sisnals that inform the lock detector 30 when the present blivet is soing to cause an "up" or "down" sisnal.
Referring now to ~IG. ', the W~Z frequency detector 50 contains four flip-flops 52, 57, 58 and 59 that are clocked by the risins edge of the blivet signal and one flip-f10p 53 that is clocked by the falling edge of the blivet signal, due to the operation of inverter 51. Flip-f1OpS 5~ and 53 monitor the clock output of the VCO 2~ on opposite edges of the blivet signal. A "good" blivet is a blivet whose rising edge occurs while the ~CO O
cloc~ out is hish and whose falling edge occurs while it is low. A good blivet therefore sets fli~-flop 52 an~ resets flip-f10F 53. NA~ sate 54 monitors the Q output of f lip-Çlop 52 and the /Q
output of flip-Flop 5~, anc pro~uces a low output whenever a good blivet has been detected by these two flip-flops.
The Following table summari~es the information inferred by the NRZ frequency detector 5~ circuitry from the relationship be.ween the clock output level and the blivet edges:
CLCCK/BLIVET IMFE~ENCES
High-going Low-ccing Fre-uency Edae (F~5~) Edge ~FF5~ Inference 1 ~ good 1 1 Too Low 0 0 Too High 1 Unclear (?) NAMr sate 55 mcnitors the states of fli?-flops 52 and 53 for the "too-low" condition, i.e., both flip-flops having a high ou'pu.. The ~hird input to this NAND gate 55 is fro~ the D-C fiip-flop created by r1AND sates 62 an~ 63, which produccs a high out?ut from NAND ~ate 62 and a low output from NAND gate 63 when the f equency is toc high. ~he reason for this thied input to NAr~D sate 55 is to prevent going directly from a "too-high" ccnditio~.
to a "too-low" condition without soing through a good state in-between. Thus, when the last state out of the ~RZ frequency detec.or was "Down ~too-high)", NAND gate 55 is prevented from producing an active-low output indicating that the next state should be "Up (toc-low)" by the low out?ut of NAND
gate 63.
Similarly, NAr!~ gate 56 monit^rs the states of flip-flops 5~ and 53 for the "too-high" condition, i.e., bo'h flip-flcps having a low outpu,. An~, i the same way as before, a high output of ~AND
gate 61 in the other D-C flip-flcp, in~.icating that the last state was not "'oo-low", is reqltired befoee an active "~co-hish" sicnzl can bc produced.
~he C-C f'ip-flops 6~ and 6~, co~p~ising ~!AND
gates 60,61 and 5~,63, hold the last '~no~n frec~enc~ direc~ion, ~Go-low, ~oc-hi~,h ^r neither, until a ~sood blivet oc-urs. An active low ~good sisnal a'tempts tc clear bcth of the D-C flip-flo?s, and will do so in the absence of an active low ,~tco-low or ~too-hi~h sisn21.
The states of the ~t~.?~D sates 54, 55 and 56 are cloc~ed intc flip-flo?s 5 " 5~ and ~9, respec-tively, by the rising edse of the next blivet, if the signal lock is inactive low, indic tins that the ?hase lock 1oo? is not loc~ed. If the sisnal lock is active high, indicatin~ that a r,hase lock loop loc~ has beer. detected, the signal lock ~orces the /good flip-flop 57 to its cleared state, producins an inactive hish output from that flip-flip, which indicates that the state is "good'.
The active high lock signal also forces the "too-70W" an~ "too-hish" flip-flops tc isncre their inputs from NA~D sates 55 and 56, respectively, and produce inactive hish outputs. Thus, uhile the loop is locked, the NRZ frequency detector cannot produce any out?ut to disrupt that lock, leavins control entirely up to the ~RZ phase detector 10.
Note, however, that even while the loop is locked and the up and down outputs of the ~RZ
frequency detector 50 are disabled, it can still produce /sood (blivet) signal outputs from NAND
gate 5~. Thus, as will be further explained below, if enough ~good blivets are detected, the lock detector 3~ can respon~ by decre~entins down ~o below its one half full level and soing ~o an unlocked condition.
Referrins now to FTG. ~, a series o' blivets prc~uces appro?riate results in the p.ese~ce of 3S three differen~ cloc~. ~requencies, one correc~, one t too-hish and onc too-low. In FIG. 6, socd rela-ticnships ~etween blive~s an~ clcck levels are indicated by "O"s abcve clock sisnal, while too low re7ationshi?s are indica.ed with ar. "L" and too-highs ~ith an "II". Unc7ear resu'ts, where the clock is low fcr the risins blivet edse and high for fallin~ blivet ed~e, are indicated ~ith a "?".
