Abstract:
Disclosed herein are exemplary embodiments of an improved write address shift register structure useful for example in a DDR3 DRAM having read/write latency. The disclosed shift register structure propagates write addresses from an address bus outside the device to array decoders to allow latent data to be written into the cells in the memory array at a proper time. The register structure comprises a reduced number of registers (e.g., four) thus eliminating the need for extraneous registers which might otherwise be used to propagate “don&#39;t care” addresses. The registers are clocked, and the addresses propagated though the registers, in accordance with a latency bus through which a user defines the desired read/write latency in accordance with user preferences and the desired clock speed of the device. The clock for each register is preferably decoded from the latency bus and hence each register preferably has its own unique clock.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to write address synchronization in integrated circuit devices having read/write latency, and is particularly but not exclusively useful in DDR SDRAMs using bit prefetching schemes.  
       BACKGROUND  
       [0002]     Synchronous dynamic random access memories (SDRAMs) are well known. In an SDRAM, and referring to  FIG. 1A , data is written into and read out from the SDRAM  10  in synchronization with an external system clock signal (CLK). More specifically, when writing data to the array of cells  12  in the SDRAM  10 , data is presented at the data lines (DQx) and at some discrete point during the clock&#39;s cycle—for example, on the clock&#39;s rising edge—this data enters the SDRAM  10  for eventual storage in the array  12  at an address specified by the address lines (Ax). Conversely, when reading data from the array  12 , data is presented to the data lines (DQx) in accordance with the queried address on the address lines Ax, again at some discrete portion of the clock&#39;s cycle. Whether reading or writing is taking place is determined by the status of the row access strobe (RAS), column access strobe (CAS), and write enable (W/E) signals as is well known in DRAM technology. Ultimately these external signals are received from a system, such as a microprocessor system.  
         [0003]     However, in an SDRAM, data can neither be written to nor read from the array  12  instantaneously. For example, it takes time for the data read from the array  12  to reach the data lines DQx. For example, in a typical device, it typically takes approximately 15 nanoseconds (ns) for data to move from the array  12  to the data lines DQx when a read has been initiated. Thus, and referring to  FIG. 1B , if the system clock signal CLK has a period (p) of 5 ns, then if a read request is specified at time T 0  at a first address (addr  1 ), the data from that address will appear at the data lines DQx after three clock cycles at T 3 . Accordingly, the device is said to have a “read latency” (or Column Access Strobe (CAS) latency) of 3 clock cycles. Thereafter, the next entered address (addr n) can be processed and its data presented at the data lines at T 4 .  
         [0004]     A synchronous device which effectively provides for faster data transfer is a double data read (DDR) SDRAM. In a DDR SDRAM (hereinafter DDR), two data pipe lines are present in the device, one of which is active on the rising edge of the clock signal, and one of which is active on the falling edge of the clock signal. This is illustrated in  FIG. 2 . In a DDR, the address entered at T 0 , addr  1 , is used to “prefetch” the data at that address and at the next sequential address (addr  2 ). The prefetched data at the two addresses are then output at the data lines DQx on the rising edge of T 3  (T 3   r ) and the falling edge of T 3  (T 3   f ). Thereafter, the next address (addr n) can be entered at T 1 , or the DDR can be configured to output the next sequential address (addr  3 ; not shown). The benefit of this approach is that twice the amount data can be output using the same clock signal, as data is output on both the rising and falling edges. In other words, for a 5 ns clock signal, two bits can be output on each data line DQx.  
         [0005]     DDR2 improves up the technology of DDR by prefetching four bits instead of two, as illustrated in  FIG. 3 . DDR2 provides the benefit that the prefetched data can be presented at the data lines more quickly, allowing the clock cycle period (p) to be decreased by half (e.g., to 2.5 ns) when compared with a DDR device. Accordingly, due to the natural delay in reading the array, the first address of data occurs at the rising edge of T 6  (T 6   r ), thus establishing a read latency of 6. Moreover, the next address (addr n) cannot be presented until time T 2 , else the device will become “backed up” with data. In any event, using DDR2 technology, two bits can be output on each data line every 2.5 ns.  
