Abstract:
A device generally comprising a memory array and a burst sequence generator. The memory array may be configured to store data. The burst sequence generator may be configured to generate a burst sequence in response to address information received by the device. The burst sequence may be configured to identify a plurality of locations for storing data in the memory array. The device may have a maximum operating current of 50 milliamps and/or a maximum standby current of about 25 microamps.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/191,853, filed Mar. 24, 2000, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and/or architecture for a random access memory device generally and, more particularly, to a synchronous random access memory device having a burst transfer capability. 
     BACKGROUND OF THE INVENTION 
     Certain microprocessors and memory devices are designed to transfer data using a burst type transfer. The burst type transfer causes the data to be transferred at multiple consecutive addresses without having to present all addresses to the memory device. The burst type transfers may be use a linear sequence where the data at consecutive addresses ate serially transferred. The burst type transfers may use an interleaved sequence where the data at interleaved addresses are serially transferred. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a device comprising a memory array and a burst sequence generator. The memory array may be configured to store data. The burst sequence generator may be configured to generate a burst sequence in response to address information received by the device. The burst sequence may be configured to identify a plurality of locations for storing data in the memory array. The device may have a maximum operating current of 50 milliamps and/or a maximum standby current of about 25 microamps. 
     The objects, features and advantages of the present invention include providing a synchronous burst random access memory that may (i) operate in a linear burst sequence, (ii) operate in an interleaved burst sequence, (iii) support single read accesses, (iv) support single write accesses, (v) consume extremely low power, and/or (vi) interface with a memory controller with minimal glue logic. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which: 
     FIG. 1 is a block diagram of the present invention; 
     FIG. 2 is a detailed block diagram of the present invention; 
     FIG. 3 is a state transition diagram; 
     FIG. 4 is a timing diagram showing a single read access and a burst read sequence; 
     FIG. 5 is a timing diagram showing a single write access and a burst write sequence; 
     FIG. 6 is a timing diagram showing a burst read sequence immediately followed by a burst write sequence; 
     FIG. 7 is a timing diagram showing a burst write sequence immediately followed by a burst read sequence; 
     FIG. 8 is a second state transition diagram; 
     FIG. 9 is a timing diagram showing a single read access and a burst read sequence; 
     FIG. 10 is a timing diagram showing a single write access and a burst write sequence; 
     FIG. 11 is a timing diagram showing a burst read sequence immediately followed by a burst write sequence; and 
     FIG. 12 is a timing diagram showing a burst write sequence immediately followed by a burst read sequence. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention generally concerns a synchronous static random access memory with a burst sequence capability. The burst sequence capability may provide high bandwidth using a synchronous interface while providing extremely low power consumption. 
     Referring to FIG. 1, a block diagram of a memory device  100  is shown according to a preferred embodiment of the present invention. The memory device  100  may have an input  102  for receiving a signal (e.g., ADD). The memory device  100  may have an input/output interface  104  for receiving and presenting a signal (e.g., DATA). The memory device  100  may have another input  106  for receiving a signal (e.g., CLK). The memory device  100  may have an input  108  for receiving a group of signals (e.g., CONTROL SIGNALS). The memory device  100  may have an input  110  for receiving power. 
     The signal CLK may serve as a clock used to establish synchronization. The synchronization may be to either a rising edge or a falling edge of the signal CLK. The signal CLK may be implemented as a periodic signal having an approximately 50% duty cycle. Other duty cycles may be implemented to meet the design criteria of a particular application. The input  106  may also be referred to as an input CLK. 
     The signal ADD may serve as an address signal. The signal ADD may be a synchronous type input signal sampled by the memory device  100  on a rising edge of the signal CLK. In a preferred embodiment, the signal ADD may be implemented as an eighteen-bit wide address. Consequently, the input  102  may be implemented as eighteen discrete input lines collectively referred to as an input A. 
