US RE36090 E Abstract A device synchronizes an internal signal with respect to a reference signal, each signal comprising pulses normally occurring at a rated frequency. The device uses a phase comparator to analyze the phase of the internal signal and the reference signal and produce one logic state if the phase of the internal signal is in advance of the phase of the reference signal and a second logic state otherwise. A programmable frequency divider divides an internal clock signal by a first number if the phase comparator signal produces the first logic state or by a second number if the phase comparator produces the second logic state. A multiplexer provides the programmable divider with either the first number or the second number depending on the logic state produced the phase comparator. The device also includes a storage element for sequentially storing a predetermined number of the latest logic states of the phase comparator. The device also includes circuitry for decrementing the first number when the latest stored logic states of the phase comparator have a single occurrence of the first logic state and for incrementing the second number when the latest stored logic states of the phase comparator have a single occurrence of the second state.
Claims(30) 1. A method of synchronizing an internal signal with a reference signal, comprising the steps of:
a) providing a phase comparison signal having a first logic state when the phase of the internal signal is in advance with respect to the reference signal, and having a second logic state otherwise; b) producing the internal signal by .Iadd.frequency .Iaddend.dividing a clock signal by a first variable divisor when said phase comparison signal is in said first logic state or a second variable divisor when said phase comparison signal is in said second logic state, the value of said first divisor initially being greater than the value of said second divisor; and c) altering the . .value.!. .Iadd.values .Iaddend.of said first and second variable divisors based on the . .value.!. .Iadd.values .Iaddend.of successive states of said phase comparison signal. 2. The method of claim 1, further including the step of storing a predetermined number of successive states of said phase comparison signal.
3. The method of claim 2 wherein the step of altering the . .value.!. .Iadd.values .Iaddend.of said first and second variable divisors alters the . .value.!. .Iadd.values .Iaddend.by selectively incrementing or decrementing the value of said first or second variable divisor based on the . .value.!. .Iadd.values .Iaddend.of said predetermined number of successive states.
4. The method of claim 3, further including the step of preventing the value of said second .Iadd.variable .Iaddend.divisor from exceeding the value of said first .Iadd.variable .Iaddend.divisor by comparing the value of said first .Iadd.variable .Iaddend.divisor with the value of said second .Iadd.variable .Iaddend.divisor and disabling the decrementing of the value of said first .Iadd.variable .Iaddend.divisor and the incrementing of the value of said second .Iadd.variable .Iaddend.divisor when the value of said first .Iadd.variable .Iaddend.divisor equals the value of said second .Iadd.variable .Iaddend.divisor.
5. The method of claim 3, further including the step of preventing the value of said first .Iadd.variable .Iaddend.divisor from exceeding a predetermined maximum first divisor value by comparing the value of said first .Iadd.variable .Iaddend.divisor with . .the value of.!. said maximum first divisor .Iadd.value .Iaddend.and disabling the incrementing of the value of said first .Iadd.variable .Iaddend.divisor when the value of said first .Iadd.variable .Iaddend.divisor equals . .the value of.!. said maximum first divisor .Iadd.value.Iaddend..
6. The method of claim 3, further including the step of preventing the value of said second .Iadd.variable .Iaddend.divisor from falling below a predetermined minimum second divisor value by comparing the value of said second .Iadd.variable .Iaddend.divisor with . .the value of.!. said minimum second divisor .Iadd.value .Iaddend.and disabling the decrementing of the value of said second .Iadd.variable .Iaddend.divisor when the value of said second .Iadd.variable .Iaddend.divisor equals . .the value of.!. said minimum second divisor .Iadd.value.Iaddend..
