Abstract:
A method and apparatus for handling the refresh of a DRAM array or other memory array requiring periodic refresh operations so that the refresh does not require explicit control signaling nor handshake communication between the memory array and an external accessing client. The method and apparatus handles external accesses and refresh operations such that the refresh operations do not interfere with the external accesses under any conditions. As a result, an SRAM compatible device can be built from DRAM or 1-Transistor cells. A clock division scheme is implemented to allow N external accesses and one refresh operation to be performed during N consecutive clock cycles.

Description:
RELATED APPLICATIONS  
       [0001]    The present application is a continuation-in-part of U.S. patent application Ser. No. 09/846,093, by Wingyu Leung, entitled “Method And Apparatus For Completely Hiding Refresh Operations In A DRAM Device Using Clock Division” filed Apr. 30, 2001, which is a continuation-in-part of U.S. patent application Ser. No. 09/405,607, by Wingyu Leung, entitled “Read/Write Buffers for Complete Hiding of the Refresh of a Semiconductor Memory and Method of Operating Same” filed Sep. 24, 1999, which is a continuation-in-part of U.S. Pat. No. 5,999,474, by Wingyu Leung and Fu-Chieh Hsu, entitled “Method and Apparatus for Complete Hiding of the Refresh of a Semiconductor Memory” issued Dec. 7, 1999.  
         [0002]    The present application is further related to U.S. Pat. No. 6,028,804, by Wingyu Leung, entitled “Method and Apparatus for 1-T SRAM Compatible Memory” and issued Feb. 22, 2000; U.S. Pat. No. 6,222,705, by Wingyu Leung, entitled “Method and Apparatus For Refreshing A Semiconductor Memory using Idle Memory Cycles” issued Apr. 24, 2001; and U.S. Pat. No. 6,075,740, by Wingyu Leung, entitled “Method and Apparatus for Increasing The Time Available for Refresh For 1-T SRAM Compatible Devices”, issued Jun. 13, 2000. These patents are hereby incorporated by reference.  
     
    
     
       FIELD OF THE INVENTION  
         [0003]    The present invention is related to semiconductor memories, especially dynamic random access memory (DRAM). In particular, the present invention relates to a method and apparatus of handling refresh operations in a semiconductor memory such that the refresh operations do not interfere with external access operations.  
         DISCUSSION OF RELATED ART  
         [0004]    A conventional DRAM memory cell, which consists of one transistor and one capacitor, is significantly smaller than a conventional SRAM cell, which consists of 4 to 6 transistors. However, data stored in a DRAM cell must be periodically refreshed, while the data stored in an SRAM cell has no such requirement. Each refresh operation of a DRAM cell consumes memory bandwidth. It is possible for an external access to be initiated at the same time that a refresh access is required. In this case, the external access must be delayed until after the refresh access has been performed. In general, this prevents DRAM cells from being operated as SRAM cells, because SRAM cells do not require refresh operations.  
           [0005]    Previous attempts to use DRAM cells in SRAM applications have been of limited success for various reasons. For example, one such DRAM device has required an external signal to control refresh operations. (See, 131,072-Word by 8-Bit CMOS Pseudo Static RAM, Toshiba Integrated Circuit Technical Data (1996).) Moreover, external accesses to this DRAM device are delayed during the memory refresh operations. As a result, the refresh operations are not transparent and the resulting DRAM device cannot be fully compatible with an SRAM device.  
           [0006]    In another prior art scheme, a high-speed SRAM cache is used with a relatively slow DRAM array to speed up the average access time of the memory device. (See, U.S. Pat. No. 5,559,750 by Katsumi Dosaka et al, and “Data Sheet of 16 Mbit Enhanced SDRAM Family 4Mx4, 2Mx8, 1Mx16” by Enhanced Memory Systems Inc., 1997.) The actual access time of the device varies depending on the cache hit rate. Circuitry is provided to refresh the DRAM cells. However, the refresh operation is not transparent to external accesses. That is, the refresh operations affect the memory access time. Consequently, the device cannot meet the requirement of total deterministic random access time.  
           [0007]    Other prior art schemes use multi-banking to reduce the average access time of a DRAM device. Examples of multi-banking schemes are described in “Data sheet, MD904 To MD920, Multi-bank DRAM (MDRAM) 128Kx32 to 656Kx32” by MoSys Inc., 1996, and in “An Access-Sequence Control Scheme to Enhance Random-Access Performance of Embedded DRAM&#39;s” by Kazushige Ayukawa et al, IEEE JSSC, vol. 33, No. May 5, 1998, pp. 800-806. These multi-banking schemes do not allow an individual memory bank to delay a refresh cycle. Another prior art scheme uses a read buffer and a write buffer to take advantage of the sequential or burst nature of an external access. An example of such a prior art scheme is described in U.S. Pat. No. 5,659,515, entitled “Semiconductor Memory Device Capable of Refresh Operation in Burst Mode” by R. Matsuo and T. Wada. In this scheme, a burst access allows a register to handle the sequential accesses of a transaction while the memory array is being refreshed. However, this scheme does not allow consecutive random accesses. For example, the memory cannot handle a random access per clock cycle.  
           [0008]    Another prior art scheme that attempts to completely hide refresh operations in a DRAM cell includes the scheme described in U.S. Pat. No. 5,642,320, entitled “Self-Refreshable Dual Port Dynamic CAM Cell and Dynamic CAM Cell Array Refreshing Circuit”, by H. S. Jang. In this scheme, a second port is added to each of the dynamic memory cells so that refresh can be performed at one port while a normal access is carried out at the other port. The added port essentially doubles the access bandwidth of the memory cell, but at the expense of additional silicon area.  
           [0009]    Another prior art scheme that attempts to completely hide the refresh operations in an asynchronous DRAM is described in U.S. Pat. No. 4,549,284, entitled “Dynamic MOS Random Access Memory”, by Kunihiko Ikuzaki. In this scheme, an automatic refresh circuit is incorporated in an asynchronous DRAM to generate a refresh cycle after an external access cycle is performed. In the absence of an external access, an internal oscillator continues to generate refresh cycles. Thus, the memory device is constantly performing refresh operations, thereby wasting power.  
           [0010]    Moreover, the oscillation period of the oscillator is set by the transconductance of an MOS transistor and a capacitor, which varies with process and temperature. Within a typical process and commercial temperature range, the oscillation period varies by up to a factor of two. As a result, it becomes difficult to synchronize the external accesses and the refresh operations. For this reason, the memory device is not suitable for high-speed operations in the auto-refresh mode.  
           [0011]    Accordingly, it would be desirable to have a DRAM device that handles refresh operations in a manner that is completely transparent to an external accessing memory client for both low-speed and high-speed operations. It would further be desirable if such a DRAM device only performed refresh operations at the times when the memory cells need to be refreshed (i.e., at a rate determined mainly by the charge leakage mechanism of the memory cells, and not by the circuit operation of the automatic refresh circuit).  
         SUMMARY  
         [0012]    Accordingly, the present embodiment provides a memory device (or an embedded memory block) that includes a plurality of memory cells, which must be periodically refreshed in order to retain data values. In one embodiment, the memory cells are DRAM cells arranged in an array having a plurality rows and columns. In a particular embodiment, the array is divided into a plurality of banks.  
           [0013]    The memory device includes a plurality of terminals for receiving signals from an external accessing client. These signals can include a clock signal, an address signal, a write/read indicator signal, and address strobe signal and a reset signal. However, these signals do not include a signal that indicates that a refresh operation must be performed.  
           [0014]    To implement refresh operations, the memory device includes a refresh controller that periodically asserts a refresh request signal, which is used to indicate that a refresh operation is pending. The refresh controller also provides a refresh address identifying one of the rows of the array. The refresh controller increments the refresh address each time that the refresh request signal is asserted.  
           [0015]    The memory device also includes a memory array sequencer for controlling the timing of external accesses and refresh accesses within the memory device. In general, the memory array sequencer ensures that the required refresh accesses are performed without interfering with any external accesses.  
           [0016]    More specifically, the memory array sequencer synchronizes external accesses and refresh accesses with different edges of an external clock signal. In one embodiment, external accesses are synchronized (initiated) in response to rising edges of the external clock signal. The external accesses are then completed during the first half cycle of the clock period (e.g., while the clock signal has a high state). In this embodiment, pending refresh accesses are synchronized (initiated) in response to falling edges of the external clock signal. The refresh accesses are then completed during the second half cycle of the clock period (e.g., while the clock signal has a low state).  
           [0017]    In another embodiment, external accesses are synchronized (initiated) in response to rising edges of the external clock signal. The external accesses are performed as quickly as possible. Pending refresh accesses are then synchronized (initiated) in response to the end of the external accesses. The refresh accesses are completed prior to the next rising edge of the external clock signal. This embodiment allows the external accesses and refresh accesses to be performed as quickly as possible, without being dependent on the duty cycle of the external clock signal.  
           [0018]    In yet another embodiment, a clock division scheme is implemented to allow N external accesses and one refresh operation to be performed during N consecutive clock cycles. In this embodiment, the memory system can include a memory array sequencer configured to enable N external accesses and one refresh access to be consecutively performed during N cycles of the clock signal, wherein N is an integer equal to two or more. This memory system can include a clock divider circuit configured to provide a divided clock signal that is activated for one period of every N periods of the clock signal, and means for initiating the refresh access only when the divided clock signal is activated.  
           [0019]    A corresponding method includes the steps of operating the memory system in response to a clock signal, and enabling N external accesses and one refresh access to be consecutively performed during N cycles of the clock signal, wherein N is an integer equal to two or more. In one embodiment, the clock signal is divided to create a divided clock signal that is activated for one period of every N periods of the clock signal, and a refresh access only enabled when the divided clock signal is activated.  
           [0020]    The refresh access can be performed at the beginning of the N cycle period. Alternately, the refresh access can be performed only during the second half of one of the N cycles of the clock signal. In another variation, each of the N external accesses is initiated only during a first half of a corresponding one of the N cycles. In another variation, the refresh access is initiated in response to an end of a first one of the N external accesses, and a second one of the N external accesses is initiated in response to an end of the refresh access. 
       
    
    
       [0021]    The present invention will be more fully understood in view of the following description and drawings.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a block diagram of a memory system in accordance with one embodiment of the present invention.  