     The signal DATA may serve as data words. The signal DATA may be a synchronous type bidirectional signal. As a synchronous type input signal, the signal DATA may be sampled by the memory device  100  on a rising edge of the signal CLK. As a synchronous type output signal, the signal DATA may become valid a delay time after a rising edge of the signal CLK. In a preferred embodiment, the signal DATA may be implemented as 32-bit wide data. Consequently, the input/output  104  may be implemented as thirty-two discrete bidirectional lines collectively referred to as an interface DG. The signal DATA generally comprises read data during read cycles and write data during write cycles. 
     The signal CONTROL SIGNALS may serve as a configuration, enable, and/or mode signal that direct operations of the memory device  100 . The signal CONTROL SIGNALS may be a collection of several signals (e.g., SCE, OE, ADV, SWE, MODE, ZZ, B 3 , and L 3 ). The signals SCE, OE, ADV, SWE, MODE, ZZ, B 3 , and L 3  may be received at input  108  on discrete lines (shown in more detail in FIG.  2 ). The discrete lines for the signals SCE, OE, ADV, SWE, MODE, ZZ, B 3 , and L 3  may be referred to as inputs /SCE, /OE, /ADV, /SWE, MODE, ZZ, B 3 , and L 3  respectively. The prefix “/” to the alphanumeric reference for an input generally indicates that the respective input is considered active low. An active low input may also be indicated by a bar over the alphanumeric reference of the input. 
     The signal SCE may serve as a synchronous chip enable signal. The signal SCE may be a synchronous type input signal sampled by the memory device  100  on the rising edge of the signal CLK. In a preferred embodiment, the signal SCE may be asserted in a logical LOW state and deasserted in a logical HIGH state. While the signal SCE is in the logical LOW state, the memory device  100  may store the signal ADD. The signal SCE is generally used to initiate all read and write operations for the memory device  100 . 
     The signal OE may serve as an output enable signal. The signal OE may be an asynchronous type input signal independent of the signal CLK. In a preferred embodiment, the signal OE may be asserted in the logical LOW state and deasserted in the logical HIGH state. While the signal OE is in the logical LOW state, the interface  104  (interface DQ) may behave as an output to present the data read from the memory device  100 . While the signal OE is in the logical HIGH state, the interface  104  may behave as a tri-state or three-state input to receive the data being written into the memory device  100 . 
     The signal ADV may serve as an advance signal. The signal ADV may be a synchronous type input signal by the memory device  100  on a rising edge of the signal CLK. In one embodiment, the signal ADV may be asserted in the logical LOW state and deasserted in the logical HIGH state. While the signal ADV is in the logical LOW state, the memory device  100  may internally increment the signal ADD during a burst sequence. In an alternative embodiment, the signal ADV may unused during normal functioning of the memory device  100 . 
     The signal SWE may serve as a synchronous write enable signal. The signal SWE may be a synchronous type input signal sampled by the memory device  100  on a rising edge of the signal CLK. The signal SWE may be asserted in the logical LOW state and deasserted in the logical HIGH state. While the signal SWE is in the logical LOW state, the memory device  100  may initiate a write cycle. The memory device  100  may ignore the signal SWE while the signal SWE is in the logical HIGH state. 
     The signal MODE may serve as a mode control signal. The signal MODE may be a static type input signal strapped to either the logical LOW state or the logical HIGH state at all times. In an alternative embodiment, the logical HIGH state may be achieved by floating the input MODE. While the signal MODE is in the logical LOW state, the memory device  100  may operate with a linear burst sequence. While the signal MODE is in the logical HIGH state, the memory device  100  may operate with an interleaved burst sequence. 
     The signal ZZ may serve as a snooze signal. The signal ZZ may be an asynchronous type input signal independent of the signal CLK. The signal ZZ may be asserted in the logical HIGH state and deasserted in the logical LOW state. While the signal ZZ is in the logical HIGH state, the memory device  100  may be placed in a low power consumption standby mode. While the signal ZZ is in the logical LOW state, the memory device  100  may be placed in a normal operation mode. If the input ZZ is left not connected, then the signal ZZ may default to the logical LOW state. 
     The signals B 3  and L 3  may serve as a burst counter selection signal. The signals B 3  and L 3  may be static type input signals strapped to either the logical LOW state or the logical HIGH state at all times. In one embodiment, the signals B 3  and L 3  generally determine a size of a burst sequence. In an alternative embodiment, the signals B 3  and L 3  may not be required. 
     The power may be divided into one set of power for inputs and outputs (I/O) of the memory device  100  and another set of power for a core of the memory device  100 . The power for I/O may include a source (e.g., V DD ) and a ground (e.g., V SS ). The power for the core may include another source (e.g., V DDQ ) and another ground (e.g., V SSQ ). A summary of the signals is provided in TABLE 1. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Name 
                 I/O 
                 Description 
               