Everythin~ is simple for the "Corrcct Clock Freq" line, with each hlivet producins a "O". The "Clock Low" line is much more interesting, producing the sequence of relationshipc:
"OCL~??'IOC?'-. The unclear, ?, indications are totally unrecognized by the circuitry shown in FIG. 5, so the "H" at blivet 7 is ignored because the last indication was a ~lr " and so the output of NA~7D aate 61 is low disa~ling MAMD gate 56 which would otherwise detect this "H". Even though the "H" is not detected, the .ailure tc satisfy ~A~D
gate 54 because the Q out?ut of flip-flop 52 is low causes the output of MAND sate 54 tc co high, and :~ this hi~h is clocked into flip-f7o? 5' by the next blivet (blivet 8~. The continui~g hish output cf flip-flop 57 has no effect on the D-C f'ip-flops comprising NAND gates 60,61 and 6~,6'.
The sood blivet at blivet ~ is detectcd by NA~D sate 54, whose output goes low in response.
The sood state of flip-10ps 52 and 53 also means that neither of the other MAMD gates 55 or 56 are satisfied. The low out?ut of M~ND gate 54 is clccked into flip-flop 5' on the rising edae of blivet 9. The same rising clock edge of blivet 9 also cloc!~s the high levels that are present on the output of MA;~D qates 55 and 56 into flip-flo?s '3 and 59, respective~y. The low out?ut of ~'ip-flQp 57 immediately clears the two D-C flip-flo~s 6~,61 r! ~
and 6~ , so that their outputs on N~!D gates 6C
and 62 become lo~, since both of both of their inputs are now high. ~hQ cleare~ states of the D-C
lip-flops 60,61 and 62,63 enable the third inputs tc ~A~D gateC '5 and 56, allo-~ing them tc detect the next high or low s~ate. lf the frequency were now correct, ~ostly gocd blivets would be detected from now on and the N~Z frecuency detector wouLd behave as it does on the line labelQd "Correct Cloc~ Freq".
In the absence of ,itter, an incorrect frequency will produce indications like those shown in F G. 6, in which the desired indica.ion always follows a good blivet, and any erroneous indica-tions always follow a desired indication anC are locke~ out by it. In the presence of jitter, however, erroneous indications sometimes follow a good blivet and reach the out2ut. Even in the presence of heavy jitter, though, the desired indi-cations always heavily out-weish the erroneous ones and the system successfully locks onto the correct frequency.
For the NRZ frequency detector ~0 to work correctly, the blivet length has to be less than one half of the bit interval. And, since resistGrs implemented in silicon and metal capacitors both have tolerances of up to +/- 2Q~, and the delay elements 2 and 4 are based on an RC product of such devices, the worst case delay tolerance could be as bad as +/- 40~. Thus, if the delay lines are designed to produce a ccmbined delay of 35~ of the bit interval, the worst case deviation still leaves the resulting tota' delay un~er half of the bit interval, thereby permittins the N~ fre~uency detector ~ to function properly.
ReCerrins now tc F_G. ', the lock detectcr ~C
receives as its inputs the blivet sisnal and the ~goo~ signal. Counter 1~ is a saturating up/down counter. A saturatins counter is a counter that does not wrap aroun~; when it reaches a ful1 coun', it s.ays there as additional in~re~ent sisnals are received. The saturatins up~dcwn ccunt_r 42 is incremented b~ incrementing means 38 whene~er a "sood" blivet is detecte~ by ~ sate ' , and decremented by ~ecrementing means 40 whenever a "bac" blivet is detected by A~D sate 36. lnver'er 34 chanses the /sood signal into a sood signal for ~ND sate 32. When the count in the counter reaches a threshold, for instance, more than half full, a "lock" sisnal is generate~.
The ratio of the increment step size (typically one) tc the ecrement step size (typicall~ some multiple of increment size) deter-mines the tolerance of the lock detector 30 to jitter. A 1:4 ratio has been found to provide a suita~le jitter tolerance. The size of the saturating u~down counter 42 (in bits) determines the integration time of the lock detec~or ~0, ~nd hence its response time to a locked oz unlocked concition. ~here is a trade-ofc between the speed with which a lock is identified and the certainty of that identiCication. Tn the present applica-tion, 12 bits of binary, for a total count of 40a6, has been found to be a suitable length for this counter, so that a lock indication appears after a count of 2048. If jitter is minimal, a loc~ is achie~ed in a~out l~ microseconds at ~?0 Megabits per second.