         [0023]    [0023]FIG. 2 is a block diagram of a refresh controller used in the memory system of FIG. 1 in accordance with one embodiment of the present invention.  
         [0024]    [0024]FIG. 3 is a circuit diagram of a memory array sequencer used in the memory system of FIG. 1 in accordance with one embodiment of the present invention.  
         [0025]    [0025]FIG. 4 is a waveform diagram illustrating the timing of various signals in the memory system of FIG. 1 in accordance with one embodiment of the present invention.  
         [0026]    [0026]FIG. 5 is a circuit diagram of another memory array sequencer, which can be used to replace the memory array sequencer of FIG. 3 in another embodiment of the present invention.  
         [0027]    [0027]FIG. 6 is a waveform diagram illustrating the timing of various signals in the memory system of FIG. 1, when the memory array sequencer of FIG. 5 is used in place of the memory array sequencer of FIG. 3.  
         [0028]    [0028]FIG. 7 is a block diagram of a 1-T SRAM system in accordance with another embodiment of the present invention.  
         [0029]    [0029]FIG. 8 is a circuit diagram of memory array sequencer used in the 1-T SRAM system of FIG. 7 in accordance with one embodiment of the invention, to support one refresh operation every two clock cycles.  
         [0030]    [0030]FIG. 9 is a waveform diagram illustrating the operation of the 1-T SRAM system of FIG. 7, when using the memory array sequencer of FIG. 8.  
         [0031]    [0031]FIG. 10 is a circuit diagram of a memory access sequencer, which can be used in place of the memory access sequencer of FIG. 8, in accordance with another embodiment of the present invention, to support one refresh operation every three clock cycles.  
         [0032]    [0032]FIG. 11 is a waveform diagram illustrating the operation of the 1-T SRAM system of FIG. 7, when using the memory array sequencer of FIG. 10.  
     
    
     DETAILED DESCRIPTION  
       [0033]    In accordance with the present invention, an SRAM compatible device is designed using DRAM cells. This SRAM compatible device is hereinafter referred to as a one-transistor SRAM (1-T SRAM).  
         [0034]    [0034]FIG. 1 is a block diagram of a 1-T SRAM system  100  in accordance with one embodiment of the present invention. 1-T SRAM system  100  includes DRAM array  101 , 2-to-1 multiplexer  102 , AND gate  103 , input signal registers  120 , refresh controller  121  and memory array sequencer  122 . DRAM array  101  includes 64 DRAM memory banks  0 - 63 , read data amplifier  70  and write data buffer  80 . Each of DRAM banks  0 - 63  includes 32 rows and 512 columns of DRAM memory cells, as well as the associated address decoders, word line drivers, sense-amplifiers and column multiplexers, which are not shown, but are understood by those of ordinary skill in the art. The column multiplexers of DRAM memory banks  0 - 63  are connected in parallel to read data amplifiers  70  and write data buffer  80 . Each of DRAM banks  0 - 63  is capable of storing 512 32-bit words. The total capacity of DRAM memory banks  0 - 63  is therefore 32K 32-bit words.  
         [0035]    The external interface of 1-T SRAM system includes a 32-bit input data bus for providing an input data word DIN[ 31 : 0 ] to write data buffer  80 , a 32-bit output data bus for providing an output data word DOUT[ 31 : 0 ] from read data amplifier  70 , a write/read indicator line for receiving write/read indicator signal WR#, a clock input terminal for receiving an external clock signal CLK, a 14-bit address bus for receiving external address bits EA[ 14 : 0 ], an address strobe line for receiving an address strobe signal ADS#, and a reset line for receiving a reset signal RST#. As used herein, the # symbol indicates that a signal is active low. Note that the signals listed above do not include any signals specifically relating to refresh accesses of DRAM banks  0 - 63 .  
         [0036]    All of the external signals are referenced to the rising edge of the CLK signal. The external address signal EA[ 14 : 0 ] is divided into 3 fields: a 4-bit word (column) address EA[ 14 : 11 ] (which identifies one of the sixteen 32-bit words in a row), a 6-bit bank address EA[ 10 : 5 ] (which identifies one of the 64 memory banks  0 - 63 ), and a 5-bit row address A[ 4 : 0 ] (which identifies one of the 32 rows in a memory bank).  
         [0037]    An external device initiates an access to 1-T SRAM system  100  by asserting a logic low address strobe signal ADS#, driving the write/read indicator signal WR# to the desired state (high for write and low for read), and providing an external address EA[ 14 : 0 ]. The ADS#, WR# and EA[ 14 : 0 ] signals are all registered (i.e., latched) into input signal registers  120  at the rising edge of CLK signal. Input signal registers  120  provide the latched ADS#, WR# and EA[ 14 : 0 ] signals as latched output signals LADS#, LWR# and LEA[ 14 : 0 ], respectively. The latched external address LEA[ 14 : 0 ] is divided into two groups of address signals. Address signal LEA[ 14 : 11 ] contains the column address of the memory cells selected for the external access, and address signal LEA[ 10 : 0 ] contains the row and bank address of the memory cells selected for the external access. Address signal LEA[ 14 : 11 ] is provided to the column decoder circuitry in each of DRAM banks  0 - 63 . Address signal LEA[ 10 : 0 ] is provided to an input bus of 2-to-1 multiplexer  102 .  
         [0038]    The latched address strobe signal LADS# is provided to memory array sequencer  122  and to an inverting input terminal of AND gate  103 . The latched write/read indicator signal LWR# is provided to read data amplifier  70  and write data buffer  80 .  
         [0039]    Refresh accesses to DRAM array  101  are managed by refresh controller  121 . Refresh controller  121  initiates a refresh access by activating a refresh request signal RREQ high in response to the external clock signal CLK. As described in more detail below, refresh controller  121  activates the refresh request signal RREQ high once every 64 clock periods. Refresh controller  121  also provides an 11-bit refresh address RFA[ 10 : 0 ], which identifies the bank and row to be refreshed in DRAM array  101 . The refresh address RFA[ 10 : 0 ] is incremented each time the refresh request signal is activated.  
         [0040]    [0040]FIG. 2 is a block diagram of refresh controller  121  in accordance with one embodiment of the present invention. Refresh controller  121  includes a modulo-64 counter  201  for generating the refresh request signal RREQ, and an 11-bit binary counter  202  for generating the bank and row address RFA[ 10 : 0 ] for the current refresh operation. Counters  201  and  202  are reset to zero counts when the reset signal RST# is activated low during the initialization of memory system  100 . After the RST# signal is de-activated high, modulo-64 counter  201  begins counting from 0 to 63, with counter  201  being incremented by one in response to each rising edge of the external clock signal CLK. When modulo-64 counter  201  reaches a full count of 63, this counter  201  drives the refresh request signal RREQ high for one period of the CLK signal. Modulo-64 counter  201  then wraps around to a zero count in response to the next rising edge of the CLK signal.  
         [0041]    The falling edge of the refresh request signal RREQ increments the count of 11-bit binary counter  202  by one. The output of 11-bit binary counter  202 , provides the refresh address RFA[ 10 : 0 ], which identifies the bank and row to be refreshed in DRAM array  101 . After 11-bit binary counter  202  reaches a full count of “111 1111 1111”, the count will automatically wrap around to a zero count in response to the next rising edge of the refresh request signal RREQ. In this manner, refresh controller  121  provides a refresh address RFA[ 10 : 0 ] that traverses the entire row and bank address space of DRAM array  101 .  
         [0042]    Memory array sequencer  122  generates the conventional DRAM control signals RAS# (row access), SEN# (sense amplifier enable), CAS# (column access), and PRC# (precharge) for controlling the operations of memory array  101 . The functionality of the RAS#, SEN#, CAS# and PRC# control signals in accessing a DRAM array is understood by those of ordinary skill in the art. For example, the functionality of these control signals is described in detail in U.S. Pat. No. 6,078,547, “Method and Structure for Controlling Operation of a DRAM Array”, by Wingyu Leung, which is hereby incorporated by reference.  
         [0043]    [0043]FIG. 3 is a circuit diagram of memory array sequencer  122  in accordance with one embodiment of the present invention. Memory array sequencer  122  includes delay lines  301 - 304 , OR gates  311 - 318 , one-shot circuits  331 - 332 , AND gate  341 , and flip-flops  351 - 352 . Flip-flop  351  includes cross-coupled NAND gates  321 - 322 , and flip-flop  352  includes cross-coupled NAND gates  323 - 324 . Delay lines  301 ,  302 ,  303  and  304  introduce signal delays d 1 , d 2 , d 3 , and d 4 , respectively. In general, memory array sequencer  122  activates the RAS#, SEN#, CAS# and PRC# signals at predetermined times during half of a CLK period. As a result, an external access (read or write) may be performed during one half of a CLK period, and a refresh access may be performed during the other half of the same CLK period. In the described embodiment, external accesses are performed during the half clock period that the CLK signal has a logic high state, and refresh accesses are performed during the half clock period that the CLK signal has a logic low state.  
         [0044]    When 1-T SRAM system  100  is started, the RST# signal is activated low, thereby setting flip-flops  351  and  352 , such that these flip-flops provide logic high output signals to AND gate  341 . The RST# signal is then de-activated high, thereby allowing flip-flops  351  and  352  to operate in response to the other applied signals. When there is no external access to system  100 , the latched address strobe signal LADS# is deactivated high. When there is not refresh access pending in system  100 , the refresh request signal RREQ is de-activated low. Under these conditions, OR gates  311  and  312  provide logic high signals to one-shot circuits  331  and  332 , respectively. In response, one-shot circuits  331 - 332  provide logic high signals to flip-flops  351 - 352 , respectively, thereby causing flip-flops  351 - 352  to continue to provide logic high output signals to AND gate  341 . The logic high output of AND gate  341  de-activates the RAS#, SEN#, CAS# and PRC# control signals high. At this time, memory array  101  is idle.  
