US 20080126688 A1
A memory subsystem is provided including an interface circuit adapted for communication with a system and a majority of address or control signals of a first number of memory circuits. The interface circuit includes emulation logic for emulating at least one memory circuit of a second number.
1. A sub-system, comprising:
an interface circuit adapted for communication with a system and a majority of address or control signals of a first number of memory circuits, the interface circuit including emulation logic for emulating at least one memory circuit of a second number.
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19. A method, comprising:
interfacing a majority of address or control signals of a first number of memory circuits and a system; and
emulating at least one memory circuit of a second number.
20. An apparatus, comprising:
a first number of memory circuits; and
an interface circuit in communication with the memory circuits, the interface circuit including emulation logic for emulating at least one memory circuit of a second number;
wherein the interface circuit interfaces a majority of address or control signals of the memory circuits.
This application is a continuation of commonly-assigned U.S. patent application Ser. No. 11/762,010 entitled “Memory Device with Emulated Characteristics” filed Jun. 12, 2007 by Rajan, et al., which, in turn, is a continuation-in-part of commonly-assigned U.S. patent application Ser. No. 11/461,420 entitled “System and Method for Simulating a Different Number of Memory Circuits” filed Jul. 31, 2006 by Rajan, et al., which are incorporated by reference as if fully set forth herein.
1. Technical Field of the Invention
This invention relates generally to digital memory such as used in computers, and more specifically to organization and design of memory modules such as DIMMs.
2. Background Art
Digital memories are utilized in a wide variety of electronic systems, such as personal computers, workstations, servers, consumer electronics, printers, televisions, and so forth. Digital memories are manufactured as monolithic integrated circuits (“ICs” or “chips”). Digital memories come in several types, such as dynamic random access memory (DRAM), static random access memory (SRAM), flash memory, electrically erasable programmable read only memory (EEPROM), and so forth.
In some systems, the memory chips are coupled directly into the system such as by being soldered directly to the system's main motherboard. In other systems, groups of memory chips are first coupled into memory modules, such as dual in-line memory modules (DIMMs), which are in turn coupled into a system by means of slots, sockets, or other connectors. Some types of memory modules include not only the memory chips themselves, but also some additional logic which interfaces the memory chips to the system. This logic may perform a variety of low level functions, such as buffering or latching signals between the chips and the system, but it may also perform higher level functions, such as telling the system what are the characteristics of the memory chips. These characteristics may include, for example, memory capacity, speed, latency, interface protocol, and so forth.
Memory capacity requirements of such systems are increasing rapidly. However, other industry trends such as higher memory bus speeds, small form factor machines, etc. are reducing the number of memory module slots, sockets, connectors, etc. that are available in such systems. There is, therefore, pressure for manufacturers to use large capacity memory modules in such systems.
However, there is also an exponential relationship between a memory chip's capacity and its price. As a result, large capacity memory modules may be cost prohibitive in some systems.
What is needed, then, is an effective way to make use of low cost memory chips in manufacturing high capacity memory modules.
A memory subsystem is provided including an interface circuit adapted for communication with a system and a majority of address or control signals of a first number of memory circuits. The interface circuit includes emulation logic for emulating at least one memory circuit of a second number.
The invention will be understood more fully from the detailed description given below and from the accompanying drawings of embodiments of the invention which, however, should not be taken to limit the invention to the specific embodiments described, but are for explanation and understanding only.
The system device may be any type of system capable of requesting and/or initiating a process that results in an access of the memory circuits. The system may include a memory controller (not shown) through which it accesses the memory circuits.
The interface circuit may include any circuit or logic capable of directly or indirectly communicating with the memory circuits, such as a buffer chip, advanced memory buffer (AMB) chip, etc. The interface circuit interfaces a plurality of signals 108 between the system device and the memory circuits. Such signals may include, for example, data signals, address signals, control signals, clock signals, and so forth. In some embodiments, all of the signals communicated between the system device and the memory circuits are communicated via the interface circuit. In other embodiments, some other signals 110 are communicated directly between the system device (or some component thereof, such as a memory controller, an AMB, or a register) and the memory circuits, without passing through the interface circuit. In some such embodiments, the majority of signals are communicated via the interface circuit, such that L>M.