ThiC lock detectcr 30 implemertation is completely digital and therefore does not require an an31Os integ!ation capacitcr an~ associate~ IC
?in. This is a significant benefit in some appli-caticns where 2 shortage of IC pin- is an important ~esisn constraint.
~h~s, i. can be seen that the present inven-tion provides an improved ~RZ clock and data recover~ syctem that len~s itself to in~ecra~icn, that includes a NRZ phase ~etector, an `i~Z
frequency de'ector anc a lcck detec'cr, and that provides au~omatic centering of the clock edge withiq the bit interval in a manner tha~ is inde?endent of analog delays and process and temperat~re variations.
While a preferred embodiment cf the present invention has been shown and described, it will be a?~arent to those skilled in the art that many changes and modifications may be made without departing frcm the invention in its broader aspects. The claims that follow are therefore intended to cover all such changes and modifica-tions as fall within the true s?irit and scope of the invention.
Claims (42)
1. An NRZ clock recovery system comprising:
a blivet generator (8) coupled to receive an NRZ input and producing a blivet signal in response to a transition of the NRZ input;
a controllable oscillator (20) coupled to receive an oscillator control signal and producing a recovered NRZ clock signal; and a control signal generator (10, 6) coupled to receive the recovered NRZ clock signal and the blivet signal and producing the oscillator control signal.
a blivet generator (8) coupled to receive an NRZ input and producing a blivet signal in response to a transition of the NRZ input;
a controllable oscillator (20) coupled to receive an oscillator control signal and producing a recovered NRZ clock signal; and a control signal generator (10, 6) coupled to receive the recovered NRZ clock signal and the blivet signal and producing the oscillator control signal.
2. An NRZ clock recovery system according to claim 1 wherein the blivet generator (3) comprises:
a delay element (2,4) coupled to receive an NRZ input and producing a delayed NRZ output;
and an exclusive-OR gate (6) coupled to receive the NRZ input and the delayed NRZ output and producing a blivet signal when the state of the NRZ input is different from the state of the twice delayed NRZ output.
a delay element (2,4) coupled to receive an NRZ input and producing a delayed NRZ output;
and an exclusive-OR gate (6) coupled to receive the NRZ input and the delayed NRZ output and producing a blivet signal when the state of the NRZ input is different from the state of the twice delayed NRZ output.
3. An NRZ clock recovery system according to claim 2 wherein the delay element comprises:
a first delay element (2) coupled to receive the NRZ input and producing a once delayed NRZ output; and a second delay element (4) coupled to receive the once delayed NRZ output of the first delay element and producing a twice delayed NRZ
output.
a first delay element (2) coupled to receive the NRZ input and producing a once delayed NRZ output; and a second delay element (4) coupled to receive the once delayed NRZ output of the first delay element and producing a twice delayed NRZ
output.
4. An NRZ clock recovery system according to claim 3, wherein the first (2) and second (4) delay elements produce substantially equal delays.
5. An NRZ clock recovery system according to claim 1 wherein the control signal generator (10, 26) comprises:
a phase detector (10) coupled to receive the recovered NRZ clock signal and the blivet signal and producing an up signal and a down signal; and oscillator control means (26) coupled to receive the up and down signals from the phase detector (10) and producing the oscillator control signal.
a phase detector (10) coupled to receive the recovered NRZ clock signal and the blivet signal and producing an up signal and a down signal; and oscillator control means (26) coupled to receive the up and down signals from the phase detector (10) and producing the oscillator control signal.
6. An NRZ clock recovery system according to claim 5. wherein the phase detector (10) comprises:
first gating means (12) coupled to receive the recovered NRZ clock signal and the blivet signal and producing an up output while the state of the recovered NRZ clock signal is the same as the state of the blivet signal;
an inverter (16) coupled to receive the recovered NRZ clock signal and producing an inverted recovered NRZ clock signal; and second gating means (14) coupled to receive the inverted recovered NRZ clock signal and the blivet signal and producing a down signal while the state of the inverted recovered NRZ clock signal is the same as the state of the blivet signal.
first gating means (12) coupled to receive the recovered NRZ clock signal and the blivet signal and producing an up output while the state of the recovered NRZ clock signal is the same as the state of the blivet signal;
an inverter (16) coupled to receive the recovered NRZ clock signal and producing an inverted recovered NRZ clock signal; and second gating means (14) coupled to receive the inverted recovered NRZ clock signal and the blivet signal and producing a down signal while the state of the inverted recovered NRZ clock signal is the same as the state of the blivet signal.