         [0045]    As described in more detail below, the latched address strobe signal LADS# is activated low during an external access. The low state of the LADS# signal, along with the high state of the CLK signal causes the output of OR gate  311  to go low. The falling edge at the output of OR gate  311  causes one-shot circuit  331  to generate a negative going pulse having a width substantially shorter than half of the CLK period. This negative pulse resets the output of flip-flop  351  to a logic low value. The logic low value provided by flip-flop  351  causes AND gate  341  to activate the RAS# signal low. The low state of the RAS# signal propagates through delay lines  301 ,  302 ,  303  and  304  (as well as OR gates  315 - 317 ) to drive signals SEN#, CAS# and PRC# low in sequence. More specifically, the low RAS# signal propagates through delay line  301  and OR gate  315  to activate the SEN# signal low after a delay of d 1 . The low SEN# signal then propagates through delay line  302  and OR gates  316 - 317  to activate the CAS# signal low after an additional delay of d 2 . Note that the logic high CLK signal applied to the inverting input terminal of OR gate  317  allows the logic low signal from OR gate  316  to be passed as the low CAS# signal. The low CAS# signal then propagates through delay line  303  to activate the PRC# signal low after an additional delay of d 3 .  
         [0046]    The logic low signal provided by delay line  303  propagates through delay line  304  after an additional delay of d 4 . The logic low output of delay line  304  is provided to the inverting input terminal of OR gate  318 . As a result, the PRC# signal is de-activated high a delay d 4  after being activated low.  
         [0047]    The low state of the output of delay line  303  and the high state of the CLK signal cause OR gate  313  to provide a logic low value to flip-flop  351 , thereby setting the output of flip-flop  351  to a logic high state. In response, AND gate  341  deactivates the RAS# signal to a logic high state. The logic high RAS# signal is applied to OR gates  315  and  316 , thereby causing the SEN# and CAS# signals, respectively, to be immediately de-activated high. The logic high value provided by OR gate  316  propagates through delay line  303  after a delay of d 3 , thereby causing OR gate  318  to continue to de-activate the PRC# signal high. The logic high value provided by delay line  303  propagates through delay line  304  after a delay of d 4 , and is applied to the inverting input terminal of OR gate  318 . In this manner, the output signals provided by delay lines  301 - 304  are reset to their original logic high states, such that these delay lines are ready for the next access.  
         [0048]    The total delay introduced by delay lines  301 ,  302 ,  303 , and  304  is designed to be less than or equal to a half period of the CLK signal. Notice that the RAS# signal is recovered to a logic high state before the CLK signal transitions from the high state to a low state. As a result, another memory cycle can be started at the falling edge of the CLK signal. To ensure that the SEN# and CAS# signals are generated properly during back-to-back memory cycles, the outputs of delay lines  301  and  302  are required to be deactivated high before the RAS# signal is activated low again.  
         [0049]    In the present embodiment, the delay d 4  introduced by delay line  304  is longer than the delay d 1  introduced by delay line  301  or the delay d 2  introduced by delay line  302 . As a result, when the PRC# signal is de-activated high, delay lines  301  and  302  are already providing logic high output signals. In other embodiments, delay line  301  or delay line  302  can have a longer delay than delay line  304 . However, in such embodiments, a fast reset must be provided on delay lines  301  and  302 , thereby ensuring that the output signals provided by delay lines  301  and  302  recover to logic high states before the activation period of the PRC# signal expires.  
         [0050]    A memory refresh operation is performed when the refresh request signal RREQ is activated to a logic high value and the CLK signal has a logic low value. That is, refresh operations are only performed during the half period that the CLK signal has a logic low state. The high state of the RREQ signal and the low state of CLK signal causes OR gate  312  to provide a logic low output signal to one-shot circuit  332 . In response, one-shot circuit  332  provides a negative going pulse having a pulse width substantially shorter than a half period of the CLK signal. The negative pulse resets the output of flip-flop  352  to a logic low state. This logic low output signal provided by flip-flop  352  is applied to an input terminal of AND gate  341 . In response, AND gate  341  activates the RAS# signal low. The low state of RAS# signal propagates through delay line  301 , thereby causing the SENT signal to be activated low after delay d 1 . The low state of the SEN# signal propagates through delay line  302  after delay d 2 , thereby causing OR gate  316  to provide a logic low output signal to OR gate  317 . Because the inverting input terminal of OR gate  317  receives a logic low CLK signal at this time, the CAS# signal remains de-activated high. The CAS# signal is thereby suppressed during the refresh access (because the refresh access does not involve a column access).  
         [0051]    The logic low signal provided by OR gate  316  propagates through delay line  303 , thereby providing a low signal to the non-inverting input terminal of OR gate, and causing the PRC# signal to be activated low after delay d 3 . The logic low state of the output signal provided by delay line d 3  propagates through delay line  304 , thus providing a logic low signal to the inverting input terminal of OR gate  318  after delay d 4 . OR gate  318  de-activates the PRC# signal high in response to the logic low signal provided to the inverting input terminal of OR gate. The low state of the output signal provided by delay line  303  and the low state of the CLK signal cause OR gate  314  to provide a logic low output signal to flip-flop  352 . In response, flip-flop  352  provides a logic high signal to AND gate  341 , thereby causing AND gate  341  to deactivate the RAS# signal high. The high state of the RAS# signal causes OR gate  315  to provide a logic high output signal, thereby causing the SEN# signal to be deactivated high. The high state of the RAS# signal also causes OR gate  316  to provide a logic high output signal. After a delay of d 3 , the logic high output signal provided by OR gate  316  propagates through delay line  303 , to the non-inverting input terminal of OR gate  318 , thereby causing OR gate  318  to continue to de-activate the PRC# signal high. The logic high value provided by delay line  303  propagates through delay line  304  after a delay of d 4 , and is applied to the inverting input terminal of OR gate  318 . In this manner, the output signals provided by delay lines  301 - 304  are reset to their original logic high states, such that these delay lines are ready for the next access.  
         [0052]    Returning now to FIG. 1, multiplexer  102  routes either the latched external address LEA[ 10 : 0 ] or the refresh address RFA[ 10 : 0 ] to memory array  101  as the row/bank address RBA[ 10 : 0 ]. Multiplexer  102  is controlled by the output signal provided by AND gate  103 . During an external access, the LADS# signal is activated low and the CLK signal is high, thereby causing AND gate  103  to provide a logic high signal to the control terminal of multiplexer  102 . In response, multiplexer  102  routes the latched external address LEA[ 10 : 0 ] to array  101 . If an external access is not being performed, AND gate  103  provides a logic low signal to the control terminal of multiplexer  102 , thereby causing the refresh address RFA[ 10 : 0 ] to be routed to array  101 .  
         [0053]    [0053]FIG. 4 is a waveform diagram that illustrates the timing of various signals in 1-T SRAM system  100  in accordance with one embodiment of the present invention. In the described example, a read access is performed during the first half of clock cycle T 1 . A refresh operation is performed during the second half of clock cycle T 1 . A write access is performed during the first half of clock cycle T 2 . Memory system  100  is idle during the second half of clock cycle T 2 , as there is no pending refresh at this time. To simplify the timing in the description below, all of the logic gates are assumed to have a negligible delay compared to the period the CLK signal and the delays d 1 -d 4  introduced by delay lines  301 - 304 .  
         [0054]    Read Access  
         [0055]    The external accessing client provides a low ADS# signal, a low WR# signal and an external address signal EA[ 14 : 0 ] before the rising edge of clock cycle T 1 . The low WR# signal specifies a read operation, and the external address signal EA[ 14 : 0 ] specifies the read address within DRAM array  101 . At the rising clock-edge of cycle T 1 , the ADS#, WR# and EA[ 14 : 0 ] signals are latched into input signal registers  120 , and provided as the LADS#, LWR# and LEA[ 14 : 0 ] signals, respectively.  
         [0056]    Within refresh controller  121 , the rising clock-edge of cycle T 1  increments modulo-64 counter  201  to a full count thereby causing the refresh request signal RREQ to be activated high. The high state of the RREQ signal increments 11-bit binary counter by one count, such that the refresh address RFA[ 10 : 0 ] has a value represented by “FA”. The refresh address FA identifies the bank and row address for the pending refresh operation. The refresh address FA is provided to multiplexer  102 , as the refresh address signal RFA[ 10 : 0 ].  
         [0057]    The logic low LADS# signal and the logic high CLK signal cause AND gate  103  (FIG. 1) to provide a logic high control signal to multiplexer  102 . In response, multiplexer  102  routes the bank/row information of the latched address LEA[ 10 : 0 ] to memory array  101  as the RBA[ 10 : 0 ] signal. The column information of the latched address LEA[ 14 : 11 ] is also provided to memory array  101 . In response, decoders (not shown) identify an addressed bank, an addressed row and an addressed set of columns of the present read access.  
         [0058]    The logic low LADS# signal and the logic high RREQ are provided to memory array sequencer  122  (FIG. 3). The low state of the LADS# signal and the high state of the CLK signal causes the RAS# signal to be activated low in the manner described above. The low state of the RAS# signal propagates through delay line  301 , thereby causing the SEN# signal to be activated low after a delay of d 1 . The low state of the SEN# signal propagates through delay line  302 , thereby causing the CAS# signal to be activated low after a delay of d 2 . Finally, the low state of the CAS# signal propagates through delay line  303 , thereby causing the PRC# signal to be activated low after a delay of d 3 . In this manner, the RAS#, SEN#, CAS# and PRC# signals are sequentially activated low.  
         [0059]    The logic low RAS# signal causes the data in the addressed row of the addressed bank (as specified by LEA[ 10 : 0 ]) to be driven to the sense amplifiers of the addressed bank. The logic low SEN# signal causes this row of data to be latched in the sense amplifiers of the addressed bank. The logic low CAS# signal causes the sense amplifiers corresponding with the addressed set of columns (as specified by LEA[ 14 : 11 ]) to be coupled to read data amplifier  70 . The logic low PRC# signal causes the data word (RDA) read from DRAM array  101  to be latched into read data amplifier  70  and provided as the data output signal DOUT[ 31 : 0 ].  
         [0060]    Within memory array sequencer  122 , the low state of the PRC# signal also causes the RAS#, SEN# and CAS# signals to be deactivated high in the manner described above. Within memory array  101 , the low state of the PRC# signal turns off the word line of the addressed row in the addressed bank, turns off the sense amplifiers in the addressed bank, and precharges the bit lines of the addressed bank, thereby preparing memory array  101  for the next operation. After a delay of d 4 , the logic low output signal provided by delay line  303  propagates through delay line  304 , thereby deactivating the PRC# signal high in the manner described above, and completing the read access.  