7. A method for synchronizing an internal signal with respect to a reference signal having a reference period, said internal signal having a long period or a short period between which said reference period is normally included, comprising the following steps:
a) analyzing the duration of the reference period with respect to the durations of the long and short periods; b) when the reference period is closer to the long period than the short period, incrementing the short period; c) when the reference period is closer to the short period than the long period, decrementing the long period; d) when the reference period is higher than the long period, incrementing the long period; and e) when the reference period is lower than the short period, decrementing the short period. 8. A method according to claim 7, wherein step a) consists in analyzing a predetermined number of the latest successive differences of phase between the internal signal and the reference signal and wherein the requirements of steps b) to e) are met when, respectively:
the number of phases .Iadd.where the internal signal is .Iaddend.in advance is higher than the number of . .lagged.!. phases .Iadd.where the internal signal is lagging.Iaddend.; the number of . .lagged.!. phases .Iadd.where the internal signal is lagging .Iaddend.is higher than the number of phases .Iadd.where the internal signal is .Iaddend.in advance; there is no phase lag .Iadd.of the internal signal.Iaddend.; and there is no phase advance .Iadd.of the internal signal.Iaddend.. 9. A method according to claim 8, .Iadd.further .Iaddend.comprising the following steps:
when the number of successive phases in advance is higher than a predetermined number, incrementing the long period; and when the number of successive phase lags is higher than the predetermined number, decrementing the short period. 10. A method according to claim 7, .Iadd.further .Iaddend.comprising the following steps:
comparing the long and short periods; and inhibiting the decrementation of the long period and the incrementation of the short period when the difference between these periods is lower than a predetermined threshold. 11. A method according to claim 7, .Iadd.further .Iaddend.comprising the following steps:
comparing the long period with a maximum period and inhibiting the incrementation of the long period when said periods are equal; and comparing the short period with a minimum period and inhibiting the decrementation of the short period when said periods are equal. 12. A device for synchronizing an internal signal with a reference signal, the device comprising:
a phase comparator providing a phase comparison signal with a first logic state when the phase of the internal signal is in advance with respect to the reference signal, and a second logic state otherwise; a programmable frequency divider producing the internal signal by dividing a clock signal by a first variable divisor when said phase comparison signal is in said first logic state or a second variable divisor when said phase comparison signal is in said second logic state, the value of said first divisor initially being greater than the value of said second divisor; and control means for altering the . .value.!. .Iadd.values .Iaddend.of said first and second variable . .divisor.!. .Iadd.divisors based on the values of successive states of said phase comparison signal.Iaddend.. 13. The device of claim 12, further including storage means for storing a predetermined number of successive states of said phase comparison signal.
14. The device of claim 13 wherein said control means alters the . .value.!. .Iadd.values .Iaddend.of said first and second variable . .divisor.!. .Iadd.divisors .Iaddend.by selectively incrementing or decrementing the value of said first or second variable divisor based on the . .value.!. .Iadd.values .Iaddend.of said predetermined number of successive states.
15. The device of claim 14, further including means for preventing the value of said second .Iadd.variable .Iaddend.divisor from exceeding the value of said first .Iadd.variable .Iaddend.divisor by comparing the value of said first .Iadd.variable .Iaddend.divisor with the value of said second .Iadd.variable .Iaddend.divisor and disabling the portion of said control means for decrementing the value of said first .Iadd.variable .Iaddend.divisor and incrementing the value of said second .Iadd.variable .Iaddend.divisor when the value of said first .Iadd.variable .Iaddend.divisor equals the value of said second .Iadd.variable .Iaddend.divisor.
16. The device of claim 14, further including means for preventing the value of said first .Iadd.variable .Iaddend.divisor from exceeding a predetermined maximum first divisor value by comparing the value of said first .Iadd.variable .Iaddend.divisor with . .the value of.!. said maximum first divisor .Iadd.value .Iaddend.and disabling the portion of said control means for incrementing the value of said first .Iadd.variable .Iaddend.divisor when the value of said first .Iadd.variable .Iaddend.divisor equals . .the value of.!. said maximum first divisor .Iadd.value.Iaddend..
17. The device of claim 14, further including means for preventing the value of said second .Iadd.variable .Iaddend.divisor from falling below a predetermined minimum second divisor value by comparing the value of said second .Iadd.variable .Iaddend.divisor with . .the value of.!. said minimum second divisor .Iadd.value .Iaddend.and disabling the portion of said control means for decrementing the value of said second .Iadd.variable .Iaddend.divisor when the value of said second .Iadd.variable .Iaddend.divisor equals . .the value of.!. said minimum second divisor .Iadd.value.Iaddend..
18. A device for synchronizing an internal signal with respect to a reference signal, said signals each comprising pulses normally occurring at a rated frequency, comprising:
a phase comparator providing a phase comparison signal at a predetermined logic state when the phase of said internal signal is in advance with respect to the phase of . .the synchronization.!. .Iadd.said reference .Iaddend.signal, and at a complementary logic state otherwise; a programmable frequency divider fed by a clock and providing said internal signal; a multiplexer providing to said divider, in order to program it, a high binary number when said phase comparison signal is at said predetermined state, and a low binary number, otherwise; a means for sequentially storing a predetermined number of the latest states of said phase comparison signal; a first counting means clocked by said internal signal and providing said high binary number to said multiplexer; a first detection means setting said first counting means in down-counting position when said predetermined number of latest stored states of said phase comparison signal have a single state at said predetermined state; a second counting means, clocked by said internal signal and providing said low binary number to said multiplexer; and a second detection means setting said second counting means in up-counting position when said predetermined number of latest stored states of said phase comparison signal have a single state at said complementary state. 19. A device according to claim 18, further including:
a third detection means setting said first counting means in up-counting position when said predetermined number of latest stored states of said phase comparison signal are all at said predetermined state; and a fourth detection means setting said second counting means to down-counting position when said predetermined number of latest stored states of said phase comparison signal are at said complementary state. 20. A device according to claim 19, .Iadd.further .Iaddend.comprising a first comparator for comparing the high and low binary numbers and setting said first and second counting means in a standby state when the high binary number is lower than or equal to the low .Iadd.binary .Iaddend.number.