7. An NRZ clock recovery system according to claim 1 further comprising:
a frequency detector (50) coupled to receive the recovered NRZ clock signal and the blivet signal and producing an up signal and a down signal; and up/up-down/down combining means (24,22), coupled to receive the up and down signals from the phase detector (10) and the up and down signals from the frequency detector (50) and producing a combined up signal and a combined down signal.
a frequency detector (50) coupled to receive the recovered NRZ clock signal and the blivet signal and producing an up signal and a down signal; and up/up-down/down combining means (24,22), coupled to receive the up and down signals from the phase detector (10) and the up and down signals from the frequency detector (50) and producing a combined up signal and a combined down signal.
8. An NRZ clock recovery system according to claim 7, wherein the frequency detector (50) comprises:
means for determining (52,51,53) the states of the clock signal at the times that blivet edges occur; and means for ascertaining (55,56) from the determined states when the frequency of the recovered NRZ clock signal is apparently low or apparently high, and producing the up signal when the frequency is apparently low and the down signal when the frequency is apparently high.
means for determining (52,51,53) the states of the clock signal at the times that blivet edges occur; and means for ascertaining (55,56) from the determined states when the frequency of the recovered NRZ clock signal is apparently low or apparently high, and producing the up signal when the frequency is apparently low and the down signal when the frequency is apparently high.
9. An NRZ clock recovery system according to claim 8, wherein the means for ascertaining comprises:
means for ignoring (55,62,63) an apparently low frequency when the result of an immediately preceding ascertaining was that the frequency was apparently high; and means for ignoring (56,60,61) an apparently high frequency when the result of the immediately preceding ascertaining was that the frequency was apparently low.
means for ignoring (55,62,63) an apparently low frequency when the result of an immediately preceding ascertaining was that the frequency was apparently high; and means for ignoring (56,60,61) an apparently high frequency when the result of the immediately preceding ascertaining was that the frequency was apparently low.
10. An NRZ clock recovery system according to claim 7, wherein the frequency detector (50) is further coupled to receive a lock signal, and while the lock signal is active the frequency detector does not produce (58,59) the up signal or the down signal.
11. An NRZ clock recovery system according to claim 7, wherein the frequency detector (50) further produces (54,57) a not-good blivet signal and wherein the system further comprises:
a lock detector (30) coupled to receive the blivet signal and the not-good blivet signal and producing a lock signal when the number of blivet signals received bears a predetermined rela-tionship to the number of not-good blivet signals received.
a lock detector (30) coupled to receive the blivet signal and the not-good blivet signal and producing a lock signal when the number of blivet signals received bears a predetermined rela-tionship to the number of not-good blivet signals received.
12. An NRZ clock recovery system according to claim 11, wherein the lock detector (30) comprises:
a saturating up/down counter (42), the counter incrementing (38) by a first constant in response to each good blivet (32,34) signal and decrementing (40) by a second constant in response to each not-good (36) blivet signal.
a saturating up/down counter (42), the counter incrementing (38) by a first constant in response to each good blivet (32,34) signal and decrementing (40) by a second constant in response to each not-good (36) blivet signal.
13. An NRZ clock recovery system according to claim 4 comprising:
means for recovering data (28) coupled to receive the once delayed NRZ output of the first delay element (2) and the recovered NRZ clock (20) and producing a recovered NRZ data output.
means for recovering data (28) coupled to receive the once delayed NRZ output of the first delay element (2) and the recovered NRZ clock (20) and producing a recovered NRZ data output.
14. An NRZ clock and data recovery system according to claim 13 wherein the means for recovering NRZ data (28) comprises:
means for holding data (28) having a data input and a clock input, the data input being coupled to receive the once delayed NRZ output and the clock input being coupled to receive the recovered NRZ clock signal, the holding means (28) producing the recovered NRZ data output.
means for holding data (28) having a data input and a clock input, the data input being coupled to receive the once delayed NRZ output and the clock input being coupled to receive the recovered NRZ clock signal, the holding means (28) producing the recovered NRZ data output.