As will be explained in greater detail below, the interface circuit presents to the system device an interface to emulated memory devices which differ in some aspect from the physical memory circuits which are actually present. For example, the interface circuit may tell the system device that the number of emulated memory circuits is different than the actual number of physical memory circuits. The terms “emulating”, “emulated”, “emulation”, and the like will be used in this disclosure to signify emulation, simulation, disguising, transforming, converting, and the like, which results in at least one characteristic of the memory circuits appearing to the system device to be different than the actual, physical characteristic. In some embodiments, the emulated characteristic may be electrical in nature, physical in nature, logical in nature, pertaining to a protocol, etc. An example of an emulated electrical characteristic might be a signal, or a voltage level. An example of an emulated physical characteristic might be a number of pins or wires, a number of signals, or a memory capacity. An example of an emulated protocol characteristic might be a timing, or a specific protocol such as DDR3.
In the case of an emulated signal, such signal may be a control signal such as an address signal, a data signal, or a control signal associated with an activate operation, precharge operation, write operation, mode register read operation, refresh operation, etc. The interface circuit may emulate the number of signals, type of signals, duration of signal assertion, and so forth. It may combine multiple signals to emulate another signal.
The interface circuit may present to the system device an emulated interface to e.g. DDR3 memory, while the physical memory chips are, in fact, DDR2 memory. The interface circuit may emulate an interface to one version of a protocol such as DDR2 with 5-5-5 latency timing, while the physical memory chips are built to another version of the protocol such as DDR2 with 3-3-3 latency timing. The interface circuit may emulate an interface to a memory having a first capacity that is different than the actual combined capacity of the physical memory chips.
An emulated timing may relate to latency of e.g. a column address strobe (CAS) latency, a row address to column address latency (tRCD), a row precharge latency (tRP), an activate to precharge latency (tRAS), and so forth. CAS latency is related to the timing of accessing a column of data. tRCD is the latency required between the row address strobe (RAS) and CAS. tRP is the latency required to terminate an open row and open access to the next row. tRAS is the latency required to access a certain row of data between an activate operation and a precharge operation.
The interface circuit may be operable to receive a signal from the system device and communicate the signal to one or more of the memory circuits after a delay (which may be hidden from the system device). Such delay may be fixed, or in some embodiments it may be variable. If variable, the delay may depend on e.g. a function of the current signal or a previous signal, a combination of signals, or the like. The delay may include a cumulative delay associated with any one or more of the signals. The delay may result in a time shift of the signal forward or backward in time with respect to other signals. Different delays may be applied to different signals. The interface circuit may similarly be operable to receive a signal from a memory circuit and communicate the signal to the system device after a delay.
The interface circuit may take the form of, or incorporate, or be incorporated into, a register, an AMB, a buffer, or the like, and may comply with Joint Electron Device Engineering Council (JEDEC) standards, and may have forwarding, storing, and/or buffering capabilities.
In some embodiments, the interface circuit may perform operations without the system device's knowledge. One particularly useful such operation is a power-saving operation. The interface circuit may identify one or more of the memory circuits which are not currently being accessed by the system device, and perform the power saving operation on those. In one such embodiment, the identification may involve determining whether any page (or other portion) of memory is being accessed. The power saving operation may be a power down operation, such as a precharge power down operation.
The interface circuit may include one or more devices which together perform the emulation and related operations. The interface circuit may be coupled or packaged with the memory devices, or with the system device or a component thereof, or separately. In one embodiment, the memory circuits and the interface circuit are coupled to a DIMM.
The memory subsystem includes a buffer chip 202 which presents the host system with emulated interface to emulated memory, and a plurality of physical memory circuits which, in the example shown, are DRAM chips 206A-D. In one embodiment, the DRAM chips are stacked, and the buffer chip is placed electrically between them and the host system. Although the embodiments described here show the stack consisting of multiple DRAM circuits, a stack may refer to any collection of memory circuits (e.g. DRAM circuits, flash memory circuits, or combinations of memory circuit technologies, etc.).
The buffer chip buffers communicates signals between the host system and the DRAM chips, and presents to the host system an emulated interface to present the memory as though it were a smaller number of larger capacity DRAM chips, although in actuality there is a larger number of smaller capacity DRAM chips in the memory subsystem. For example, there may be eight 512 Mb physical DRAM chips, but the buffer chip buffers and emulates them to appear as a single 4 Gb DRAM chip, or as two 2 Gb DRAM chips. Although the drawing shows four DRAM chips, this is for ease of illustration only; the invention is, of course, not limited to using four DRAM chips.