         [0006]     DDR3 represents the next generation of DDR technology, and essentially amounts to a further extension of DDR2. As illustrated in  FIG. 4 , DDR3 allows for an eight-bit prefetch and theoretically can run at an even further decreased clock cycle period (p) of 1.25 ns, although in reality the clock may run slightly slower in an actual DDR3 as will be explained momentarily. Accordingly, due to the natural delay in reading the array, the first address of data occurs at the rising edge of T 12  (T 12   r ), thus establishing a read latency of 12. Moreover, the next address (addr n) cannot be presented until a minimum time of T 4 .  
         [0007]     DDR SDRAMs, such as DDR3, also have a write latency as well as a read latency. Write latency defines the number of clock cycles between presentation of an address at the address lines Ax and the actual presentation of the data at the data lines DQx to be stored at that address. Write latency is required because of the device&#39;s natural read latency. However, per user specifications, the write latency and read latency are generally not equal, but instead usually vary by one in an attempt to maximize usage of the data bus to which the data lines DQx are attached. More specifically, the write latency is preferably the read latency minus one.  
         [0008]     In current DDR3s under development, it turns out that clock periods of 1.25 ns are difficult to achieve, although future reductions in sizes and capacitances will certainly make such clock speeds achievable in the near future. A more appropriate and slightly slower clock speed of 1.5 ns is thus currently targeted for such devices. Due to the natural 15 ns delay in reading the array as explained earlier, the read latency in a device with such a clock speed is 10 (15/1.5). Accordingly, the write latency is preferably 9 for the reasons noted above.  
         [0009]     Thus, and as illustrated in  FIG. 5 , when an address (addr  1 ) is presented to the DDR3 at T 0  for writing data to the device, it will be 9 clock cycles until the data corresponding to that address (or those addresses in sequence) is presented by the data bus to the data lines DQx. As with reading, eight bits of data are written into the device at one time, and are captured from T 9   r  to T 12   f . Once captured, these eight bits are written in parallel into the array at the next fraction of the clock cycle (T 13   r ) in response to a write array pulse  20 . Again, so as not to back up the device, a new address (addr n) cannot be fed to the address lines Ax until at least T 4   r , similarly to what was described when reading the device and as illustrated in  FIG. 4 . This new address&#39;s data is captured on the data lines DQx at T 13   r  though T 16   f , etc.  
         [0010]     This DDR3 writing scheme however requires tracking the write addresses through the device. This is cumbersome, as the actual address at which the data will be stored is not needed until thirteen clock cycles later at T 13   r . In the prior art, the addresses were organized and flowed through the device in a series of registers, as shown in  FIG. 6 . Essentially, these registers  22 , preferably D flip flops, but can constitute other structures for holding data as well such as latches, and in this sense “register” is used generically throughout this disclosure. The registers shift the pertinent write addresses between the address lines Ax (i.e., address bus) and the array decoders on each clock cycle. Thus, addr  1  would be present in register  22   0  at T 0 , then would propagate to register  22   1  at T 1 , to register  22   2  at T 2 , etc. At T 4 , the next address, if any or if changed, would be presented to the first register  22   0 , etc. In this way, the device would have the correct address presented from register  22   12  to the array decoders at T 13  in time for writing to the array in accordance with the write array signal  20  (see  FIG. 5 ).  
         [0011]     This solution of propagating the addresses through the DDR3 is beneficial in that it is relatively simple to implement. However, it also suffers from drawbacks. First, the register structure takes up quite a bit of space on the DDR3 integrated circuit. Specifically, 13 registers are needed in the above-illustrated example, each of which is “k” bits long in accordance with the size of the write addresses that are being propagated. In a device having 17 address bits, a 17-by-13 bit space would thus be used for the register structures. Stated more generically, and assuming P equals the number of bits prefetched by the device, and WL max  equals the maximum write latency, the number of registers needed would be the value WL max +P/2 rounded up to the next integer (or 9+8/2=13 registers in this example).  