               
                   
               
             
             
               
                 A [17:0]   
                 Input- 
                 Address Inputs used to select one of the (262, 144 or 524, 288) address locations. Sampled 
               
               
                   
                 Synchronous 
                 at the rising edge of the CLK if {overscore (SCE)} is sampled active. A [1:0]  are also fed into the Burst counter 
               
               
                   
                   
                 and used to increment through the burst sequence when {overscore (ADV)} is asserted. 
               
               
                 DQ [31:0]   
                 I/O- 
                 Bidirectional Data I/O lines. As inputs, they feed into an on-chip data register that is triggered 
               
               
                   
                 Synchronous 
                 by the rising edge of CLK. As outputs, they deliver the data contained in the memory location 
               
               
                   
                   
                 specified by A [17:0] . The direction of the pins is controlled by {overscore (OE)} and the internal logic of the 
               
               
                   
                   
                 device. When {overscore (OE)} is asserted LOW, the pins can behave as outputs. When HIGH, DQ [0:31]  are 
               
               
                   
                   
                 placed in a three-state condition. 
               
               
                 CLK 
                 Input-Clock 
                 Clock Input. Used to capture all synchronous inputs to the device. 
               
               
                 {overscore (SCE)}  
                 Input- 
                 Synchronous Chip Enable, active LOW, sampled on the rising edge of CLK. When asserted 
               
               
                   
                 Synchronous 
                 LOW, A [x:0]  is captured in the address registers. A [1:0]  are also loaded into the burst counter. 
               
               
                   
                   
                 {overscore (SCE)} is used to initiate all Read and Write accesses. 
               
               
                 {overscore (OE)}  
                 Input- 
                 Output Enable, asynchronous input, active LOW. Controls the direction of the I/O pins. When 
               
               
                   
                 Asynchronous 
                 LOW, the I/O pins behave as outputs. When deasserted HIGH, I/O pins are three-stated, and 
               
               
                   
                   
                 act as input data pins. {overscore (OE)} is masked during the beginning of a read cycle when emerging from 
               
               
                   
                   
                 a deselected state. Data is not driven until after /CE is driven high. 
               
               
                 {overscore (ADV)}  
                 Input- 
                 Advance Input signal. Not used during normal functioning of the device. This could be used in 
               
               
                   
                 Synchronous 
                 future revisions of the device. 
               
               
                 {overscore (SWE)}  
                 Input- 
                 Synchronous Write Enable Input. When asserted LOW on the rising edge of CLK. to initiate a 
               
               
                   
                 Asynchronous 
                 write cycle, SWE is asserted at the start if a write cycle only. When {overscore (SCE)} is deasserted HIGH, 
               
               
                   
                   
                 {overscore (SWE)} is ignored. 
               
               
                 ZZ 
                 Input- 
                 Snooze: This active HIGH input puts the device in low power consumption standby mode. For 
               
               
                   
                 Asynchronous 
                 normal operation, this input has to be either LOW or NC (No Connect). 
               
               
                 MODE 
                 Input- 
                 Selects Burst Order. When tied to GND selects linear burst sequence. When tied to V DDQ  or 
               
               
                   
                 Static 
                 left floating selects interleaved burst sequence. This is a strap pin and should remain static 
               
               
                   
                   
                 during device operation. 
               
               
                 NC 
                 No Connect 
                 No Connect. Some of these pins are not connected to the die. Pins B3, L3 are used as strap 
               
               
                   
                   
                 pins to select the type of burst (1, 4, 8 or continous). These pins are read on reset. 
               
               
                 V DD   
                 Power Supply 
                 Power supply inputs to the core of the device. 
               
               
                 V SS   
                 Ground 
                 Ground for the core of the device. Should be connected to ground of the system. 
               
               
                 V DDQ   
                 Power Supply 
                 Power supply inputs to the I/O&#39;s of the device. 
               
               
                 V SSQ   
                 Ground 
                 Ground for the I/O&#39;s of the device. Should be connected to ground of the system. 
               