15. An NRZ frequency detector (50) for a phase lock loop circuit, the NRZ frequency detector having a first terminal for receiving a blivet signal and a second terminal for receiving a clock signal, the NRZ frequency detector comprising:
means for determining (52,53) the states of the clock signal at the times that edges of the blivet signal occur; and means for ascertaining (55,56) from the determined states when the frequency of the clock signal is apparently low or apparently high, and producing a too-low signal when the frequency is apparently low and a too-high signal when the frequency is apparently high.
means for determining (52,53) the states of the clock signal at the times that edges of the blivet signal occur; and means for ascertaining (55,56) from the determined states when the frequency of the clock signal is apparently low or apparently high, and producing a too-low signal when the frequency is apparently low and a too-high signal when the frequency is apparently high.
16. An NRZ frequency detector according to claim 15, wherein the means for determining comprises:
a first flip-flop (52) having the clock signal as a data input and the blivet signal as a clock input, the first flip-flop (52) producing a first indication signal if the clock signal was high at the time of a first edge of the blivet signal;
an inverter (51) coupled to receive the blivet signal and producing an inverted blivet signal; and a second flip-flop (53) having the clock signal as a data input and the inverted blivet signal as a clock input, the second flip-flop producing a second indication signal if the clock signal was high at the time of a second edge of the blivet signal.
a first flip-flop (52) having the clock signal as a data input and the blivet signal as a clock input, the first flip-flop (52) producing a first indication signal if the clock signal was high at the time of a first edge of the blivet signal;
an inverter (51) coupled to receive the blivet signal and producing an inverted blivet signal; and a second flip-flop (53) having the clock signal as a data input and the inverted blivet signal as a clock input, the second flip-flop producing a second indication signal if the clock signal was high at the time of a second edge of the blivet signal.
17. An NRZ frequency detector according to claim 15, wherein the means for ascertaining comprises:
first gating means (55) coupled to receive a first indication signal if the clock signal was high at a time of a first edge of the blivet signal and a second indication signal if the clock signal was high at a time of a second edge of the blivet signal, the first gating means (55) producing a too-low signal in the first and second indication signals are both active; and second gating means (56) coupled to receive the first indication signal and the second indication signal, the second gating means producing a too-high signal if the first and second indication signals are both inactive.
first gating means (55) coupled to receive a first indication signal if the clock signal was high at a time of a first edge of the blivet signal and a second indication signal if the clock signal was high at a time of a second edge of the blivet signal, the first gating means (55) producing a too-low signal in the first and second indication signals are both active; and second gating means (56) coupled to receive the first indication signal and the second indication signal, the second gating means producing a too-high signal if the first and second indication signals are both inactive.
18. An NRZ frequency detector according to claim 1?, wherein the means for ascertaining further comprises:
means for holding (58) a previously produced too-low signal; and means for holding (59) a previously produced too-high signal.
means for holding (58) a previously produced too-low signal; and means for holding (59) a previously produced too-high signal.
19. An NRZ frequency detector according to claim 18 further comprising a lock signal generating means (30), and the means for ascertaining (55,56,53,59) is coupled (53,59) to receive the lock signal and while the lock signal is active the means for ascertaining does not produce (58,59) the up signal or the down signal.
20. An NRZ frequency detector according to claim 18, wherein the means for ascertaining (55,56,58,59) further comprises:
means for ignoring (55,62,63) an apparently low frequency when the result of an immediately preceding ascertaining was that the frequency was apparently high; and means for ignoring (56,60,61) an apparently high frequency when the result of the immediately preceding ascertaining was that the frequency was apparently low.
means for ignoring (55,62,63) an apparently low frequency when the result of an immediately preceding ascertaining was that the frequency was apparently high; and means for ignoring (56,60,61) an apparently high frequency when the result of the immediately preceding ascertaining was that the frequency was apparently low.
21. A lock detector (30) for 2 phase lock loop circuit, the lock detector having a first terminal for receiving blivet pulses and a second terminal for receiving not-good pulses, the lock detector (30) comprising:
a saturating up/down counter (42);
means for incrementing (38) the counter by a first constant in response to each blivet pulse that is not accompanied by a not-good pulse (32,34);
means or decrementing (40) the counter by a second constant in response to each blivet that is accompanied by a not-good pulse (36); and means for determining (44) if a count in the counter (42) is above a predetermined threshold value and producing a lock signal when the count is above the predetermined threshold.
a saturating up/down counter (42);
means for incrementing (38) the counter by a first constant in response to each blivet pulse that is not accompanied by a not-good pulse (32,34);
means or decrementing (40) the counter by a second constant in response to each blivet that is accompanied by a not-good pulse (36); and means for determining (44) if a count in the counter (42) is above a predetermined threshold value and producing a lock signal when the count is above the predetermined threshold.