         [0012]     Moreover, the relatively large amount of space needed for the register structures seems particularly wasteful when it is realized that a unique address is not input at every clock cycle. As noted above, an address can be entered into the device at a minimum of every four clock cycles, and as shown in the example of  FIG. 5 , valid addresses are captured by the device at times T 0  and T 4 . For the intervening time periods (T 1  through T 3 ), the device merely captures “don&#39;t care” address data, which is then propagated through the shift register structure of  FIG. 6 . Of course, propagating these don&#39;t care address values through the register structure is not ideal.  
         [0013]     Furthermore, as noted above, the read latency (and thus the write latency) might ideally change depending on the clock speed to be used with the DDR3. For example, while the above example contemplates a clock speed of 1.5 ns and a read latency of 10 (write latency of 9), a 3 ns clock would require only a read latency of 5 (write latency of 4). When this is recognized, it is seen that all thirteen registers  22  are not required, and that a fewer number of registers would be more optimal for use with lower clock speeds. In short, use of lower clock speeds and lower latency values may render some of the registers  22  unnecessary, which is again wasteful. Moreover, the design of the DDR3 device, and specifically the register structure, becomes dictated by the clock speed and latency values to be used. This hampers user flexibility, as a new design would be needed for each new clock speed or latency value to be used, which again is not ideal.  
         [0014]     Moreover, as write addresses (including “don&#39;t care” addresses) are propagated through the register structure at each clock cycle, current will be necessarily drawn by each register at each clock cycle. This excess current draw is significant, and is preferably minimized.  
         [0015]     Accordingly, the art would be benefited by a solution to this problem, and specifically to a solution that simplifies and renders more flexible the write register structure on a DDR3 device. This disclosure provides such a solution.  
       SUMMARY  
       [0016]     Disclosed herein are exemplary embodiments of an improved write address shift register structure useful for example in a DDR3 DRAM having read/write latency. The disclosed shift register structure propagates write addresses from an address bus outside the device to array decoders to allow latent data to be written into the cells in the memory array at a proper time. The register structure comprises a reduced number of registers (e.g., four) thus eliminating the need for extraneous registers which might otherwise be used to propagate “don&#39;t care” addresses. The registers are clocked, and the addresses propagated though the registers, in accordance with a latency bus through which a user defines the desired read/write latency in accordance with user preferences and the desired clock speed of the device. The clock for each register is preferably decoded from the latency bus and hence each register preferably has its own unique clock. The disclosed write address shift register structure takes up less area on the DDR3 device, eliminates use of the registers to propagate “don&#39;t care” addresses, reduces power requirements, and improves user flexibility by allowing latency values to be programmed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     Embodiments of the inventive aspects of this disclosure will be best understood with reference to the following detailed description, when read in conjunction with the accompanying drawings, in which:  
         [0018]      FIG. 1A  illustrates a block diagram of a prior art SDRAM.  
         [0019]      FIG. 1B  illustrates the read latency of the SDRAM of  FIG. 1A .  
         [0020]      FIGS. 2-4  respectively illustrate the read latency of a DDR SDRAM, a DDR2 SDRAM and a DDR3 SDRAM.  
         [0021]      FIG. 5  illustrates the write latency of a DDR3 SDRAM.  
         [0022]      FIG. 6  illustrates the write address shift register structure between the address bus and the array decoders in the DDR3 SDRAM of  FIGS. 4 and 5 .  
         [0023]      FIGS. 7-10  illustrate logic for decoding a user programmable latency bus and for generating register clock signals to send to the disclosed register structure of  FIG. 11 .  
         [0024]      FIG. 11  illustrates the disclosed improved write address shift register structure.  
         [0025]      FIGS. 12A and 12B  respectively illustrate timing diagrams for the register clock signals for the shift register of  FIG. 11  for a read latency of 10 and 5. 