In the example shown, the buffer chip is coupled to send address, control, and clock signals 208 to the DRAM chips via a single, shared address, control, and clock bus, but each DRAM chip has its own, dedicated data path for sending and receiving data signals 210 to/from the buffer chip.
Throughout this disclosure, the reference number I will be used to denote the interface between the host system and the buffer chip, the reference number 2 will be used to denote the address, control, and clock interface between the buffer chip and the physical memory circuits, and the reference number 3 will be used to denote the data interface between the buffer chip and the physical memory circuits, regardless of the specifics of how any of those interfaces is implemented in the various embodiments and configurations described below. In the configuration shown in
In the example shown, the DRAM chips are physically arranged on a single side of the buffer chip. The buffer chip may, optionally, be a part of the stack of DRAM chips, and may optionally be the bottommost chip in the stack. Or, it may be separate from the stack.
Initially, first information is received (702) in association with a first operation to be performed on at least one of the memory circuits (DRAM chips). Depending on the particular implementation, the first information may be received prior to, simultaneously with, or subsequent to the instigation of the first operation. The first operation may be, for example, a row operation, in which case the first information may include e.g. address values received by the buffer chip via the address bus from the host system. At least a portion of the first information is then stored (704).
The buffer chip also receives (706) second information associated with a second operation. For convenience, this receipt is shown as being after the storing of the first information, but it could also happen prior to or simultaneously with the storing. The second operation may be, for example, a column operation.
Then, the buffer chip performs (708) the second operation, utilizing the stored portion of the first information, and the second information.
If the buffer chip is emulating a memory device which has a larger capacity than each of the physical DRAM chips in the stack, the buffer chip may receive from the host system's memory controller more address bits than are required to address any given one of the DRAM chips. In this instance, the extra address bits may be decoded by the buffer chip to individually select the DRAM chips, utilizing separate chip select signals (not shown) to each of the DRAM chips in the stack.
For example, a stack of four ×4 1 Gb DRAM chips behind the buffer chip may appear to the host system as a single ×4 4 Gb DRAM circuit, in which case the memory controller may provide sixteen row address bits and three bank address bits during a row operation (e.g. an activate operation), and provide eleven column address bits and three bank address bits during a column operation (e.g. a read or write operation). However, the individual DRAM chips in the stack may require only fourteen row address bits and three bank address bits for a row operation, and eleven column address bits and three bank address bits during a column operation. As a result, during a row operation (the first operation in the method 702), the buffer chip may receive two address bits more than are needed by any of the DRAM chips. The buffer chip stores (704) these two extra bits during the row operation (in addition to using them to select the correct one of the DRAM chips), then uses them later, during the column operation, to select the correct one of the DRAM chips.
The mapping between a system address (from the host system to the buffer chip) and a device address (from the buffer chip to a DRAM chip) may be performed in various manners. In one embodiment, lower order system row address and bank address bits may be mapped directly to the device row address and bank address bits, with the most significant system row address bits (and, optionally, the most significant bank address bits) being stored for use in the subsequent column operation. In one such embodiment, what is stored is the decoded version of those bits; in other words, the extra bits may be stored either prior to or after decoding. The stored bits may be stored, for example, in an internal lookup table (not shown) in the buffer chip, for one or more clock cycles.
As another example, the buffer chip may have four 512 Mb DRAM chips with which it emulates a single 2 Gb DRAM chip. The system will present fifteen row address bits, from which the buffer chip may use the fourteen low order bits (or, optionally, some other set of fourteen bits) to directly address the DRAM chips. The system will present three bank address bits, from which the buffer chip may use the two low order bits (or, optionally, some other set of two bits) to directly address the DRAM chips. During a row operation, the most significant bank address bit (or other unused bit) and the most significant row address bit (or other unused bit) are used to generate the four DRAM chip select signals, and are stored for later reuse. And during a subsequent column operation, the stored bits are again used to generate the four DRAM chip select signals. Optionally, the unused bank address is not stored during the row operation, as it will be re-presented during the subsequent column operation.