21. A device according to claim 18, .Iadd.further .Iaddend.comprising:
a . .second.!. .Iadd.first .Iaddend.comparator comparing said high binary number with a maximum . .binary.!. number and setting said first counting means in a standby state when said maximum number is exceeded; and a . .third.!. .Iadd.second .Iaddend.comparator comparing said low binary number with a minimum number and setting said second counting means in said standby state when said .Iadd.low binary number is less than said .Iaddend.minimum number . .is exceeded.!.. 22. A device according to claim 18, wherein said programmable divider comprises a divider by two distinct numbers, either one . .is.!. .Iadd.being .Iaddend.selected by the output signal of a pulse generator programmed by the number provided by said multiplexer. .Iadd.23. A phase-locked loop, comprising:
a phase comparator having a reference-signal input, a synchronized-signal input, and a phase output; a shift register having a clock input, a signal input coupled to said phase output of said phase comparator, and a plurality of register outputs; a programmable frequency-divider circuit having first and second divisor inputs, a control input coupled to said phase output of said phase comparator, and a synchronized-signal output that is coupled to said synchronized-signal input of said phase comparator; a high-divisor counter having a clock input coupled to said synchronized-signal output of said phase comparator, a count-down input, and a high-count output coupled to said first divisor input of said frequency-divider circuit; a low-divisor counter having a clock input coupled to said synchronized-signal output of said phase comparator, a count-up input, and a low-count output coupled to said second divisor input of said frequency-divider circuit; a first logic circuit having a plurality of inputs, one of said inputs coupled to said phase output of said phase comparator and the remaining of said inputs each coupled to a corresponding one of said register outputs, said first logic circuit having an output coupled to said count-down input of said high-divisor counter; and a second logic circuit having a plurality of inputs, one of said inputs coupled to said phase output of said phase comparator and the remaining of said inputs each coupled to a corresponding one of said register outputs, said second logic circuit having an output coupled to said count-up input of said low-divisor counter. .Iaddend..Iadd.24. The phase-locked loop of claim 23 wherein said first and second logic circuits each comprises an AND gate. .Iaddend..Iadd.25. The phase-locked loop of claim 23 wherein: said first logic circuit comprises a first AND gate that has an inverting input for every other input, the number of inverting inputs greater than or equal to the number of noninverting inputs; and said second logic circuit comprises a second AND gate that has an inverting input for every other input, the number of noninverting inputs of the second AND gate greater than or equal to the number of inverting inputs of the second AND gate. .Iaddend..Iadd.26. A phase-locked loop, comprising:
a phase comparator having a reference-signal input, a synchronized-signal input, and a phase output; a shift register having a clock input, a signal input coupled to said phase output of said phase comparator, and a plurality of register outputs; a programmable frequency-divider circuit having first and second divisor inputs, a control input coupled to said phase output of said phase comparator, and a synchronized-signal output that is coupled to said synchronized-signal input of said phase comparator; a high-divisor counter having a clock input coupled to said synchronized-signal output of said phase comparator, a count-up input, and a high-count output coupled to said first divisor input of said frequency-divider circuit; a low-divisor counter having a clock input coupled to said synchronized-signal output of said phase comparator, a count-down input, and a low-count output coupled to said second divisor input of said frequency-divider circuit; a first logic circuit having a plurality of inputs, one of said inputs coupled to said phase output of said phase comparator and the remaining of said inputs each coupled to a corresponding one of said register outputs, said first logic circuit having an output coupled to said count-up input of said high-divisor counter; and a second logic circuit having a plurality of inputs, one of said inputs coupled to said phase output of said phase comparator and the remaining of said inputs each coupled to a corresponding one of said register outputs, said second logic circuit having an output coupled to said count-down input of said low-divisor counter. .Iaddend..Iadd.27. The phase-locked loop of claim 26 wherein said first and second logic circuits each comprises an AND gate. .Iaddend..Iadd.28. A phase-locked loop, comprising:
a phase comparator having a reference-signal input, a synchronized-signal input, and a phase output; a shift register having a clock input, a signal input coupled to said phase output of said phase comparator, and a plurality of register outputs; a programmable frequency-divider circuit having a clock input, first and second divisor inputs, a control input coupled to said phase output of said phase comparator, and a synchronized-signal output that is coupled to said synchronized-signal input of said phase comparator; a high-divisor counter having a clock input coupled to said synchronized-signal output of said phase comparator, a count-down input, a count-up input, and a high-count output coupled to said first divisor input of said frequency-divider circuit; a low-divisor counter having a clock input coupled to said synchronized-signal output of said phase comparator, a count-down input, a count-up input, and a low-count output coupled to said second divisor input of said frequency-divider circuit; a first logic circuit having a plurality of inputs, one of said inputs coupled to said phase output of said phase comparator and the remaining of