     
    
     DETAILED DESCRIPTION  
       [0026]     The disclosed improved write address shift register structure is shown in  FIG. 11 . Like the prior art, the shift register structure comprises a series of registers  30 , which preferably comprise D flip flops, but which could comprises other structures as well, such as latches, etc., as mentioned earlier. Four registers are shown, a number significantly reduced from the number of registers occurring in the prior art (see  FIG. 6 ). However, the exact number of registers to be used according to the disclosed techniques will depend upon the worst-case scenario, i.e., the fastest clock speed to be used (and hence the read/write latency) and the DDR technology at issue. For a DDR3, 8-bit prefetch device and assuming a maximum read latency of ten as discussed earlier, a minimum of four registers are needed because that many addresses might be presented to and processed by the device at one time. However, one skilled in the art will appreciate that differing minimum numbers of registers will be useful in other applications and with other types of DDR devices. More specifically, assuming P equals the number of bits prefetched by the device, and WL max  equals the maximum write latency, the number of registers needed would be the value (WL max +P/2)/(P/2) rounded up to the next integer. Thus, the for disclosed example, where WL max =9, and P=8, this formula yields (9+8/2)/(8/2)=3.25, which when rounded up to the next integer yields 4 registers.  
         [0027]     As shown in  FIG. 11 , and denoting another diversion from the prior art, each of the registers  30  is preferably clocked to move the address data forward in the register structure by a unique register clock signal (RdWrCmd, ClkA 0 , ClkA 1 , and ClkA 2 ). It is not imperative that a unique clock signal be used for each register  30  (some but less than all of the registers may use a common clock signal in other useful embodiments), but this is preferred. So designed, the register structure of  FIG. 11  holds only the pertinent write addresses truly needed by the array decoders to perform an array write operation  20 . In short, no “don&#39;t care” addresses are propagated through the register structure.  
         [0028]     The generation of the register clock signals is accomplished in a useful embodiment by the logic circuitry disclosed in  FIGS. 7-10 . As one skilled in the art will well understand how such logic circuitry operates, such circuitry is only briefly discussed. It should be noted that the disclosed circuitry is merely exemplary, and constitutes only one of many embodiments for generating the register clock signals. Other logic circuits and configurations may be used to generate the register clock signals, and these clock signals may be applied to the respective registers  30  at different times than those disclosed in  FIGS. 12A and 12B  (to be discussed later) while still providing the benefits of the disclosed write address propagation scheme.  
         [0029]     A central component in the generation of the register structure control signals is the read latency bus  35 , as shown in  FIG. 7 . As shown, the latency bus  35  comprises four lines (RL&lt;0:3&gt;) which indicates the read latency to be used with the device. For example, if a 1.5 ns clock is to be used, requiring a read latency of 10, the signals of the latency bus  35  will represent the number 10 in a binary fashion, e.g.: RL&lt;3&gt;=‘1’, RL&lt;2&gt;=‘0’, RL&lt;‘1’&gt;=‘1’, and RL&lt;0&gt;=‘0’. If a 3.0 ns clock is to be used, requiring a read latency of 5, the signals of the latency will represent the number 5 in a binary fashion, e.g., RL&lt;3&gt;=‘0’, RL&lt;2&gt;=‘1’, RL&lt;1&gt;=‘0’, and RL&lt;0&gt;=‘1’. As disclosed in the examples herein, a read latency from 5 to 10 is contemplated, although one skilled in the art will realize that other read latencies could be used, and therefore that this range of values is merely exemplary.  
         [0030]     In a preferred embodiment, the read latency as reflected by the latency bus  35  is user programmable. Such user programmability can be accomplished in any number of ways using well-known techniques which don&#39;t require detailed elaboration and thus are only briefly discussed. First, the value of the latency bus  35  can be set to fixed values by the user and stored in an appropriate register (not shown) on the device using a special access mode. Second, the latency bus  35  may be mask programmable by selectively coupling the bus lines to power or ground on the device. This approach is less preferred as it requires a mask change for each device requiring a different latency period. Third, fuses or anti-fuses can be opened or shorted to set the value of the latency bus  35 . Fourth, the speed of the clock (CLK) may be sensed by the device to allow the appropriate read latency to be determined and set. For example, if the device senses a 1.5 ns clock, it will understand that a read latency is 10 is needed, and will automatically set the bus accordingly. Such schemes and circuitry for sensing clock speeds are well known. This latter approach is beneficial in that it allows the device to be used when the clock speed in the microprocessor system in which the device resides is not known or may vary. While the latency bus  35  preferably constitutes bits indicative of read latency, the bus may also reflect write latency as this is related to the read latency as noted earlier.  