As yet another example, addresses may be mapped between four 1 Gb DRAM circuits to emulate a single 4 Gb DRAM circuit. Sixteen row address bits and three bank address bits come from the host system, of which the low order fourteen address bits and all three bank address bits are mapped directly to the DRAM circuits. During a row operation, the two most significant row address bits are decoded to generate four chip select signals, and are stored using the bank address bits as the index. During the subsequent column operation, the stored row address bits are again used to generate the four chip select signals.
A particular mapping technique may be chosen, to ensure that there are no unnecessary combinational logic circuits in the critical timing path between the address input pins and address output pins of the buffer chip. Corresponding combinational logic circuits may instead be used to generate the individual chip select signals. This may allow the capacitive loading on the address outputs of the buffer chip to be much higher than the loading on the individual chip select signal outputs of the buffer chip.
In another embodiment, the address mapping may be performed by the buffer chip using some of the bank address signals from the host system to generate the chip select signals. The buffer chip may store the higher order row address bits during a row operation, using the bank address as the index, and then use the stored address bits as part of the DRAM circuit bank address during a column operation.
For example, four 512 Mb DRAM chips may be used in emulating a single 2 Gb DRAM. Fifteen row address bits come from the host system, of which the low order fourteen are mapped directly to the DRAM chips. Three bank address bits come from the host system, of which the least significant bit is used as a DRAM circuit bank address bit for the DRAM chips. The most significant row address bit may be used as an additional DRAM circuit bank address bit. During a row operation, the two most significant bank address bits are decoded to generate the four chip select signals. The most significant row address bit may be stored during the row operation, and reused during the column operation with the least significant bank address bit, to form the DRAM circuit bank address.
The column address from the host system memory controller may be mapped directly as the column address to the DRAM chips in the stack, since each of the DRAM chips may have the same page size, regardless any differences in the capacities of the (asymmetrical) DRAM chips.
Optionally, address bit A may be used by the memory controller to enable or disable auto-precharge during a column operation, in which case the buffer chip may forward that bit to the DRAM circuits without any modification during a column operation.
In various embodiments, it may be desirable to determine whether the simulated DRAM circuit behaves according to a desired DRAM standard or other design specification. Behavior of many DRAM circuits is specified by the JEDEC standards, and it may be desirable to exactly emulate a particular JEDEC standard DRAM. The JEDEC standard defines control signals that a DRAM circuit must accept and the behavior of the DRAM circuit as a result of such control signals. For example, the JEDEC specification for DDR2 DRAM is known as JESD79-2B. If it is desired to determine whether a standard is met, the following algorithm may be used. Using a set of software verification tools, it checks for formal verification of logic, that protocol behavior of the simulated DRAM circuit is the same as the desired standard or other design specification. Examples of suitable verification tools include: Magellan, supplied by Synopsys, Inc. of 700 E. Middlefield Rd., Mt. View, Calif. 94043; Incisive, supplied by Cadence Design Systems, Inc., of 2655 Sealy Ave., San Jose, Calif. 95134; tools supplied by Jasper Design Automation, Inc. of 100 View St. #100, Mt. View, Calif. 94041; Verix, supplied by Real Intent, Inc., of 505 N. Mathilda Ave. #210, Sunnyvale, Calif. 94085; 0-1n, supplied by Mentor Graphics Corp. of 8005 SW Boeckman Rd., Wilsonville, Oreg. 97070; and others. These software verification tools use written assertions that correspond to the rules established by the particular DRAM protocol and specification. These written assertions are further included in the code that forms the logic description for the buffer chip. By writing assertions that correspond to the desired behavior of the emulated DRAM circuit, a proof may be constructed that determines whether the desired design requirements are met.
For instance, an assertion may be written that no two DRAM control signals are allowed to be issued to an address, control, and clock bus at the same time. Although one may know which of the various buffer chip/DRAM stack configurations and address mappings (such as those described above) are suitable, the verification process allows a designer to prove that the emulated DRAM circuit exactly meets the required standard etc. If, for example, an address mapping that uses a common bus for data and a common bus for address, results in a control and clock bus that does not meet a required specification, alternative designs for buffer chips with other bus arrangements or alternative designs for the sideband signal interconnect between two or more buffer chips may be used and tested for compliance. Such sideband signals convey the power management signal, for example.