said inputs each coupled to a corresponding one of said register outputs, said first logic circuit having an output coupled to said count-up input of said high-divisor counter; a second logic circuit having a plurality of inputs, one of said inputs coupled to said phase output of said phase comparator and the remaining of said inputs each coupled to a corresponding one of said register outputs, said second logic circuit having an output coupled to said count-down input of said high-divisor counter; a third logic circuit having a plurality of inputs, one of said inputs coupled to said phase output of said phase comparator and the remaining of said inputs each coupled to a corresponding one of said register outputs, said third logic circuit having an output coupled to said count-up input of said low-divisor counter; and a fourth logic circuit having a plurality of inputs, one of said inputs coupled to said phase output of said phase comparator and the remaining of said inputs each coupled to a corresponding one of said register outputs, said fourth logic circuit having an output coupled to said count-down input of said low-divisor counter. .Iaddend..Iadd.29. The phase-locked loop of claim 28 wherein said shift register comprises three storage cells. .Iaddend..Iadd.30. The phase-locked loop of claim 28 wherein said programmable frequency-divider circuit comprises: a programmable frequency divider having a clock input coupled to said clock input of said frequency-divider circuit, a divisor input, and a synchronized-signal output that is coupled to said synchronized-signal output of said frequency-divider circuit; and a multiplexer having first, second, and control inputs respectively coupled to said first, second, and control inputs of said frequency-divider circuit, said multiplexer having an output coupled to said divisor input of said programmable frequency divider. .Iaddend..Iadd.31. The phase-locked loop of claim 28 wherein:
said shift register has first second, and third register outputs and has three sequentially arranged memory cells each having an output that is coupled to a corresponding one said register outputs; said first logic circuit comprises an AND gate having a first input coupled to said phase output of said phase comparator and having second, third, and fourth inputs respectively coupled to said first, second, and third register outputs; said second logic circuit comprises an AND gate having a first terminal coupled to said phase output of said phase comparator, a second input coupled to said first register output, and a third input coupled to said second register output, said second terminal inverted with respect to said first and third terminals; said third logic circuit comprises an AND gate having a first terminal coupled to said phase output of said phase comparator, a second input coupled to said first register output, and a third input coupled to said second register output, said first and third terminals inverted with respect to said second terminal; and said fourth logic circuit comprises an AND gate having a first input coupled to said phase output of said phase comparator and having second, third, and fourth inputs respectively coupled to said first, second, and third register outputs, said inputs of said fourth logic circuit logically inverted with respect to said inputs of said first logic circuit. .Iaddend..Iadd.32. The phase-locked loop of claim 28, further comprising: a first disable circuit intercoupled between said high-divisor counter and said first logic circuit, said first disable circuit having an input coupled to said output of said first logic circuit, a disable input, and an output coupled to said count-up terminal of said high-divisor counter; a second disable circuit intercoupled between said high-divisor counter and said second logic circuit, said second disable circuit having an input coupled to said output of said second logic circuit, a disable input, and an output coupled to said count-down terminal of said high-divisor counter; a third disable circuit intercoupled between said low-divisor counter and said third logic circuit, said third disable circuit having an input coupled to said output of said third logic circuit, a disable input, and an output coupled to said count-up terminal of said low-divisor counter; a fourth disable circuit intercoupled between said low-divisor counter and said fourth logic circuit, said fourth disable circuit having an input coupled to said output of said fourth logic circuit, a disable input, and an output coupled to said count-down terminal of said low-divisor counter; a first comparator having a high-divisor input coupled to said output of said high-divisor counter, a maximum-high-divisor input, and an output coupled to said disable input of said first disable circuit; a second comparator having a high-divisor input coupled to said output of said high-divisor counter, a low-divisor input coupled to said output of said low-divisor, and an output coupled to said disable inputs of said second and third disable circuits; and a third comparator having a low-divisor input coupled to said output of said low-divisor counter, a minimum-low-divisor input, and an output coupled to said disable input of said fourth disable circuit. .Iaddend..Iadd.33. The phase-locked loop of claim 28 wherein said programmable frequency-divider circuit comprises: a selectable-divisor frequency divider having a clock input coupled to said clock input of said programmable frequency-divider circuit, a divisor input, and a signal output; a multiplexer having first, second, and control inputs respectively coupled to said first divisor, second divisor, and control inputs of said programmable frequency-divider circuit, said multiplexer having an output; a fixed frequency divider having an input coupled to said signal output of said selectable-divisor frequency divider, and having a synchronized-signal output coupled to said synchronized-signal output of said programmable frequency-divider circuit; and a programmable pulse generator having