         [0031]     Regardless of the technique used to set the value of the read latency bus  35 , the bits of that bus are decoded and processed using the exemplary logic circuitry of  FIG. 7  to  10  to allow the register clock signals to be determined. As one skilled in the art understands how the disclosed logic circuitry functions, such circuitry is only briefly discussed. As shown in  FIG. 7 , the bit of bus  35  are sent to various logic gates (AND and NOR gates) to activate a signal indicative of whether a read latency of 5 or 8, 6 or 9, or 7 or 10 has been user programmed (e.g., RL5 or 8). In parallel, and as shown in  FIG. 8 , an internal write command (WriteCmd) is sent through a series of flip flops clocked by the main clock signal (CLK) to generate a sequential series of time delayed write signals (Write&lt;X&gt;). One skilled in the art will realize that the WriteCmd signal is generated as a function of the RAS, CAS, and W/E signals external to the device.  
         [0032]     The signals from  FIGS. 7 and 8  are sent to the circuit of  FIG. 9 , which is used to generate various intermediary write signals (IWrX). Specifically, the RLXorY signals gate buffers allowing one of the time delayed versions of Write&lt;X&gt; signals to pass through to a series of flip flops or latches. (As one skilled in the art will recognize, the flip flops route the input (D) to the output (Q) on the rising edge of the clock, while the latches pass the input to the output whenever the latch clock signal is low). The intermediary write signals IWrX are sent to various multiplexers as shown in  FIG. 10 , which is controlled by the most significant bit RL&lt;3&gt; of the read latency bus  35  to generate the various register  30  clocks (ClkA 0 , ClkA 1 , and ClkA 2 ) and signals indicative of when writing of data to the array decoders should start (WriteArray  20 ) and stop (WriteStop 40 ) (See  FIGS. 5, 12A , and  12 B). In this example, register clock ClkA 0  is logically equivalent to the Write&lt;4&gt; signal.  
         [0033]     Accordingly, and as noted earlier, the generated register clock signals are used to clock the registers  11  as shown in  FIG. 11 . (The first register  30   0  in this embodiment is actually clocked by a signal indicative of whether the device is in read or write mode (RdWrCmd), which like the WriteCmd signal is generated as a function of the RAS, CAS, and W/E signals external to the device). The timing of these generated register clock signals are shown respectively in  FIGS. 12A and 12B  for the assumed extreme cases of read latency  10  and read latency  5 .  
         [0034]     Thus, as noted earlier, the disclosed write synchronization scheme requires fewer write address registers than does the prior art, while requiring only minimal logic to allow such size reduction to occur. The write addresses propagate through the write address register structure without needlessly propagating “don&#39;t care” addresses. Additionally current is only drawn when addresses are propagated in response to a write command, and is not necessarily drawn upon every iteration of the system clock. Moreover, the read latency value is programmable, thus allowing the minimal number of registers to be used over a latency range without wasted resources, reducing device power requirements.  
         [0035]     While disclosed as being particularly useful with DDR3 technologies, one skilled in the art will appreciate that the disclosed write synchronization scheme can be used with similar benefits to SDRAMs, other generations of DDR technology including those to be developed in the future, other memory devices, and generically to any integrated circuit device requiring write address synchronization such as microprocessors.  
         [0036]     It should be understood that the inventive concepts disclosed herein are capable of many modifications. To the extent such modifications fall within the scope of the appended claims and their equivalents, they are intended to be covered by this patent.