In one embodiment, the buffer chip may cause a one-half clock cycle delay between the buffer chip receiving address and control signals from the host system memory controller (or, optionally, from a register chip or an AMB), and the address and control signals being valid at the inputs of the stacked DRAM circuits. Data signals may also have a one-half clock cycle delay in either direction to/from the host system. Other amounts of delay are, of course, possible, and the half-clock cycle example is for illustration only.
The cumulative delay through the buffer chip is the sum of a delay of the address and control signals and a delay of the data signals.
In the specific example shown, the memory controller issues the write operation at t0. After a one clock cycle delay through the buffer chip, the write operation is issued to the DRAM chips at t1. Because the memory controller believes it is connected to memory having a read CAS latency of six clocks and thus a write CAS latency of five clocks, it issues the write data at time t0+5=t5. But because the physical DRAM chips have a read CAS latency of four clocks and thus a write CAS latency of three clocks, they expect to receive the write data at time t1+3=t4. Hence the problem, which the buffer chip may alleviate by delaying write operations.
The waveform “Write Data Expected by DRAM” is not shown as belonging to interface 1, interface 2, or interface 3, for the simple reason that there is no such signal present in any of those interfaces. That waveform represents only what is expected by the DRAM, not what is actually provided to the DRAM.
It should be noted that extra delay of j clocks (beyond the inherent delay) which the buffer chip deliberately adds before issuing the write operation to the DRAM is the sum j clocks of the inherent delay of the address and control signals and the inherent delay of the data signals. In the example shown, both those inherent delays are one clock, so j=2.
In the example shown, the memory controller issues the write operation at t0. After a one clock inherent delay through the buffer chip, the write operation arrives at the DRAM at t1. The DRAM expects the write data at t1+3=t4. The industry specification would suggest a nominal write data time of t0+5=t5, but the AMB (or memory controller), which already has the write data (which are provided with the write operation), is configured to perform an early write at t5−2=t3. After the inherent delay 1203 through the buffer chip, the write data arrive at the DRAM at t3+1=t4, exactly when the DRAM expects it—specifically, with a three-cycle DRAM Write CAS latency 1204 which is equal to the three-cycle Early Write CAS Latency 1202.
An example is shown, in which the memory controller issues a write operation 1302 at time t0. The buffer chip or AMB delays the write operation, such that it appears on the bus to the DRAM chips at time t3. Unfortunately, at time t2 the memory controller issued an activate operation (control signal) 1304 which, after a one-clock inherent delay through the buffer chip, appears on the bus to the DRAM chips at time t3, colliding with the delayed write.
For example, a buffered stack that uses 4-4-4 DRAM chips (that is, CAS latency=4, tRCD=4, and tRP=4) may appear to the host system as one larger DRAM that uses 6-6-6 timing.
Since the buffered stack appears to the host system's memory controller as having a tRCD of six clock cycles, the memory controller may schedule a column operation to a bank six clock cycles (at time t6) after an activate (row) operation (at time t0) to the same bank. However, the DRAM chips in the stack actually have a tRCD of four clock cycles. This gives the buffer chip time to delay the activate operation by up to two clock cycles, avoiding any conflicts on the address bus between the buffer chip and the DRAM chips, while ensuring correct read and write timing on the channel between the memory controller and the buffered stack.
As shown, the buffer chip may issue the activate operation to the DRAM chips one, two, or three clock cycles after it receives the activate operation from the memory controller, register, or AMB. The actual delay selected may depend on the presence or absence of other DRAM operations that may conflict with the activate operation, and may optionally change from one activate operation to another. In other words, the delay may be dynamic. A one-clock delay (1402A, 1502A) may be accomplished simply by the inherent delay through the buffer chip. A two-clock delay (1402B, 1502B) may be accomplished by adding one clock of additional delay to the one-clock inherent delay, and a three-clock delay (1402C, 1502C) may be accomplished by adding two clocks of additional delay to the one-clock inherent delay. A read, write, or activate operation issued by the memory controller at time t6 will, after a one-clock inherent delay through the buffer chip, be issued to the DRAM chips at time t7. A preceding activate or precharge operation issued by the memory controller at time t0 will, depending upon the delay, be issued to the DRAM chips at time t1, t2, or t3, each of which is at least the tRCD or tRP of four clocks earlier than the t7 issuance of the read, write, or activate operation.