an input coupled to said output of said multiplexer, an output coupled to said divisor input of said selectable-divisor frequency divider, and a clock input coupled to said signal output of said selectable-divisor frequency divider. .Iaddend..Iadd.34. A method, comprising: performing a number of sequential comparisons between the phase of a reference signal having a reference period and the phase of a synchronized signal that has first and second periods; causing said synchronized signal to have said first period if said reference signal lags said synchronized signal during a first of said comparisons; lengthening said first period if said reference signal lags said synchronized signal during said first comparison and during a first predetermined number of comparisons that immediately precede said first comparison; shortening said first period if said phase of said reference signal alternates leading and lagging said phase of said synchronized signal according to a first predetermined sequence during said first comparison and during a second predetermined number of comparisons that immediately precede said first comparison; causing said synchronized signal to have said second period if said reference signal leads said synchronized signal during said first comparison; shortening said second period if said reference signal leads said synchronized signal during said first comparison and during a third predetermined number of comparisons that immediately precede said first comparison; and lengthening said second period if said phase of said reference signal alternates leading and lagging said phase of said synchronized signal according to a second predetermined sequence during said first comparison and during a fourth predetermined number of comparisons that immediately precede said first comparison. .Iaddend..Iadd.35. The method of claim 34, further comprising:
inhibiting said lengthening of said first period if said first period is equal to or longer than a predetermined maximum period; and inhibiting said shortening of said second period if said second period is equal to or shorter than a predetermined minimum period. .Iaddend..Iadd.36. The method of claim 34, further comprising: comparing the lengths of said first and second periods; and inhibiting said shortening of said first period and said lengthening of said second period if said second period is equal to or greater than said first period. .Iaddend..Iadd.37. The method of claim 34 wherein: said first predetermined number equals said third predetermined number; and said second predetermined number equals said fourth predetermined number. .Iaddend..Iadd.38. The method of claim 34 wherein: said first and third predetermined numbers equal 3; and said second and fourth predetermined numbers equal 2. .Iaddend..Iadd.39. The method of claim 34 wherein: said first predetermined sequence equals lead-lag-lead of said phase of said reference signal with respect to said phase of said synchronized signal; and said second predetermined sequence equals lag-lead-lag of said phase of said reference signal with respect to said phase of said synchronized signal. .Iaddend..Iadd.40. The method of claim 34, further comprising:
generating said synchronized signal from a clock signal; wherein said causing said synchronized signal to have said first period includes dividing said clock signal by a first divisor; and said causing said synchronized signal to have said second period includes dividing said clock signal by a second divisor. .Iaddend..Iadd.41. The method of claim 34, further comprising: generating said synchronized signal from a clock signal; wherein said causing said synchronized signal to have said first period includes dividing said clock signal by a first divisor; said causing said synchronized signal to have said second period includes dividing said clock signal by a second divisor; wherein said lengthening said first period includes incrementing said first divisor; wherein said shortening said first period includes decrementing said first divisor; wherein said shortening said second period includes decrementing said second divisor; and wherein said lengthening said second period includes incrementing said second divisor. .Iaddend..Iadd.42. The method of claim 34, further comprising:
frequency dividing a clock signal by a first number to generate an intermediate signal; frequency dividing said intermediate signal by a second number to generate said synchronized signal; generating a first value for said first number in response to each of a third number of pulses of said intermediate signal; and generating a second value for said first number in response to each of a fourth number of pulses of said intermediate signal, said fourth number being equal to the difference between a predetermined number and said third number. .Iaddend..Iadd.43. The method of claim 34, further comprising:
frequency dividing a clock signal by a first number to generate an intermediate signal; frequency dividing said intermediate signal by a second number to generate said synchronized signal; generating a first value for said first number in response to each of a third number of pulses of said intermediate signal; generating a second value for said first number in response to each of a fourth number of pulses of said intermediate signal, said fourth number being equal to the difference between a predetermined number and said third number; wherein said causing said synchronized signal to have said first period includes generating a third value for said third number; and wherein said causing said synchronized signal to have said second period includes generating a fourth value for said third number. .Iaddend..Iadd. . The method of claim 34, further comprising:
frequency dividing a clock signal by a first number to generate an intermediate signal; frequency dividing said intermediate signal by 768 to generate said synchronized signal; generating a third number that has a value that depends upon said phase of said reference signal with respect to said phase of said synchronized signal, said value ranging from 0 to 511; setting said first number to 10 in response to each of said third number of pulses of said intermediate signal; and setting said first number to 9 in response to each of a fourth number of pulses of said intermediate signal, said fourth number being equal to the difference between 1024 and said third number. .Iaddend. Description The present invention relates to phase locked loops (PLL) and particularly to a digital PLL. A PLL is a device providing a signal synchronized with respect to a reference signal, that is, having the same phase and frequency as the reference signal. In the absence of a reference signal, the PLL provides a frequency approximately equal to the rated frequency of the reference signal. The function of the PLL is to follow the reference signal and to provide a signal similar to the reference signal in case of an accidental absence of the latter. FIG. 1A shows a conventional simple digital PLL, intended to synchronize the pulses of an internal signal Fint with respect to the pulses of a reference signal SYNC. This PLL comprises a digital phase comparator (PHASE-COMP) 10 receiving signals Fint and SYNC and providing at its output a signal SGN. Signal SGN assumes a first logic state (1) if the phase of signal Fint is in advance to the phase of signal SYNC, i.e. if a pulse of signal Fint occurs prior to a pulse of signal SYNC, and assumes a second state (0) if signal Fint has a delayed phase. Signal Fint is obtained by dividing a clock signal CK through a divider (DIV N Number N FIG. 1B shows signals SYNC, Fint and SGN in an exemplary application of the PLL of Figure IA to TV signals, where signal Fint is used for generating the line scan. Signal SYNC is then generated by a receiving circuit or a magnetic tape recorder and its rated period is 64 microseconds. Period T of signal SYNC, especially in the case of a video tape recorder where it is determined by the running speed of the tape, can vary within a substantially high range. Number N FIG. 1B shows the signals obtained in steady state assuming that signal SYNC is at its rated period (64 microseconds). At a time t0, a pulse from signal Fint is detected prior to a synchronization pulse from signal SYNC, which indicates that signal SYNC has a phase lag with respect to signal Fint. The signal SGN of the phase comparator goes to "1" and selects the division of the clock signal CK by the high number N At a time t1, the next synchronization pulse of signal SYNC is detected before a pulse from signal Fint. This indicates that the signal Fint has a phase lag with respect to signal SYNC. Signal SGN goes to "0" and selects the division by the low number N At a time t2, a pulse from signal Fint is detected prior to the next synchronization pulse from signal SYNC, indicating a phase lag. Signal SGN switches to state "1" and the process is resumed in the same way as at time t0. In the above case where the period T of the reference signal SYNC is constant and equal to the average value of the long and short periods of signal Fint, the period of signal Fint oscillates at each period between 63 and 65 microseconds. If the period T is slightly higher than 64 microseconds, the period of signal Fint oscillates in the same way with, however, from time to time, a few successive periods of 65 microseconds. If the period T is slightly lower than 65 microseconds, signal Fint will have a period of 65 microseconds with, from time to time, a period of 63 microseconds. In fact, the long or short periods of signal Fint successively occur so that the average frequency calculated over a large number of periods of signal Fint is equal to that of signal SYNC. It can be demonstrated that the duty cycle of signal SGN is expressed by (T-T If period T is higher than 65 microseconds, the period of signal Fint is blocked at 65 microseconds and is therefore false. Complementary cases occur when period T ranges from 63 to 64 microseconds. Thus, this PLL is capable of following a reference signal SYNC only if its period T ranges within determined limits (here, 63 and 65 microseconds). It is therefore advantageous to separate as much as possible the extreme limits of signal Fint in order to increase the tracking range. However, by increasing this range, the resolution of the PLL is reduced. In fact, assuming that signal SYNC is the line scan synchronization signal of a TV set, the period jumps between the extreme periods of signal Fint, from which scanning is generated, appear in the form of lines on the screen alternatively beginning forward or backward one with respect to the other. If the extreme periods of signal Fint are too high, these line differences become visible and unpleasant (a vertical line will be indented). An object of the intention is to provide a simple digital PLL with a large tracking range and a good resolution. The invention is embodied in a method of synchronizing an internal signal to a reference signal by comparing the phase of the internal signal with the phase of the reference signal and producing a first logic state if the phase of the internal signal is in advance of the phase of the reference signal, or a second logic state otherwise. The internal signal is produced by dividing a clock signal by a first or a second variable divisor depending on the logic state of the phase comparison signal. The value of the variable divisors may be altered based on the value of successive states of the phase comparison signal. Alternatively, this object is achieved with a method for synchronizing an internal signal with respect to a reference signal having a reference period, the internal signal having a long or short period between which the reference period is normally included, comprising the following steps: a) analyzing the duration of the reference period with respect to the durations of the long and short periods; b) if the reference period is closer to the long period than to the short period, incrementing the short period; c) if the reference period is closer to the short period