22. A method for NRZ clock recovery comprising comprising the steps of:
generating a blivet (8) in response to a transition of an NRZ input;
producing a recovered NRZ clock signal (20) according to an oscillator control signal; and generating a control signal (10,26) based on the timing relationship between the recovered NRZ clock signal and the blivet signal.
generating a blivet (8) in response to a transition of an NRZ input;
producing a recovered NRZ clock signal (20) according to an oscillator control signal; and generating a control signal (10,26) based on the timing relationship between the recovered NRZ clock signal and the blivet signal.
73. A method for NRZ clock recovery according to claim 22 wherein the step of generating a blivet (8) comprises the steps of:
delaying (2,4) an NRZ input to produce a delayed NRZ output; and producing (6) a blivet signal when the state of the NRZ input is different from the state of the delayed NRZ output.
delaying (2,4) an NRZ input to produce a delayed NRZ output; and producing (6) a blivet signal when the state of the NRZ input is different from the state of the delayed NRZ output.
24. A method for NRZ clock recovery according to claim 23 wherein the delaying step comprises the steps of:
delaying once (2) the NRZ input to produce a once delayed NRZ output; and delaying again (4) the once delayed NRZ
output to produce a twice delayed NRZ output.
delaying once (2) the NRZ input to produce a once delayed NRZ output; and delaying again (4) the once delayed NRZ
output to produce a twice delayed NRZ output.
25. A method for NRZ clock recovery according to claim 24 wherein the delaying once step (2) and the delaying again step (4) produce substantially equal delays.
26. A method for NRZ clock recovery according to claim 22 wherein the step of generating a control signal (10,20) comprises:
detecting a phase relationship (10) between the recovered NRZ clock signal and the blivet signal and producing an up signal and a down signal according to the detected relationship; and producing an oscillator control signal (26) according to the up and down signals.
detecting a phase relationship (10) between the recovered NRZ clock signal and the blivet signal and producing an up signal and a down signal according to the detected relationship; and producing an oscillator control signal (26) according to the up and down signals.
27. A method for NRZ clock recovery according to claim 26 wherein the step of detecting a phase relationship (10) comprises:
producing (12) an up output while the state of the recovered NRZ clock signal is the same as the state of the blivet signal; and producing (14) a down signal while the state of an inverted recovered NRZ clock signal is the same as the state of the blivet signal.
producing (12) an up output while the state of the recovered NRZ clock signal is the same as the state of the blivet signal; and producing (14) a down signal while the state of an inverted recovered NRZ clock signal is the same as the state of the blivet signal.
28. A method for NRZ clock recovery according to claim 22 further comprising the steps of:
detecting a frequency relationship (50) between the recovered NRZ clock signal and the blivet signal to produce an up signal and a down signal; and combining (24,22) the up and down signals from the phase detecting step (10) and the up and down signals from the frequency detecting step (50) to produce a combined up signal and a combined down signal.
detecting a frequency relationship (50) between the recovered NRZ clock signal and the blivet signal to produce an up signal and a down signal; and combining (24,22) the up and down signals from the phase detecting step (10) and the up and down signals from the frequency detecting step (50) to produce a combined up signal and a combined down signal.
29. A method for NRZ clock recovery according to claim 28, wherein the detecting step (50) comprises the steps of:
determining (52,51,53) the states of the recovered NRZ clock signal at the times that blivet edges occur; and ascertaining (55,55) from the determined states when the frequency of the recovered NRZ
clock signal is apparently low or apparently high, to produce the up signal when the frequency is apparently low and the down signal when the frequency is apparently high.
determining (52,51,53) the states of the recovered NRZ clock signal at the times that blivet edges occur; and ascertaining (55,55) from the determined states when the frequency of the recovered NRZ
clock signal is apparently low or apparently high, to produce the up signal when the frequency is apparently low and the down signal when the frequency is apparently high.
30. A method for NRZ clock recovery according to claim 29, wherein the step of ascertaining comprises the steps of:
ignoring (55,62,63) an apparently low frequency when the result of an immediately preceding ascertaining step was that the frequency was apparently high; and ignoring (56,60,61) an apparently high frequency when the result of the immediately preceding ascertaining step was that the frequency was apparently low.
ignoring (55,62,63) an apparently low frequency when the result of an immediately preceding ascertaining step was that the frequency was apparently high; and ignoring (56,60,61) an apparently high frequency when the result of the immediately preceding ascertaining step was that the frequency was apparently low.