Since the buffered stack appears to the memory controller to have a tRP of six clock cycles, the memory controller may schedule a subsequent activate (row) operation to a bank a minimum of six clock cycles after issuing a precliarge operation to that bank. However, since the DRAM circuits in the stack actually have a tRP of four clock cycles, the buffer chip may have the ability to delay issuing the precharge operation to the DRAM chips by up to two clock cycles, in order to avoid any conflicts on the address bus, or in order to satisfy the tRAS requirements of the DRAM chips.
In particular, if the activate operation to a bank was delayed to avoid an address bus conflict, then the precharge operation to the same bank may be delayed by the buffer chip to satisfy the tRAS requirements of the DRAM. The buffer chip may issue the precharge operation to the DRAM chips one, two, or three clock cycles after it is received. The delay selected may depend on the presence or absence of address bus conflicts or tRAS violations, and may change from one precharge operation to another.
Although the multiple DRAM chips appear to the memory controller as though they were a single, larger DRAM, the combined power dissipation of the actual DRAM chips may be much higher than the power dissipation of a monolithic DRAM of the same capacity. In other words, the physical DRAM may consume significantly more power than would be consumed by the emulated DRAM.
As a result, a DIMM containing multiple buffered stacks may dissipate much more power than a standard DIMM of the same actual capacity using monolithic DRAM circuits. This increased power dissipation may limit the widespread adoption of DIMMs that use buffered stacks. Thus, it is desirable to have a power management technique which reduces the power dissipation of DIMMs that use buffered stacks.
In one such technique, the DRAM circuits may be opportunistically placed in low power states or modes. For example, the DRAM circuits may be placed in a precharge power down mode using the clock enable (CKE) pin of the DRAM circuits.
A single rank registered DIMM (R-DIMM) may contain a plurality of buffered stacks, each including four ×4 512 Mb DDR2 SDRAM chips and appear (to the memory controller via emulation by the buffer chip) as a single ×4 2 Gb DDR2 SDRAM. The JEDEC standard indicates that a 2 Gb DDR2 SDRAM may generally have eight banks, shown in
The memory controller may open and close pages in the DRAM banks based on memory requests it receives from the rest of the host system. In some embodiments, no more than one page may be able to be open in a bank at any given time. In the embodiment shown in
The clock enable inputs of the DRAM chips may be controlled by the buffer chip, or by another chip (not shown) on the R-DIMM, or by an AMB (not shown) in the case of an FB-DIMM, or by the memory controller, to implement the power management technique. The power management technique may be particularly effective if it implements a closed page policy.
Another optional power management technique may include mapping a plurality of DRAM circuits to a single bank of the larger capacity emulated DRAM. For example, a buffered stack (not shown) of sixteen ×4 256 Mb DDR2 SDRAM chips may be used in emulating a single x4 4 Gb DDR2 SDRAM. The 4 Gb DRAM is specified by JEDEC as having eight banks of 512 Mbs each, so two of the 256 Mb DRAM chips may be mapped by the buffer chip to emulate each bank (whereas in
However, since only one page can be open in a bank at any given time, only one of the two DRAM chips emulating that bank can be in the active state at any given time. If the memory controller opens a page in one of the two DRAM chips, the other may be placed in the precharge power down mode. Thus, if a number p of DRAM chips are used to emulate one bank, at least p-1 of them may be in a power down mode at any given time; in other words, at least p-1 of the p chips are always in power down mode, although the particular powered down chips will tend to change over time, as the memory controller opens and closes various pages of memory.
As a caveat on the term “always” in the preceding paragraph, the power saving operation may comprise operating in precharge power down mode except when refresh is required.
In some embodiments, at least one first refresh control signal may be sent to a first subset of the physical memory circuits at a first time, and at least one second refresh control signal may be sent to a second subset of the physical memory circuits at a second time. Each refresh signal may be sent to one physical memory circuit, or to a plurality of physical memory circuits, depending upon the particular implementation.
The refresh control signals may be sent to the physical memory circuits after a delay in accordance with a particular timing. For example, the timing in which they are sent to the physical memory circuits may be selected to minimize an electrical current drawn by the memory, or to minimize a power consumption of the memory. This may be accomplished by staggering a plurality of refresh control signals. Or, the timing may be selected to comply with e.g. a tRFC parameter associated with the memory circuits.