than to the long period, decrementing the long period; d) if the reference period is higher than the long period, incrementing the long period; and e) if the reference period is smaller than the short period, decrementing the short period. According to an embodiment of the invention, step a) consists in analyzing a predetermined number of the latest successive phase differences between the internal signal and the reference signal and the requirements of steps b) to e) are respectively met if: the number of phases in advance is higher than the number of lagged phases; the number of lagged phases is higher than the number of phases in advance; there is no phase lag; and there is no phase advance. According to an embodiment of the invention, if the number of the successive phases in advance is higher than a predetermined number, the long period is incremented; and, if the number of successive phase lags is higher than the predetermined number, the short period is decremented. According to an embodiment of the invention, the following steps are achieved: comparing the long period with the short period and inhibiting the decrementation of the long period and the incrementation of the short period when the difference between these periods is lower than a predetermined threshold. According to an embodiment of the invention, the following steps are provided: comparing the long period with a maximum period and inhibiting the incrementation of the long period if these periods are equal; and comparing the short period with a minimum period and inhibiting the decrementation of the short period if these periods are equal. The invention is embodied in a device containing a phase comparator to measure the phase difference of the internal signal and the reference signal and producing a phase comparison signal. A programmable divider divides a clock signal by a first or second variable divisor depending on the logic state of the phase comparison signal. The value of the variable divisor may be altered depending on the value of successive logic states of the phase comparison signal. Alternatively, the invention provides a device for synchronizing an internal signal with respect to a reference signal, these signals each comprising pulses normally occurring at a rated frequency, comprising a phase comparator providing a phase comparison signal at a predetermined logic state if the phase of the internal signal is in advance with respect to the phase of the synchronization signal, and to a complementary logic state otherwise; a programmable sequence divider receiving a clock signal and providing the internal signal; a multiplexer providing the divider, in order to program it, with a high binary number if the comparison signal is at a predetermined state, and a low binary number otherwise; means for sequentially storing the latest states of the comparison signal; first counting means clocked by the internal signal and providing the high binary number to the multiplexer; first detection means selling the first counting means to down-counting position when the latest stored states of the comparison signal have a single state at the predetermined state; second counting means clocked by the internal signal and providing the low binary number to the multiplexer; and second detection means setting the second counting means to up-counting position when the latest stored states of the comparison signal have a single state at the complementary state. According to an embodiment or the invention, this device further comprises third detection means setting the first counting means to counting position when the stored states of the comparison signal are all set to the predetermined state; and fourth detection means setting the second counting means to down-counting position when the stored states of the comparison signal are all set to the complementary state. According to an embodiment or the invention, this device further comprises a first comparator for comparing the high and low binary numbers and setting the first and second counting means to standby position if the high number is lower than or equal to the low number. According to an embodiment of the invention, this device further comprises a second comparator for comparing the high binary number with the maximum binary number and setting the first counting means to the standby position if the maximum number is exceeded; and a third comparator for comparing the low binary number with a minimum number and setting the second counting means to the standby position if the minimum number is exceeded. According to an embodiment of the invention, the programmable divider comprises a divider by two distinct numbers, either one is selected by the output signal of a pulse generator programmed by the number supplied by the multiplexer. The foregoing and other objects, features and advantages of the invention will be apparent from the following detailed description of preferred embodiments as illustrated in the accompanying figures wherein: FIGS. 1A and 1B, above described, respectively show a conventional digital PLL and signals associated with this PLL; FIG. 2 is a simplified diagram of a digital PLL according to the invention; FIG. 3 is an exemplary embodiment of the PLL of FIG. 2; and FIG. 4 shows an embodiment of a programmable divider usable in the PLL of FIGS. 2 and 3. In a synchronization system with a PLL as shown in FIG. 1A, the method according to the invention consists in modifying the long (T Following a shift of the frequency of signal SYNC, the period T of the latter is no longer included between 65 the long and short periods of signal Fint. If period T is, for example, higher than the long period, this long period is incremented until it is again higher than period T. If period T is lower than the short period, the short period is decremented until it is again lower than period T. Then, the period of signal Fint the most remote from period T is brought nearer until the other period of signal Fint is the most remote, and so forth until the long and short periods are separated from each other by a predetermined minimum value while being on both sides of period T. FIG. 2 is a simplified drawing of a PLL according to an embodiment of the invention enabling this modification of long and short periods of signal Fint by adequately modifying the division numbers N The above described components of the PLL according to the invention have the same functions as those of the conventional PLL of FIG. 1A and similarly provide a signal Fint with short periods T The conventional divider by N A control circuit (CTRL) 18 increments or decrements numbers N a) Signal SGN presents states alternating between "1" and "0" .Iadd.