         [0061]    Refresh Access  
         [0062]    In the second half of clock cycle T 1 , the low state of the CLK signal causes AND gate  103  to provide a logic low signal to the control terminal of multiplexer  102 . In response, multiplexer  102  routes the refresh address RFA[ 10 : 0 ] to memory array  101  as the RBA[ 10 : 0 ] signal. In response, decoders (not shown) identify an addressed bank and an addressed row of the present refresh access.  
         [0063]    Within memory array sequencer  122 , the low state of the CLK signal and the high state of the RREQ signal cause the RAS# signal to be activated low in the manner described above. The low state of the RAS# signal propagates through delay line  301 , thereby causing the SEN# signal to be activated low after a delay of d 1 . The low state of the SEN# signal propagates through delay line  302 , with a delay of d 2 , and then through delay line  303 , with a delay of d 3 , thereby causing the PRC# signal to be activated low after a delay of d 2  plus d 3 . Note that the logic low CLK signal prevents OR gate  317  from activating a logic low CAS# signal, as column access is not required during a refresh operation. In this manner, the RAS#, SEN# and PRC# signals are sequentially activated low.  
         [0064]    The logic low RAS# signal causes the data in the addressed row of the addressed bank (as specified by RFA[ 10 : 0 ]) to be driven to the sense amplifiers of the addressed bank. The logic low SEN# signal causes this row of data to be latched in the sense amplifiers of the addressed bank. The sense amplifiers resolve the data values to a full signal swing, thereby refreshing the data from the addressed row.  
         [0065]    Within memory array sequencer  122 , the low state of the PRC# signal causes the RAS# and SEN# signals to be deactivated high in the manner described above. Within memory array  101 , the low state of the PRC# signal turns off the word line of the addressed row in the addressed bank, turns off the sense amplifiers in the addressed bank, and precharges the bit lines of the addressed bank, thereby preparing DRAM array  101  for the next operation. After a delay of d 4 , the PRC# signal is deactivated high, thereby completing the refresh access.  
         [0066]    Write Access  
         [0067]    The external accessing client provides a low ADS# signal, a high WR# signal, a write data value DIN[ 31 : 0 ] and an external address signal EA[ 14 : 0 ] before the rising edge of clock cycle T 2 . The high WR# signal specifies a write operation, and the external address signal EA[ 14 : 0 ] specifies the write address within DRAM array  101 . At the rising clock-edge of cycle T 2 , the ADS#, WR# and EA[ 14 : 0 ] signals are latched into input signal registers  120 , and provided as the LADS#, LWR# and LEA[ 14 : 0 ] signals, respectively. In addition, the write data value DIN[ 31 : 0 ] is latched into write data buffer  80 .  
         [0068]    The logic low LADS# signal and the logic high CLK signal cause AND gate  103  (FIG. 1) to provide a logic high control signal to multiplexer  102 . In response, multiplexer  102  routes the bank/row information of the latched address LEA[ 10 : 0 ] to DRAM array  101  as the RBA[ 10 : 0 ] signal. The column information of the latched address LEA[ 14 : 11 ] is also provided to memory array  101 . In response, decoders (not shown) identify an addressed bank, an addressed row and an addressed set of columns of the present write access.  
         [0069]    Within refresh controller  121 , the modulo-64 counter  201  is incremented to a zero count in response to the rising edge of the CLK signal, thereby causing the refresh request signal RREQ to be deactivated low.  
         [0070]    The logic low LADS# signal and the logic low RREQ signal are provided to memory array sequencer  122  (FIG. 3). The low state of the LADS# signal and the high state of the CLK signal cause the RAS#, SEN#, CAS# and PRC# signals to be sequentially activated low in the manner described above.  
         [0071]    The logic low RAS# signal causes the data in the addressed row of the addressed bank (as specified by LEA[ 10 : 0 ]) to be driven to the sense amplifiers of the addressed bank. The logic low SEN# signal causes this row of data to be latched in the sense amplifiers of the addressed bank. The logic low CAS# signal causes the sense amplifiers corresponding with the addressed set of columns (as specified by LEA[ 14 : 11 ]) to be coupled to write data buffer  80 , thereby overwriting the data in the sense amplifiers corresponding with the addressed set of columns with the write data value DIN[ 31 : 0 ]. These sense amplifiers, in turn, couple the write data value to the corresponding memory cells in the addressed row.  
         [0072]    Within memory array sequencer  122 , the low state of the PRC# signal causes the RAS#, SEN# and CAS# signals to be deactivated high in the manner described above. Within DRAM array  101 , the low state of the PRC# signal turns off the word line of the addressed row in the addressed bank, turns off the sense amplifiers in the addressed bank, and precharges the bit lines of the addressed bank, thereby preparing memory array  101  for the next operation. After a delay of d 4 , the PRC# signal is deactivated high, thereby completing the write access.  
         [0073]    In the foregoing manner, 1-T SRAM system  100  implements refresh accesses without interfering with external accesses, and without requiring an external refresh indicator signal. In the embodiment described above, the RAS#, SEN#, CAS# and PRC# signals are only activated when an external access or refresh operation is being performed. This results in power savings in 1-T SRAM system  100 .  
         [0074]    Alternate Embodiment  
         [0075]    In the embodiment described above, the external accesses are performed during one half of the clock period (i.e., when the CLK signal is high), and the refresh operations are performed during the other half of the clock period (i.e., when the CLK signal is low). Operation of  1 T SRAM system  100  therefore depends on both the high period and the low period of the CLK signal. Consequently, the operation and performance of 1-T SRAM system  100  is affected by the duty-cycle of the external clock signal CLK. Because a refresh access does not involve a column access operation, the memory cycle time for a refresh access is shorter than the memory cycle time for an external access. Consequently, 1-T SRAM system  100  would be capable of operating at higher clock frequencies if the memory cycle time of refresh accesses is optimized. In general, the shortest possible clock period (i.e., the highest possible clock frequency) exists when the clock period is equal to memory cycle time of an external access plus the memory cycle time of a refresh access. It is therefore desirable to have a memory system that operates independent of the clock duty-cycle, such that the memory system can operate in response to the shortest possible clock period.  
         [0076]    [0076]FIG. 5 is a circuit diagram of memory array sequencer  500 , which is used in another embodiment of the present invention. In this embodiment, memory array sequencer  500  replaces memory array sequencer  122  (FIGS. 1, 3). The construct of the other functional blocks of memory system  100  remains the same. As described in more detail below, memory array sequencer  500  enables memory system  100  to operate independent of the duty-cycle of the CLK signal.  
         [0077]    Memory array sequencer  500  includes delay lines  501 - 505 , OR gates  511 - 520 , AND gates  521 - 523 , one-shot circuits  531 - 533 , and flip-flops  551 - 554 , which include cross-coupled NAND gate pairs  541 - 542 ,  543 - 544 ,  545 - 546  and  547 - 548 , respectively. In general, delay lines  501 - 503  introduce the same signal delays (d 1 , d 2 , d 3 ) as delay lines  301 - 303 , respectively. Delay lines  504  and  505  introduce the same delays (d 1 , d 3 ) as delay lines  501  and  503 , respectively. Delay lines  501 - 503  serve two functions. First, during an external (read/write) access, delay lines  501 - 503  generate the timing control for the memory access operations. That is, delay lines  501 - 503  control the timing of the SEN#, CAS# and PRC# signals during an external access cycle. Second, delay lines  501 - 503  control the timing of the beginning of a refresh period. Delay lines  504  and  505  generate the timing control for the refresh access operations. That is, delay lines  504 - 505  control the timing of the SEN# and PRC# signals during a memory refresh cycle. External access timing control signals aRAS#, aSEN#, aPRC# and their counterpart refresh timing control signals rRAS#, rSEN# and rPRC# are logically AND&#39;ed to form the array control timing signals RAS#, SEN# and PRC#, respectively. The CAS# signal, which controls the column operation of the array, is activated only during external access cycles.  
         [0078]    Initial Generation of aRAS#, aSEN#, CAS#, aPRC# 
         [0079]    [0079]FIG. 6 is a waveform diagram illustrating the operation of 1-T SRAM system  100  when using memory array sequencer  500 . During the first clock cycle T 1 , there are no external accesses or refresh accesses pending in the memory system. During the second clock cycle T 2 , both a read access and a refresh access are pending. Although a write access is not specifically described in FIG. 6, it is understood that memory array sequencer  500  generates the same signals during read and write accesses.  
         [0080]    The first clock cycle T 1  is representative of the state of memory array sequencer  500  after memory system  100  has been initialized (although it is understood that a refresh request would not be generated the cycle after initialization). When 1-T SRAM system  100  is started, the RST# signal is activated low, thereby setting flip-flops  551 - 554 , such that these flip-flops provide logic high output signals. During clock cycle T 1 , there is no external access or refresh access to system  100 , so the latched address strobe signal LADS# is de-activated high and the refresh request signal RREQ is de-activated low. As a result, OR gates  511  and  512  provide logic high signals to one-shot circuits  532  and  533 , respectively. In response, one-shot circuits  532 - 533  provide logic high signals to flip-flops  552 - 553 , respectively, thereby causing flip-flops  552 - 553  to continue to provide logic high output signals. The output signal of flip-flop  552  is referred to as the aRAS# signal. As described below, the aRAS# signal is used to activate the RAS# signal during an external access. The logic high aRAS# signal is provided to AND gate  521 .  
         [0081]    The logic high LADS# signal causes OR gates  518 - 520  to provide logic high output signals aSEN#, CAS# and aPRC#, respectively. The aSEN# and aPRC# signals are used to activate the SEN# and PRC# signals during an external access. The CAS# signal always corresponds with an external access (because the CAS# signal is not required during a refresh access). The logic high aSEN# and aPRC# signals are provided to AND gates  522  and  523 , respectively.  
         [0082]    Initial Generation of rRAS#, rSEN#, rPRC# 
         [0083]    The logic high output signal of flip-flops  553  and  554  cause OR gate  513  to provide a logic high signal to OR gate  516 . In response, OR gate  516  provides a logic high output signal. The output signal of OR gate  516  is referred to as the rRAS# signal. As described below, the rRAS# signal is used to activate the RAS# signal during a refresh access. The logic high rRAS# signal is provided to AND gate  521 . Because both the aRAS# and rRAS# signals initially have logic high values, AND gate  521  initially provides a logic high (deactivated) RAS# signal.  