To this end, physical DRAM circuits may receive periodic refresh operations to maintain integrity of data stored therein. A memory controller may initiate refresh operations by issuing refresh control signals to the DRAM circuits with sufficient frequency to prevent any loss of data in the DRAM circuits. After a refresh control signal is issued, a minimum time tRFC may be required to elapse before another control signal may be issued to that DRAM circuit. The tRFC parameter value may increase as the size of the DRAM circuit increases.
When the buffer chip receives a refresh control signal from the memory controller, it may refresh the smaller DRAM circuits within the span of time specified by the tRFC of the emulated DRAM circuit. Since the tRFC of the larger, emulated DRAM is longer than the tRFC of the smaller, physical DRAM circuits, it may not be necessary to issue any or all of the refresh control signals to the physical DRAM circuits simultaneously. Refresh control signals may be issued separately to individual DRAM circuits or to groups of DRAM circuits, provided that the tRFC requirements of all physical DRAMs has been met by the time the emulated DRAM's tRFC has elapsed. In use, the refreshes may be spaced in time to minimize the peak current draw of the combination buffer chip and DRAM circuit set during a refresh operation.
The interface circuit includes a system address signal interface for sending/receiving address signals to/from the host system, a system control signal interface for sending/receiving control signals to/from the host system, a system clock signal interface for sending/receiving clock signals to/from the host system, and a system data signal interface for sending/receiving data signals to/from the host system. The interface circuit further includes a memory address signal interface for sending/receiving address signals to/from the physical memory, a memory control signal interface for sending/receiving control signals to/from the physical memory, a memory clock signal interface for sending/receiving clock signals to/from the physical memory, and a memory data signal interface for sending/receiving data signals to/from the physical memory.
The host system includes a set of memory attribute expectations, or built-in parameters of the physical memory with which it has been designed to work (or with which it has been told, e.g. by the buffer circuit, it is working). Accordingly, the host system includes a set of memory interaction attributes, or built-in parameters according to which the host system has been designed to operate in its interactions with the memory. These memory interaction attributes and expectations will typically, but not necessarily, be embodied in the host system's memory controller.
In addition to physical storage circuits or devices, the physical memory itself has a set of physical attributes.
These expectations and attributes may include, by way of example only, memory timing, memory capacity, memory latency, memory functionality, memory type, memory protocol, memory power consumption, memory current requirements, and so forth.
The interface circuit includes memory physical attribute storage for storing values or parameters of various physical attributes of the physical memory circuits. The interface circuit further includes system emulated attribute storage. These storage systems may be read/write capable stores, or they may simply be a set of hard-wired logic or values, or they may simply be inherent in the operation of the interface circuit.
The interface circuit includes emulation logic which operates according to the stored memory physical attributes and the stored system emulation attributes, to present to the system an interface to an emulated memory which differs in at least one attribute from the actual physical memory. The emulation logic may, in various embodiments, alter a timing, value, latency, etc. of any of the address, control, clock, and/or data signals it sends to or receives from the system and/or the physical memory. Some such signals may pass through unaltered, while others may be altered. The emulation logic may be embodied as, for example, hard wired logic, a state machine, software executing on a processor, and so forth.
When one component is said to be “adjacent” another component, it should not be interpreted to mean that there is absolutely nothing between the two components, only that they are in the order indicated.
The physical memory circuits employed in practicing this invention may be any type of memory whatsoever, such as: DRAM, DDR DRAM, DDR2 DRAM, DDR3 DRAM, SDRAM, QDR DRAM, DRDRAM, FPM DRAM, VDRAM, EDO DRAM, BEDO DRAM, MDRAM, SGRAM, MRAM, IRAM, NAND flash, NOR flash, PSRAM, wetware memory, etc.
The physical memory circuits may be coupled to any type of memory module, such as: DIMM, R-DIMM, SO-DIMM, FB-DIMM, unbuffered DIMM, etc.
The system device which accesses the memory may be any type of system device, such as: desktop computer, laptop computer, workstation, server, consumer electronic device, television, personal digital assistant (PDA), mobile phone, printer or other peripheral device, etc.
The various features illustrated in the figures may be combined in many ways, and should not be interpreted as though limited to the specific embodiments in which they were explained and shown.
Those skilled in the art, having the benefit of this disclosure, will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present invention. Indeed, the invention is not limited to the details described above. Rather, it is the following claims including any amendments thereto that define the scope of the invention.