31. A method for NRZ clock recovery according to claim 28, wherein the detecting step (50) further comprises the steps of:
receiving a lock signal; and not producing the up signal or the down signal while the lock signal is active
receiving a lock signal; and not producing the up signal or the down signal while the lock signal is active
32. A method for NRZ clock recovery according to claim 28, wherein the detecting step (50) further comprises the steps of:
generating a not-good blivet (54, 57); and detecting a lock (30) to produce a lock signal when the number of blivet signals received bears a predetermined relationship to the number of not-good blivet signals received.
generating a not-good blivet (54, 57); and detecting a lock (30) to produce a lock signal when the number of blivet signals received bears a predetermined relationship to the number of not-good blivet signals received.
33. A method for NRZ clock recovery according to claim 32 wherein the detecting a lock step (30) comprises the step of:
incrementing (38) a saturating up/down counter (42) by a first constant in response to each good blivet (32, 34) signal; and decrementing (40) the saturating up/down counter (42) by a second constant in response to each not-good (36) blivet signal.
incrementing (38) a saturating up/down counter (42) by a first constant in response to each good blivet (32, 34) signal; and decrementing (40) the saturating up/down counter (42) by a second constant in response to each not-good (36) blivet signal.
34. A method for NRZ clock recovery according to claim 25 further comprising the step of:
recovering (28) NRZ data to produce a recovered NRZ data output.
recovering (28) NRZ data to produce a recovered NRZ data output.
35. A method for NRZ clock and data recovery according to claim 34 wherein the recovering step (28) comprises the steps of:
receiving the once delayed NRZ output from the delaying once step (2) at an input of a means for holding data; and clocking the once delayed NRZ output from the delaying once step into the means for holding data using the recovered NRZ clock (20).
receiving the once delayed NRZ output from the delaying once step (2) at an input of a means for holding data; and clocking the once delayed NRZ output from the delaying once step into the means for holding data using the recovered NRZ clock (20).
36. A method for NRZ frequency detecting in a phase lock loop circuit, the method comprising the steps of:
determining (52,53) the states of a clock signal at the times that edges of a blivet signal occur; and ascertaining (55,56) from the determined states when the frequency of the clock signal is apparently low or apparently high, producing a too-low signal when the frequency is apparently low and a too-high signal when the frequency is apparently high.
determining (52,53) the states of a clock signal at the times that edges of a blivet signal occur; and ascertaining (55,56) from the determined states when the frequency of the clock signal is apparently low or apparently high, producing a too-low signal when the frequency is apparently low and a too-high signal when the frequency is apparently high.
37. A method for NRZ frequency detecting according to claim 36, wherein the determining step comprises the steps of:
producing (52) a first indication signal if the clock signal was high at the time of a first edge of the blivet signal;
inverting (51) the blivet signal to produce an inverted blivet signal; and producing (53) a second indication signal if the clock signal was high at the time of a second edge of the blivet signal.
producing (52) a first indication signal if the clock signal was high at the time of a first edge of the blivet signal;
inverting (51) the blivet signal to produce an inverted blivet signal; and producing (53) a second indication signal if the clock signal was high at the time of a second edge of the blivet signal.
38. A method for NRZ frequency detecting according to claim 36 wherein the step of ascertaining comprises the steps of:
ANDing (55) a first signal indicating if the clock signal was high at a time of a first edge of the blivet signal with a second signal indicating if the clock signal was high at a time of a second edge of the blivet signal to produce a too-low signal if the first and second indication signals are both active; and ANDing (56) an inverted version of the first indicating signal with an inverted version of the second indicating signal to produce a too-high signal if the first and second indicating signals are both inactive.
ANDing (55) a first signal indicating if the clock signal was high at a time of a first edge of the blivet signal with a second signal indicating if the clock signal was high at a time of a second edge of the blivet signal to produce a too-low signal if the first and second indication signals are both active; and ANDing (56) an inverted version of the first indicating signal with an inverted version of the second indicating signal to produce a too-high signal if the first and second indicating signals are both inactive.
39. A method for NRZ frequency detecting according to claim 37 wherein the step of ascertaining further comprises the steps of:
holding (58) a previously produced too-low signal; and holding (59) a previously produced too-high signal.
holding (58) a previously produced too-low signal; and holding (59) a previously produced too-high signal.