         [0084]    The logic high rRAS# signal is routed through delay line  504  to OR gate  517 , thereby causing OR gate  517  to provide a logic high rSEN# signal. The rSEN# signal is used to activate the SEN# signal during a refresh access. The logic high rSEN# signal is provided to AND gate  522 . Because both the aSEN# and rSEN# signals initially have logic high values, AND gate  522  initially provides a logic high (deactivated) SEN# signal.  
         [0085]    The logic high rSEN# signal is routed through delay line  505  to provide a logic high rPRC# signal. The rPRC# signal is used to activate the PRC# signal during a refresh access. The logic high rPRC# signal is provided to AND gate  523 . Because both the aPRC# and rPRC# signals initially have logic high values, AND gate  523  initially provides a logic high (de-activated) PRC# signal.  
         [0086]    Generation of aS#, C# and aP# 
         [0087]    Initially, flip-flop  551  provides a logic high output signal to delay line  501 . This logic high output signal propagates through delay line  501  to OR gate  514 . In response, OR gate  514  provides a logic high aS# signal. As described below, the aS# signal is used to activate the aSEN# signal.  
         [0088]    The logic high aS# signal propagates through delay line  501  to OR gate  515 . In response, OR gate  515  provides a logic high C# signal. As described below, the C# signal is used to activate the CAS# signal.  
         [0089]    The logic high C# signal propagates through delay line  503 , thereby providing a logic high aP# signal. As described below, the aP# signal is used to activate the aPRC# signal, and to signal the start of a refresh access.  
         [0090]    At the rising edge of each clock cycle, including clock cycle T 1 , one-shot circuit  531  is activated, such that one-shot circuit  531  generates a negative going pulse having a duration substantially shorter than one half the CLK period. This negative pulse resets the output of flip-flop  551  to a logic low value. The logic low value provided by flip-flop  551  propagates through delay line  501 , with delay d 1 , to OR gate  514 . In response, OR gate  514  provides a logic low aS# signal.  
         [0091]    The logic low aS# signal propagates through delay line  502 , with delay d 2 , to OR gate  515 . In response, OR gate  515  provides a logic low C# signal.  
         [0092]    The logic low C# signal propagates through delay line  503 , with delay d 3 , thereby providing a logic low aP# signal. The logic low aP# signal causes the aS# and C# signals to transition to logic high states (through OR gates  514  and  515 , respectively). The logic low aP# signal also returns (sets) the output of flip-flop  551  to a logic high state. The logic high C# signal propagates through delay line  503 , with delay d 3 , thereby causing the aP# signal to transition back to a logic high state. Note that the aS#, C# and aP# signals are generated in this sequence during every cycle of the CLK signal, regardless of the states of the LADS# or RREQ signals. That is, the aS#, C# and aP# signals are generated in this sequence whether or not there is an external access and/or refresh access pending in memory system  100 .  
         [0093]    External Access  
         [0094]    The external accessing client provides a low ADS# signal, a low WR# signal and an external address signal EA[ 14 : 0 ] before the rising edge of clock cycle T 2 . The low WR# signal specifies a read operation, and the external address signal EA[ 14 : 0 ] specifies the read address within array  101 . At the rising clock-edge of cycle T 2 , the ADS#, WR# and EA[ 14 : 0 ] signals are latched into input signal registers  120 , and provided as the LADS#, LWR# and LEA[ 14 : 0 ] signals, respectively. The functionality of the WR#, LWR#, EA[ 14 : 0 ] and LEA[ 14 : 0 ] signals has been described above in connection with FIGS.  1 - 4 . Because these signals are not relevant to the operation of memory array sequencer  500 , these signals are not discussed further in the present example.  
         [0095]    Within refresh controller  121 , the rising clock-edge of cycle T 2  increments modulo-64 counter  201  to a full count thereby causing the refresh request signal RREQ to be activated high. The high state of the RREQ signal increments 11-bit binary counter by one count, such that the refresh address RFA[ 10 : 0 ] has a value represented by “FA”. Because the functionality of the refresh address RFA[ 10 : 0 ] has been described above, this description is not repeated in the present example.  
         [0096]    As described above, the LADS# signal is activated low at the rising edge of clock cycle T 2 . The low state of the LADS# signal, along with the high state of the CLK signal causes the output of OR gate  511  to go low. The falling edge at the output of OR gate  511  causes one-shot circuit  532  to generate a negative going pulse having a width substantially shorter than half of the CLK period. This negative pulse resets the output of flip-flop  552  (i.e., the aRAS# signal) to a logic low value. This logic low aRAS# signal causes AND gate  521  to activate the RAS# signal low.  
         [0097]    The rising edge of clock cycle T 2  also causes one-shot circuit  551  to generate a negative going pulse having a width substantially shorter than half of the CLK period. This negative pulse resets the output of flip-flop  551  to a logic low value. This logic low value propagates through delay lines  501 ,  502  and  503 , thereby sequentially activating and deactivating the aS#, C# and aP# signals in the manner described above for clock cycle T 1 . Because the LADS# signal has a logic low value at this time, OR gates  518 ,  519  and  520  effectively pass the aS#, C# and aP# signals as the aSEN#, CAS# and aPRC# signals, respectively. In addition, AND gates  522  and  523  effectively pass the aSEN# and aPRC# signals as the SEN# and PRC# signals, respectively. When the aP# signal is activated low, flip-flop  552  is set, thereby de-activating the aRAS# signal high. In response, to the high aRAS# signal, AND gate  522  deactivates the RAS# signal high. The read access is performed in response to the sequentially activated and deactivated RAS#, SEN#, CAS# and PRC# signals. As described in more detail below, the aP# signal is used to coordinate the timing of the refresh access.  
         [0098]    Refresh Access  
         [0099]    Turning now to the refresh access, the rising edge of clock cycle T 2  (along with the logic high RREQ signal) causes OR gate  512  to provide a logic low signal to one-shot circuit  533 . In response, one-shot circuit  553  generates a negative going pulse having a width substantially shorter than half of the CLK period. This negative pulse resets the output of flip-flop  553  to a logic low value, which is provided to OR gate  513 . However, because flip-flop  554  provides a logic high signal to OR gate  513 , this OR gate continues to provide a logic high output signal. As a result, the rRAS# signal remains deactivated high.  
         [0100]    The aP# signal is applied to an input terminal of flip-flop  554 . When the aP# signal is activated low, flipflop  554  is reset, thereby causing this flip-flop  554  to provide a logic low output signal to OR gate  513 . However, because an inverting input terminal of OR gate  513  is coupled to receive the aP# signal, OR gate  513  continues to provide a logic high output signal at this time. As a result, the rRAS# signal remains deactivated high.  
         [0101]    As described above, the aP# signal is deactivated high after a delay of d 3 . However, this transition of the aP# signal does not change the output signal of flip-flop  554 . As a result, OR gate  513  provides a logic low output signal in response to the rising edge of the aP# signal. The logic low output signal of OR gate  513  is provided to OR gate  516 . In response, OR gate  516  activates the rRAS# signal low. This logic low rRAS# signal is effectively routed through AND gate  521 , thereby providing a logic low RAS# signal for the refresh access. In this manner, the RAS# signal of the refresh access is activated in response to the deactivated aP# signal. Stated another way, the refresh access is automatically initiated at the conclusion of the external (read) access. This advantageously allows the external access and the refresh access to be implemented in the shortest possible time.  
         [0102]    The logic low rRAS# signal propagates through delay line  504  (with delay d 2 ) to OR gate  517 . In response, OR gate  517  activates the rSEN# signal low. This logic low rSEN# signal is effectively routed through AND gate  522 , thereby providing a logic low SEN# signal for the refresh access.  
         [0103]    The logic low rSEN# signal propagates through delay line  505  (with delay d 3 ), thereby activating the rPRC# signal low. This logic low rPRC# signal is effectively routed through AND gate  523 , thereby providing a logic low PRC# signal for the refresh access.  
         [0104]    The logic low rPRC# signal causes the rRAS# and rSEN# signals to be deactivated high (through OR gates  516  and  517 , respectively). The logic low rPRC# signal is also applied to input terminals of flip-flops  553  and  554 , thereby setting the output signals of these flip-flops to logic high values, and preparing these flip-flops for the next refresh access. The logic high deactivated rSEN# propagates through delay line  505 , thereby deactivating the rPRC# signal high after delay d 2 . At this time, memory array sequencer  500  has been returned to its initial state, and is ready for the next clock cycle.  
         [0105]    Notice that if a refresh access were not pending in clock cycle T 2 , the RREQ signal would not be activated high. As a result, one-shot  533  would not be activated, and flip-flop  533  would continue to provide a logic high output signal to OR gate  513 . Under these conditions, the rRAS#, rSEN# and rPRC# signals would not be activated low during this cycle, and the RAS#, SEN# and PRC# would not be activated for a second time during this cycle. That is, a refresh access will not be performed if a refresh access is not pending.  
         [0106]    Second Alternate Embodiment  
         [0107]    In the embodiments described above, a memory clock cycle is split into two portions, such that both a memory access operation and a refresh operation can be performed during a single memory clock cycle. As a result, the minimum memory clock period is limited to a memory access operation period plus a refresh operation period. Even when the refresh cycle is shorter than the memory access cycle, the amount of memory bandwidth allocated for refresh is substantial. In practical applications, refresh operations are relatively infrequent events, occurring once in multiple clock cycles. Allocating a fixed portion of each clock period in order to hide refresh operations wastes bandwidth. In accordance with another embodiment of the present invention, another way to decrease the impact of refresh operations on the operating frequency of the memory system is to allocate one refresh period among multiple clock cycles.  
         [0108]    [0108]FIG. 7 is a block diagram of a 1-T SRAM system  700 , which is used in the present embodiment of the invention. Similar elements in memory system  100  (FIG. 1) and memory system  700  (FIG. 7) are labeled with similar reference numbers. Thus, 1-T SRAM system  700  includes DRAM array  101 , 2-to-1 multiplexer  102 , input signal registers  120  and refresh controller  121 . 1-T SRAM system  700  also includes memory array sequencer  701 , which replaces memory array sequencer  122  (FIGS. 1, 3).  