(.Iaddend. its duty cycle is 0.5). As indicated in relation with FIG. 1B, the period T of signal SYNC is thus centered between the short periods T b) Signal SGN is continuously at "1" (the resulting signal Fint is blocked on the long period). This indicates that the long period T c) Signal SON presents sequences of "1's" with one "0" from time to time (its duty cycle is close to 1). This indicates that the long period T However, in the case the number of "1's" is very high, the low number N Cases d) and e) here below correspond to the complementary cases of cases b)) and c) and the same reasoning is valid. d) Signal SGN is constantly at "0". This indicates that the short period T e) Signal SGN presents successive sequences of "0's" with one "1" from time to time (its duty cycle close to 0). This indicates that the short period T Of course, the PLL according to the invention operates only if predetermined requirements are complied with. Especially, N FIG. 3 describes in detail an embodiment of the PLL of FIG. 2. The figure shows again the phase comparator 10, the multiplexer 14 and the programmable divider 13 of FIG. 2. Memories 16 and 17 containing the high number N The last three states SGN States SGN The complements of states SGN The complement of state SGN State SGN In a first step, the AND gates 38, 41, 44 and 47 and other components of the circuit not yet described will not be taken into account because they do not affect the main operation of the PLL. It is assumed that the outputs of the AND gates 37, 40, 43 and 46 are directly connected to the up and down validation inputs of the up/down counters 16 and 17 and that number N Four successive "1's" of signal SGN, which corresponds to the above mentioned cases b) and c), are detected by AND gate 37 which enables up-counting of counter 16 and therefore the incrementation by one unit of number N Four successive "0's" of signal SGN, which corresponds to cases d) and e) above mentioned, are detected by AND gate 40 which enables down-counting of counter 17 and therefore the decrementation by one unit of number N A series of states 1, 0, 1 of signal SGN, which corresponds to cases a) and c) above mentioned, is detected by gate 46 which enables the counting of counter 17 and therefore the incrementation of number N A series of states 0, 1, 0 of signal SGN, which corresponds to cases a) and e) above mentioned, is detected by gate 43 which enables down-counting of counter 16 and therefore the decrementation of number N The circuit according to the invention comprises additional components among which AND gates 38, 41, 44 and 47, preventing numbers N To avoid that counter 16 counts beyond Nmax, its output (N To avoid that counter 17 counts below Nmin, the output (N Numbers N Numbers N N FIG. 4 shows another embodiment of the programmable divider and is intended to show that various modifications can be made to the implementation of the invention. The programmable divider of FIG. 4 comprises a divider by 9 or by 10 (DIV 9/10) 60 receiving the clock signal CK and providing an intermediate signal Fi. The division by 9 (that is, the provision of a pulse every nine pulses of the clock signal CK) or by 10 (that is, the provision of a pulse every ten pulses of signal CK) is selected by a signal 10/9*. State "1" of this signal causes the division by 10 and state "0" causes the division by 9. Signal Fint is obtained from the intermediate signal Fi by dividing the latter by a divider 62. Signal 10/9* is provided by a programmable pulse generator (n-PLS) 64 receiving the intermediate signal Fi and programmed by a number n which is one of the intermediate numbers n In an example adapted to television, the clock signal CK has a 111-MHz frequency, divider 62 is a divider by 768 and the pulse generator 64 is a generator of n pulses amongst 1024 pulses of signal Fi, number n provided by counters 16 or 17 being a binary number (coded on 9 bits) ranging from 0 to 511. With this divider configuration, signal Fi presents successions or 1024 pulses, n or which are separated one from the other by 10 periods of the clock signal CK and 1024-n of which arc separated one from the other by 9 periods of signal CK. Signal Fint presents a pulse every 768 pulses of signal Fi. Thus, the frequency of signal Fint is equal to that of clock signal CK divided by N=768 (n/1024)+9!. If n=256, N=710, and the frequency of signal Fint is equal to the rated line frequency of 15625 Hz. When this divider is used in the PLL according to the invention, as soon as the high (n In addition, the PLL according to the invention can track a signal SYNC having a period varying from 62.3 to 65.7 microseconds (respectively corresponding to n In the embodiment described in relation with FIG. 3, a durable advanced or delayed phase of signal Fint is detected over four periods (AND gates 37, 40). The occurrence of an accidental different phase is detected over three periods (AND gates 43, 46). It would also have been possible to achieve a detection over three periods in both cases. The invention has been described in relation with TV signals, but those skilled in the art will note that this circuit is a digital PLL that can be used instead of any conventional PLL by adapting the frequency of clock CK, the division factors of the dividers and the differences between numbers N Patent Citations
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