40. A method for NRZ frequency detecting according to claim 39 further comprising the steps of:
generating a lock signal (30); and not producing (58,59) the too-low signal or the too-high signal while the lock signal is active.
generating a lock signal (30); and not producing (58,59) the too-low signal or the too-high signal while the lock signal is active.
41. A method for NRZ frequency detecting according to claim 39 wherein the step of ascertaining (55,56,58,59) further comprises the steps of:
ignoring (55,60,63) an apparently low frequency when the result of an immediately preceding ascertaining step was that the frequency was apparently high; and ignoring (56,60,61) an apparently high frequency when the result of the immediately preceding ascertaining step was that the frequency was apparently low.
ignoring (55,60,63) an apparently low frequency when the result of an immediately preceding ascertaining step was that the frequency was apparently high; and ignoring (56,60,61) an apparently high frequency when the result of the immediately preceding ascertaining step was that the frequency was apparently low.
42. A method of lock detecting (30) for a phase lock loop circuit, the method comprising the steps of:
incrementing (38) a saturating up/down counter (42) by a first constant in response to a blivet pulse that is not accompanied by a not-good pulse (32,34);
decrementing (40) the saturating up/down counter (42) by a second constant in response to a blivet pulse that is accompanied by a not-good pulse (36); and determining (44) if a count in the saturating up/down counter (4 ) is above a predetermined threshold value to produce a lock signal when the count is above the predetermined threshold.
incrementing (38) a saturating up/down counter (42) by a first constant in response to a blivet pulse that is not accompanied by a not-good pulse (32,34);
decrementing (40) the saturating up/down counter (42) by a second constant in response to a blivet pulse that is accompanied by a not-good pulse (36); and determining (44) if a count in the saturating up/down counter (4 ) is above a predetermined threshold value to produce a lock signal when the count is above the predetermined threshold.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US665,861 | 1991-03-07 | ||
| US07/665,861 US5164966A (en) | 1991-03-07 | 1991-03-07 | Nrz clock and data recovery system employing phase lock loop |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2061909A1 true CA2061909A1 (en) | 1992-09-08 |
Family
ID=24671857
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002061909A Abandoned CA2061909A1 (en) | 1991-03-07 | 1992-02-26 | Nrz clock and data recovery system employing phase lock loop |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US5164966A (en) |
| EP (1) | EP0502739B1 (en) |
| JP (1) | JPH0828701B2 (en) |
| CA (1) | CA2061909A1 (en) |
| DE (1) | DE69229016T2 (en) |
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| JPH01231430A (en) * | 1988-03-10 | 1989-09-14 | Nec Corp | Pll lock detecting circuit |
| US4972161A (en) * | 1989-06-28 | 1990-11-20 | Digital Equipment Corporation | Clock recovery for serial data communications system |
| US4949051A (en) * | 1989-09-01 | 1990-08-14 | General Electric Company | Phase lock clock recovery with aided frequency aquisition |
| US4984255A (en) * | 1989-11-15 | 1991-01-08 | National Semiconductor Corporation | Edge transition insensitive delay line system and method |
| US5127026A (en) * | 1990-04-05 | 1992-06-30 | Gazelle Microcircuits, Inc. | Circuit and method for extracting clock signal from a serial data stream |
| DE4019976A1 (en) * | 1990-06-22 | 1992-01-02 | Siemens Ag | Self-centring data recovery circuit for signal receiver - uses flip=flop receiving input signal and clock signal provided by phase regulating loop |
-
1991
- 1991-03-07 US US07/665,861 patent/US5164966A/en not_active Expired - Fee Related
-
1992
- 1992-02-26 CA CA002061909A patent/CA2061909A1/en not_active Abandoned
- 1992-03-05 EP EP92301912A patent/EP0502739B1/en not_active Expired - Lifetime
- 1992-03-05 DE DE69229016T patent/DE69229016T2/en not_active Expired - Fee Related
- 1992-03-06 JP JP8444892A patent/JPH0828701B2/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| EP0502739B1 (en) | 1999-04-28 |
| EP0502739A3 (en) | 1993-09-22 |
| JPH0828701B2 (en) | 1996-03-21 |
| US5164966A (en) | 1992-11-17 |
| DE69229016D1 (en) | 1999-06-02 |
| DE69229016T2 (en) | 1999-11-11 |
| JPH0591098A (en) | 1993-04-09 |
| EP0502739A2 (en) | 1992-09-09 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| FZDE | Discontinued |