         [0109]    In memory system  700 , multiplexer  102  is controlled by a refresh control signal RFR#, which is generated by memory array sequencer  701 . The generation of the RFR# signal is described in more detail below. When the RFR# signal is asserted low, multiplexer  102  routes the refresh address RFA[ 10 : 0 ] to the memory row address bus RBA[ 10 : 0 ] for a refresh operation. Conversely, when the RFR# signal is de-asserted high, multiplexer  102  routes the latched external address signal LEA[ 10 : 0 ] to the memory row address bus RBA[ 10 : 0 ] for a memory access. The operation of memory system  700  is similar to the operation of memory system  100 , with the differences in the memory array operations described below.  
         [0110]    First, an embodiment supporting one refresh cycle and two memory cycles in two clock cycles will be described. Then, an embodiment supporting one refresh cycle and three memory cycles in three clock cycles will be described. Based on the description of these two embodiments, the present invention can be extended to other embodiments, in which a refresh cycle can be carried out in a period of four or more clock cycles without interfering with access to the memory array  101 .  
         [0111]    [0111]FIG. 8 is a circuit diagram of memory array sequencer  701  in accordance with one embodiment of the invention, which supports one refresh operation and two memory access operations in two clock cycles. Memory array sequencer  701  includes delay lines  801 - 807 , NAND gates  811 - 824 , which are cross coupled as illustrated to form R-S flip-flops  851 - 857  AND gate  825 , one-shot circuits  831 - 833 , OR gates  861 - 867  and divide-by-two counter  870 .  
         [0112]    In general, one-shot circuits  831 - 833  operate in a manner similar to one-shot circuits  531 - 533  of memory array sequencer  500  (FIG. 5). Flip-flops  851 - 853  operate in a manner similar to flip-flops  551 - 553 , respectively, of memory array sequencer  500  (FIG. 5). Flip-flops  854 - 856  are used to generate the memory access control signals RAS#, SEN# and CAS#, respectively. Flip-flop  857  is used to generate the refresh control signal RFR#, which is used to control multiplexer  102  (FIG. 7).  
         [0113]    Refresh control signal RFR# stays low during a memory refresh cycle. In general, delay lines  801 - 803  introduce the same signal delays (d 1 , d 2 , d 3 ) as delay lines  301 - 303 , respectively. That is, delay lines  801 - 803  control the timing of the SEN#, CAS# and PRC# signals during an external access cycle. Delay lines  804 - 807  control the timing of the SEN#, CAS# and PRC# signals during a memory refresh cycle. Divide-by-two counter  870  divides the external clock signal CLK, thereby providing a half-clock signal HCLK having a frequency that is half the frequency of the CLK signal. As described in more detail below, the HCLK signal is used to enable one refresh cycle to be implemented every two clock cycles in accordance with the present embodiment.  
         [0114]    Memory array sequencer  701  generates the memory control signals RAS#, SEN#, CAS#, PRC# and RFR# for memory access and refresh operations. The timing is generated such that only one refresh operation is allowed for every two clock cycles. The HCLK signal is used to define two different cycle types of the CLK signal. One clock cycle type is defined during the time that counter  870  provides a logic low HCLK signal. During these clock cycles, a refresh cycle is allowed. The other clock cycle type is defined during the time that counter  870  provides a logic high HCLK signal. During these clock cycles, no refresh cycle is allowed.  
         [0115]    The refresh request signal RREQ provided by refresh controller  121  is sampled by the rising edge of the CLK signal. If the refresh request signal RREQ is high during a rising edge of the CLK signal, then OR gate  862  drives a logic low output signal, resulting in a negative pulse at the output of one-shot circuit  833 . This negative pulse resets the output of flip-flop  853  to a logic ‘0’ value, thereby activating the refresh pending signal RFP#. If the HCLK signal is also low, then delay lines  804 - 807  are enabled to generate the memory control timing for a memory refresh operation.  
         [0116]    External memory accesses, however, can be launched every cycle of the CLK signal. At the rising-edge of the CLK signal, if the ADS# signal asserted low, then OR gate  861  provides a low output signal. This low output signal causes a low-going pulse to occur at the output terminal of one-shot circuit  832 . This low-going pulse resets the output of flip-flop  852  to a logic ‘0’ value, thereby activating the access pending signal ACP# low. Also at the rising-edge of the CLK signal, one-shot circuit  831  generates a low-going pulse that sets the output of flip-flop  851  a logic low value. In the absence of a refresh operation, the refresh control signal RFR# has a logic high value. In this case, the logic low state at the output of one-shot circuit  831 , together with the logic high state of the RFR# signal causes OR gate  863  to provide a logic low output signal. The logic low output signal provided by OR gate  863 , together with the logic low state of the access pending signal ACP# causes OR gate  864  to provide a logic low aR# signal. This logic low aR# signal resets flip-flop  854 , thereby causing flip-flop  854  to assert a logic low RAS# signal. The falling edge of the RAS# signal identifies the beginning of a memory cycle. In this case, the memory cycle is a memory access (rather than a refresh access).  
         [0117]    By design, both the memory access cycle time and the refresh cycle time are shorter than one period of the CLK signal (and the time required for two memory access cycles and one refresh cycle is shorter than two periods of the CLK signal). However, a memory access together with a refresh access is longer than one clock cycle. In the described embodiment, memory access operations are positioned at the beginning of a clock cycle, and refresh operations are positioned at the end of a clock cycle. As a result, a refresh operation initiated during one clock cycle is not completed until some time during the next clock cycle. Therefore, a memory access cycle following a refresh cycle is delayed. The delay is facilitated by the low state of the refresh control signal RFR#, which keeps the output of OR gate  864  high and inhibits the launch of the access cycle following the refresh cycle.  
         [0118]    In the absence of an external access and a refresh operation, flip-flop  851  provides a logic low output signal to OR gate  863  at the rising edge of the CLK signal, thereby causing OR gate  863  to provide a logic low output signal to delay line  801 . This logic low signal propagates to the output of delay line  801  as access control signal aS#, and then to the output of delay line  802  as access control signal aC#, and then to the output of delay line  803  as access control signal aP#. If there is a pending refresh (i.e., the RFP# signal is asserted low) and the HCLK signal is low, then the logic low aP# signal further propagates through delay lines  804 ,  805 ,  806  and  807 , causing the refresh control signals rRAS#, rSEN#, rPRC#, and RPND#, respectively, to be sequentially asserted low. In the case where the refresh pending signal RFP# is high (i.e., no refresh is pending), or the HCLK signal is high, then the low aP# signal is not propagated to delay line  804  and the outputs of delay lines  804  to  807  remain at logic high states.  
         [0119]    Returning now to the top of the delay chain, the logic low aS# signal sets the output of flip-flop  851  high, and this high signal in turn propagates to the outputs of delay lines  801 - 803  (and delay lines  804 - 807  if the RFP# and HCLK signals are low). Because flip-flop  851  is reset to provide a logic low output signal in response to each rising edge of the CLK signal, the aS#, aC# and aP# signals are pulsed low in response to every rising edge of the CLK signal. As long as there is no refresh cycle being performed, the logic low aS#, aC# and aP# pulses will have a first phase relationship relative to the rising edge of the CLK signal. However, if a refresh cycle is carried out, then the phases of the logic low aS#, aC# and aP# pulses are delayed accordingly by the low state of the RFR# signal. In the present embodiment, this delay is limited such that the back-to-back cycles of access-refresh-access operations are completed in two clock cycles.  
         [0120]    The memory control signals RAS#, SEN#, CAS#, RFR#, and PRC# are predominantly derived from the falling edges of the signals at the output terminals of delay lines  801 - 807 . Accordingly, during an access cycle when the access pending signal ACP# is asserted low, the RAS# signal is activated in response to the low state of the aR# signal at the output of OR gate  864 ; the SEN# signal is activated in response to the falling edge of the aS# signal; and the PRC# signal is activated in response to the low state of the aP# signal. The RAS#, SEN#, and CAS# signals are deactivated high in response to the falling edge of the PRC# signal.  
         [0121]    Similarly, during a refresh cycle when the refresh pending signal RFP# is asserted low, both the refresh control signal RFR# and the RAS# signal are activated in response to the falling edge of the rRAS# signal; the SEN# signal is activated in response to the falling edge of the rSEN# signal; and the PRC# signal is activated in response to the low state of the rPRC# signal. The CAS# signal is not activated during a refresh cycle. Both the RAS# and SEN# signals are deactivated high by the falling edge of the PRC# signal. The refresh control signal RFR# is de-activated high at the end of a refresh cycle in response to the falling edge of refresh precharge end signal rPEND#, which is provided by delay line  807  in response to the rPRC# signal.  
         [0122]    If there is no pending memory access, the access pending signal ACP# is deactivated high, and OR gates  864 ,  866  and  867  inhibit the activation of the RAS#, SEN# and CAS# signals, respectively. The PRC# signal, however, is still activated by the low going pulse aP#. This does not cause any functional problem in the memory as the default state of memory array  101  is the precharge state.  
         [0123]    If there is no pending refresh access, the high state of the refresh pending signal RFP# inhibits the activation of the rRAS#, rSEN#, rPRC# and rPEND# signals at the output terminals of delay lines  804 - 807  and thus the activation of the memory control signals RAS#, SEN# and PRC#.  
         [0124]    [0124]FIG. 9 is a waveform diagram illustrating the operation of 1-T SRAM system  700  when using memory array sequencer  701 . During the first clock cycle T 1 , (i.e., an idle cycle) there are no external accesses or refresh accesses pending in the memory system. During the second clock cycle T 2 , both a read memory access and a refresh access are pending. During the third clock cycle T 3 , a write memory access and another refresh access are pending. As described below, the refresh access pending during the third clock cycle T 3  is not processed during the third clock cycle T 3 . Note that it is not necessary for the memory access of cycle T 2  to be a read access and the memory access of cycle T 3  to be a write access. The memory accesses performed during cycles T 2  and T 3  can include any combination of read and/or write accesses in other examples.  
         [0125]    As shown in FIG. 9, at the beginning of cycle T 1 , the high state of the address strobe signal ADS# is de-activated high, thereby indicating no external access is requested. The low state of the refresh request signal RREQ indicates that no refresh operation is requested. The high ADS# signal causes flip-flop  852  to deactivate the access pending signal ACP# high. Similarly, the low RREQ signal causes flip-flop  853  to deactivate the refresh pending signal RFP# signal high. As a result, no memory operation is carried out in this clock cycle. This is reflected by the high state of the memory control signals RAS#, SEN#, CAS#, and PRC# throughout cycle T 1 .  
         [0126]    Note that the rising edge of the CLK signal at the beginning of cycle T 1  causes flip-flop  851  to provide a logic low output signal to OR gate  863 , thereby causing OR gate  863  to provide a logic low output signal, which is propagated through delay lines  801 - 803 . As a result, the signals aS#, aC# and aP# are sequentially asserted and de-asserted during clock cycle T 1 . However, the logic high ACP# and RFP# signals ensure that these asserted aS#, aC# and aP# signals do not cause the RAS#, SEN#, CAS#, and PRC# signals to be asserted during cycle T 1 .  
         [0127]    Prior to the rising edge of cycle T 2 , the ADS# signal is activated low, the write/read control signal WR# is driven low, and an external address EA[ 14 : 0 ] having a value of “A” is provided, thereby indicating that a read memory access to address “A” is pending. Also prior to the rising edge of cycle T 2 , the RREQ signal is activated high, thereby indicating that a refresh access is also pending.  
         [0128]    At the rising edge of cycle T 2 , the low state of the ADS# signal causes the access pending signal ACP# to be activated low. In addition, the high state of the RREQ signal causes the refresh pending signal RFP# to be activated low. Moreover, the rising edge of the CLK signal causes OR gate  863  to provide a logic low output signal. OR gate  864  activates the aR# signal low in response to the logic low output of OR gate  863  and the logic low ACP# signal. This logic low aR# signal causes flip-flop  854  to assert the RAS# signal low.  
         [0129]    The logic low output signal provided by OR gate  863  is also propagated through delay lines  801 ,  802  and  803 , as access control signals aS#, aC# and aP#, respectively, with delays d 1 , d 2  and d 3 , respectively. The logic low aS# and ACP# signals cause OR gate  866  to provide a logic low signal to flip-flop  855 . In response, flip-flop  855  activates the SEN# signal low. The logic low aC# and ACP# signals cause OR gate  867  to provide a logic low signal to flip-flop  856 . In response, flip-flop  856  activates the CAS# signal low. The logic low aP# signal causes AND gate  825  to activate the PRC# signal low. In this manner, the RAS#, SEN#, CAS# and PRC# signals are activated by the falling edges of the aR#, aS#, aC#, and aP# signals, respectively. The logic low PRC# signal causes flip-flops  854 - 856  to deactivate the RAS#, SEN#, and CAS# signals high. The PRC# is deactivated high when the aP# signal is deactivated high. In this manner, the first memory access is performed during clock cycle T 2 .  
         [0130]    Because both the refresh pending signal RFP# and the half clock signal HCLK are low, the low-going pulse of the aP# signal is propagated through OR gate  865  to delay lines  804 - 807  as the rRAS#, rSEN#, rPRC#, and rPEND# signals. The activation of the rRAS# signal indicates the end of the memory access cycle and the beginning of the memory refresh cycle. The activated (low) rRAS# signal causes flip-flops  854  and  857  to activate the respective RAS# and RFR# signals low. The logic low refresh control signal RFR# causes the current refresh address (i.e., “FA”) to be provided to the memory array. The activated (low) rSEN# signal causes flip-flop  855  to activate the SEN# signal low. Similarly, the activated (low) rPRC# signal causes AND gate  825  to activate the PRC# signal low. The activated (low) PRC# signal indicates the beginning of sense-amplifier precharge. Thus, the activated PRC# signal causes flip-flops  854  and  855  to deactivate the RAS# and SEN# signals high. The rPEND# signal is activated low after the low rPRC# signal propagates through delay line  807 . The low rPEND# signal identifies the end of the refresh cycle, and causes flip-flop  857  to deactivate the refresh control signal RFR# high, thereby causing the latched external address (i.e., “B”) to be provided to the memory array. The low rPEND# signal also resets the refresh pending signal RFP# provided by flip-flop  853 . In this manner, a refresh access is performed, beginning in cycle T 2  and ending in cycle T 3 .  
         [0131]    Prior to the rising edge of cycle T 3 , the address strobe signal ADS# is activated low, the write/read control signal WR# is driven high, and the external address signal EA[ 14 : 0 ] has a value of B, thereby indicating a pending write operation. Also prior to the rising edge of cycle T 3 , the refresh request signal RREQ is activated high, thereby indicating another pending refresh request. (It is understood that the refresh request signal RREQ would not be asserted during consecutive clock cycles. However, the refresh request signal RREQ is activated high prior to cycle T 3  in the present example to illustrate the manner in which the refresh request signal RREQ is processed when the HCLK signal has a logic high value.) The low state of the ADS# signal causes the access pending signal ACP# to be activated low, and the high state of the RREQ signal causes the refresh pending signal RFP# to be activated low. However, because the refresh cycle that began in cycle T 2  is not finished by the beginning of cycle T 3  (i.e., the RFR# signal is activated low at the rising edge of cycle T 3 ), OR gate  863  provides a logic high output signal, which prevents OR gate  864  from activating the aR# signal low. Thus, the aR# signal remains deactivated in a logic high state and the new access cycle does not begin at the rising edge of cycle T 3 .  
         [0132]    The refresh control signal RFR# is deactivated high about a quarter of the way through clock cycle T 3  (in response to the activated rPEND# signal). At this time, OR gate  863  provides a logic low output signal, thereby enabling OR gate  864  to activate the aR# signal low. The logic low output signal provided by OR gate  863  also causes the sequential activation and deactivation of the aS#, aC# and aP# signals, in the manner described above. Accordingly, the RAS#, SEN#, CAS#, and PRC# signals are activated to carry out the memory access cycle. The access cycle ends with the deactivation of the RAS#, SEN#, CAS#, and PRC# signals. Note that this memory access cycle ends prior to the rising edge of the next clock cycle T 4 . As a result, two memory accesses and a refresh access are performed during two clock cycles T 2  and T 3 . Also note that if the refresh access was not present during cycle T 2 , then the second memory access would be initiated immediately at the rising edge of clock cycle T 3 .  
         [0133]    Moreover, although the refresh pending signal RFP# is activated low, the HCLK signal provided by counter  870  is deactivated high, thereby preventing OR gate  865  from propagating the aP# signal to delay line  804 . As a result, the refresh control signals rRAS#, rSEN#, rPRC# and rPEND# are not activated during cycle T 3 . The pending refresh operation is served in the following clock cycle, when the HCLK signal is activated low.  
         [0134]    [0134]FIG. 10 is a circuit diagram of a memory access sequencer  901 , which can be used in place of memory access sequencer  701 , in accordance with another embodiment of the present invention. In this embodiment, memory access sequencer  901  generates memory control timing signals which enable one refresh operation and three memory access operations to be performed in three clock cycles. Memory access sequencer  901  includes delay lines  901 - 906 , set/reset flip-flops  951 - 956 , one-shot circuits  931 - 934 , OR gates  961 - 965 , AND gates  925 - 926  and divide-by-3 counter  970 . Flip-flops  951 - 956  include cross-coupled NAND gates  911 - 922  as illustrated. OR gate  961 , one-shot circuit  931  and flip-flop  951  operate in the same manner as OR gate  862 , one-shot circuit  833  and flip-flop  853  of memory access sequencer  701  (FIG. 8). Thus, these elements drive the refresh pending signal RFP# low when the refresh request signal RREQ and the clock signal CLK are both high. Similarly, OR gate  962 , one-shot circuit  932  and flip-flop  952  operate in the same manner as OR gate  861 , one-shot circuit  832  and flip-flop  852  of memory access sequencer  701  (FIG. 8). Thus, these elements drive the access pending signal ACP# low when the address strobe signal ADS# is activated low and the clock signal CLK is high.  
         [0135]    Divide-by-three counter  970  generates a periodic signal CLK 3 # having a frequency equal to one-third of the frequency of the CLK signal. The CLK# 3  signal has a logic low state for one third of the CLK 3 # period, and a logic high state for two thirds of the CLK 3 # period. That is, the CLK 3 # signal has a low state for one period out of every three periods of the CLK signal. The CLK 3 # signal is applied to OR gate  963  and one-shot circuit  933 . OR gate  963  activates the refresh control signal RFR# when the CLK 3 # signal has a low value and the refresh pending signal RFP# is activated low. Thus, the refresh control signal RFR# can only be activated low once every three cycles of the CLK period.  
         [0136]    Flip-flops  953 - 955  generate the memory control timing signals RAS#, SEN#, and CAS#, respectively, in response to the RFR# signal and the output signals rSEN#, rPRC#, aRAS#, aCAS#, and aPRC# provided by delay lines  901 ,  902 ,  903 ,  904 ,  905  and  906 , respectively.  
         [0137]    At each falling edge of the CLK 3 # signal, one-shot circuit  933  applies a low-going pulse to the input terminal of delay line  901 . This low-going pulse propagates through delay lines  901  and  902  to AND gate  926 , thereby causing AND gate  926  to provide a logic low PRC# output signal, which resets flip-flop  956  and sets flip-flops  951  and  953 - 955 . The pulse width can be 1 ns or another period sufficient to set or reset these flip-flops.  
         [0138]    The input terminal of delay line  903  is connected to one-shot circuit  934 , which generates a low-going pulse when both the access pending signal ACP# and the access enabled signal ACEN# provided by flip-flop  956  are activated low. The access enabled signal ACEN# provided by flip-flop  956  is activated low by the falling edge of the precharge signal PRC#. Therefore, when a memory access is pending (ACP# low), a low-going pulse is generated at the input of delay line  903  when the memory array  101  enters a precharge state. The precharge state is triggered by the falling edge of the PRC# signal, which is in turn triggered by a falling edge of either the refresh precharge signal rPRC# or the access precharge signal aPRC#. The access enable signal ACEN# is deactivated high when the memory enters a refresh cycle as indicated by the falling edge of the RFR# signal, or when the memory enters a memory access cycle as indicated by the falling edge of the RAS# signal. The access pending signal ACP# is deactivated high when the memory enters a memory access cycle signaled by the falling-edge of the aRAS# signal. The refresh pending signal RFP# is deactivated high when the memory is finishing a refresh operation signaled by the falling edge of the rPRC# signal.  
         [0139]    The memory control signals RAS#, SEN#, CAS#, and PRC# are derived from the falling edges of the signals provided at the output terminals of delay lines  901 - 906 . Accordingly, during a memory access cycle when the access pending signal ACP# is activated low, the RAS# signal is activated low by the falling edge of the aRAS# provided by delay line  903 , the SEN# signal is activated low by the falling edge of the aSEN# signal provided by delay line  904 , and the PRC# signal is activated low by the falling edge of the aPRC# signal provided by delay line  906 . The activated signals RAS#, SEN#, and CAS# are deactivated in response to the falling edge of the PRC# signal.  
         [0140]    Similarly, during a refresh cycle when refresh pending signal RFP# is activated low, both the refresh control signal RFR# and the RAS# signal are activated in response to the low state of the CLK 3 # signal, the SEN# signal is activated low by the falling edge of the rSEN# signal provided by delay line  901 , and the PRC# signal is activated low by the falling edge of the rPRC# signal provided by delay line  902 . The CAS# signal is not activated during a refresh cycle. During a refresh cycle, the RFR#, RAS# and SEN# signals are all deactivated high in response to the falling edge of the rPRC# signal.  
         [0141]    When no memory access is pending, the access pending signal ACP# is deactivated high, thereby causing OR gate  964  to inhibit the aRAS#, aSEN#, aCAS# and aPRC# signals from being activated low. When no refresh is pending, the refresh pending signal RFP# is deactivated high, thereby causing both the RFR# signal and the output of OR gate  965  to have logic high values. As a result, the activation of the memory control signals RAS#, SEN#, CAS# and PRC# are inhibited. However, a low-going pulse is still generated by one-shot circuit  933  once every three periods of the CLK signal (at the falling edge of the CLK 3 # signal), and this pulse is propagated through delay lines  901  and  902  as the rSEN# and rPRC# signals.  
         [0142]    [0142]FIG. 11 is a waveform diagram illustrating the operation of 1-T SRAM system  700  when using memory array sequencer  901 . A memory access occurs in each of the clock cycles T 1 -T 4 , as indicated by the ADS# signal, which is activated low prior to the rising edge of each of the clock cycles T 1 -T 4 . In addition, a refresh request is detected during cycle T 3 , and serviced with a memory refresh cycle during cycle T 4 .  
         [0143]    At the beginning of cycle T 1 , the low state of the address strobe signal ADS# and the high state of the CLK signal causes the output of OR gate  962  to go low and a low-going pulse to appear at the output of one-shot circuit  932 , thereby activating the access pending signal ACP# low. However, the high state of the access enable signal ACEN# inhibits the low state of the ACP# signal from propagating to one-shot circuit  934 . In addition, the CLK 3 # signal goes low at the rising edge of cycle T 1 , thereby causing one-shot circuit  933  to apply a low-going pulse to the input terminal of delay line  901 . This pulse is propagated through delay lines  901  and  902  to activate the rSEN# and rPRC# signals, respectively. The low state of the rPRC# signal causes AND gate  926  to provide a logic low value to flip-flop  956 , thereby causing this flip-flop  956  to activate the access enable signal ACEN# low. The low states of the access enable signal ACEN# and the access pending signal ACP# cause OR gate  964  to provide a logic low output signal, thereby causing one-shot circuit  934  to generate a low-going pulse. This low-going pulse is propagated through delay lines  903  to  906  thereby sequentially activating and deactivating the aRAS#, aSEN#, aCAS# and aPRC# signals. The activated aRAS# signal causes AND gate  925  to provide a logic low output signal, thereby causing flip-flop  953  to activate the RAS# signal low. The activated aCAS# and aPRC# signals cause flip-flops  954  and  955  to activate the respective SEN# and CAS# signals low. The activated aPRC# signal causes AND gate  926  to provide a logic low output signal, activating the PRC# signal low, and causing flip-flops  953 - 955  to deactivate the RAS#, SEN# and CAS# signals high. The activated and deactivated RAS#, SEN#, CAS# and PRC# signals carry out a memory access cycle.  
         [0144]    The activation of the RAS# signal also causes flip-flop  956  to de-activate the access pending signal ACEN# high, thus inhibiting the start of another access cycle until the current access cycle is completed. Because the memory access cycle is not started until the second half of the CLK cycle T 1 , even though the access cycle time is shorter than a CLK period, the current access cycle does not finish until well into cycle T 2 .  
         [0145]    At the beginning of cycle T 2 , the low state of the address strobe signal ADS# and the high state of the CLK signal causes the access pending signal ACP# to be activated low. However, the high state of access enable signal ACEN# inhibits the low ACP# signal from propagating through OR gate  964  to one-shot device  934 . Thus, the second memory access is not carried out until some time close to the second half of cycle T 2 , when the PRC# signal is activated low. When the PRC# signal is activated low, flip-flop  956  resets, and causes the ACEN# signal to go low. The low ACEN# signal, combined with the low ACP# signal, causes OR gate  964  to provide a low input signal to one-shot circuit  934 , thereby causing one-shot circuit  934  to provide a low-going pulse. The low-going pulse provided by one-shot circuit  934  enables another set of access cycle timing signals aRAS#, aSEN#, aCAS# and aPRC# to be generated by delay lines  903 - 906 . The activation of the RAS# signal again deactivates the access enable signal ACEN# high and inhibits the start of another memory access cycle until the current cycle is finished. Because the access cycle time is shorter than a clock period, the current access cycle is completed closer to the beginning of next clock cycle T 3 .  
         [0146]    Prior the beginning of cycle T 3 , the refresh request signal RREQ is asserted high, thereby indicating the presence of a refresh request. In addition, the address strobe signal ADS# is activated low, thereby indicating the presence of a third consecutive access cycle. The high state of the refresh request signal RREQ causes OR gate  961  to provide a logic low signal to one-shot device  931  at the rising edge of the CLK signal. In response, one-shot device  931  provides a low-going pulse to flip-flop  951 , thereby activating the refresh pending signal RFP# low. The low state of the address strobe signal ADS# causes OR gate  962  to provide a logic low signal to one-shot device  932  at the rising edge of the CLK signal. In response, one-shot device  932  provides a low-going pulse to flip-flop  952 , thereby activating the access pending signal ACP# low. Because the CLK 3 # signal has a logic high state during cycle T 3 , OR gate  963  does not activate the refresh control signal RFR# during cycle T 3 . Thus, the refresh cycle is not performed during cycle T 3 . However, the memory access cycle initiated at the rising edge of cycle T 3  is carried out in the manner described above for the first two memory access cycles. However, the memory access cycle initiated at the rising edge of cycle T 3  is completed before the beginning of cycle T 4 .  
         [0147]    At the beginning of cycle T 4 , the low state of the address strobe signal ADS# again causes the access pending signal ACP# to be activated low. Similar to the situation in cycle T 1 , the clock signal CLK 3 # transitions to a logic low state. At this time, the logic low CLK 3 # and RFP# signals cause OR gate  963  to activate the refresh control signal RFR# low. The low RFR# signal causes AND gate  925  to provide a low signal to flip-flop  953 , thereby causing the RAS# signal to be activated low. The low state of the refresh control signal RFR# also causes flip-flop  956  to deactivate the access enable signal ACEN# high, thereby inhibiting the memory access initiated during cycle T 4  from being carried out. The falling edge of the CLK 3 # signal also causes one-shot circuit  933  to generate a low-going pulse at the input of delay line  901 . This pulse is propagated through delay line  901  as the rSEN#, and then through delay line  902  as the rPRC# signal. The falling edge of the rSEN# signal causes OR gate  965  to provide a logic low signal to flip-flop  954 . In response, flip-flop  954  activates the SEN# signal low. The activation of the RAS# and SEN# signals causes a memory refresh cycle to be carried out. The falling edge of the rPRC# signal causes flip-flop  951  to deactivate the refresh pending signal RFP# high. The high RFP# signal, in turn, causes OR gate  963  to deactivate the refresh control signal RFR# high. The falling edge of the rPRC# signal also causes AND gate  926  to provide a logic low PRC# signal, thereby causing flip-flops  953  and  954  to deactivate the RAS# and SEN# signals high. The low state of the PRC# signal resets flip-flop  956 , thereby causing the access enable signal ACEN# to be activated low.  
         [0148]    At this time, the low states of the access enable signal ACEN# and the access pending signal ACP# cause OR gate  964  to provide a logic low signal to one-shot circuit  934 . In response, one-shot circuit  934  generates a low-going pulse at the input of delay line  903 . This pulse is propagated, in sequence, to the outputs of delay lines  903  to  906  activating the memory control signals to carry out the fourth memory access cycle.  
         [0149]    In the foregoing manner, three memory access cycles and one refresh access cycle can be completed during three clock cycles. It is noted that the timing between the rising edge of the first clock cycle T 1  and the memory control signals generated during the first clock cycle T 1  is the same as the timing between the rising edge of the fourth clock cycle T 4  and the memory control signals generated during the fourth clock cycle T 4 . Thus, the refresh operations can be completed once every three cycles of the CLK signal, without disrupting the external memory access operations.  
         [0150]    Although the invention has been described in connection with several embodiments, it is understood that this invention is not limited to the embodiments disclosed, but is capable of various modifications, which would be apparent to a person skilled in the art. For example, although the read/write accesses have been described as occurring during the first half of a clock cycle, and the refresh operations have been described as occurring during the second half of a clock cycle, it is understood that this order may be reversed in other embodiments. The manner of performing this reversal would be apparent to one of ordinary skill in the art. Furthermore, although the refresh operations have been described as not activating the CAS# signal, it is understood that the CAS# signal may be activated during refresh operations in other embodiments. Moreover, although the above-described memory array sequencers are initially reset by the RST# signal, it is understood that these memory array sequencers are capable of resetting themselves in the absence of the RST# signal. In addition, the DRAM memory banks can have different sizes in different embodiments. Similarly, different numbers of DRAM banks can be used. Moreover, buses having different widths than those described can be used in other embodiments. In addition, different logic can be used to provide the same results. Thus, the invention is limited only by the following claims.