               
                   
               
             
          
         
       
     
     Referring to FIG. 2, a detailed block diagram of the memory device  100  is shown. The memory device  100  generally comprises a number of circuits  112 ,  114 ,  116 ,  118 ,  120 ,  122 ,  124 , and  126 . Other configurations of the memory device  100  may be implemented to meet the design criteria of a particular application. 
     The circuit  112  may serve as a burst sequence generator. In a preferred embodiment, the circuit  112  may be implemented as a burst counter. The burst counter  112  may receive the signal CLK. The burst counter  112  may receive a signal (e.g., J) from the circuit  114 . The burst counter may present a signal (e.g., K) to the circuit  116 . The signal K may be appended to a signal (e.g., L) presented by the circuit  114  to present a signal (e.g., KL) to the circuit  116 . The signal KL may serve as an address to the circuit  116 . 
     The circuit  114  may serve as an input register and control logic. The input register  114  may receive the signals CLK, ADV, ADD, SWE, SCE, MODE, ZZ, B 3  and L 3 . The input register  114  may present the signal J to the burst counter  112  and the signal L to the circuit  116 . The input register  114  may present a signal (e.g., M) to the circuit  118 , a signal (e.g., N) to the circuit  120 , and another signal (e.g., P) to the circuit  126 . 
     The circuit  116  may serve as a memory array having multiple locations for storing the write data. The memory array  116  may receive the signal KL from the burst counter  112  and the input register  114 . The array may receive the write data buffered by the circuit  118 . The memory array  116  may present the read data to the circuit  120  for buffering. In a preferred embodiment, the memory array  116  may be implemented as a 32-bit wide by at least 256 K array. Other dimensions of the memory array  116  may be implemented to meet the design criteria of a particular application. 
     The circuit  118  may serve as a data input register. The data-in register  118  may receive the signal CLK. The data-in register  118  may receive the write data from the circuit  122  and the signal M from the input register  114 . The data-in register  116  may present the write data to the memory array  116 . 
     The circuit  120  may serve as an output register and logic. The output register  120  may receive the signal CLK. The output register  120  may also receive the signal N from the input register  114  and the read data from the memory array  116 . The output register  120  may present the read data as buffered to the circuit  124 . 
     The circuit  122  may serve as an input buffer amplifier connected to the interface DQ. The input driver  122  may receive the signal DATA. The input buffer amplifier may present the write data from the signal DATA to the data-in register  118 . 
     The circuit  124  may serve as an output buffer amplifier connected to the interface DQ. The output driver  124  may receive the read data from the output register  120  and a signal (e.g., R) from the circuit  126 . The signal R may be asserted to the logical HIGH state and deasserted to the logical LOW state. While the signal R is in the logical HIGH state, the output driver  124  may present the read data at the interface DQ as the signal DATA. While the signal R is in the logical LOW state, the output driver  124  may appear as a high impedance at the interface DQ. 
     The circuit  126  may serve as a logical NOR function. The circuit  126  may receive the signal P from the input register  114  and the signal OE. The circuit  126  may present the signal R to the output driver  124 . The circuit  126  generally presents the signal R in the logical HIGH state when the signals P and OE are both in the logical LOW state. The circuit  126  generally presents the signal R in the logical LOW state for all other combinations of the signals P and OE in the logical HIGH and the logical LOW states. 
     The signal J may serve as a starting point for the burst counter  112 . The signal J is generally address information comprising two or three least-significant bits of the signal ADD depending upon the length of the burst. During a burst sequence, the signal K may start equal to the signal J. At each rising edge of the signal CLK, the burst counter  112  steps the signal K in a linear or interleaved fashion depending upon the signal MODE. The signal L may be additional address information comprising all except the two or three least-significant bits of the signal ADD. The combination of the two or three least-significant bits from the signal K and the other bits from the signal L create a complete address (e.g., the signal KL) for the memory array  116 . 
     The signals M, N and P may serve as controls signals to the data-in register  118 , the output register  120 , and the circuit  126  respectively. The signal R may serve as a control signal to the output driver  124 . The signals M, N, P, and R may be asserted in the logical LOW state and deasserted in the logical HIGH state. 
     All synchronous inputs may pass through the registers  114  and  118  controlled by the rising edge of the signal CLK. All data outputs may pass through the output register  120  controlled by the rising edge of the signal CLK for pipelined operation. Read accesses may be pipelined through the data-in register  118  and the output register  120 . This generally provides a 2-stage pipelined read operations. The signal SCE may be provided to deselect the memory device  100  when not used. The memory device  100  may be in a low power mode when deselected. All write accesses may be controlled with internal self-timed write operations. 
     Referring to FIG. 3, a state transition diagram is shown in accordance with one embodiment of the present invention. The three characters on each state transition line refer to the signals SCE, SWE and ADV from left to right. A truth table for the state transition lines may be seen in TABLE 2. The logical LOW and the logical HIGH states are generally represented in FIG.  3  and TABLE 2 as “0” and “1” respectively. 
     
       
         
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Inputs/ 
                   
               
               
                   
                 SCE 
                 SWE 
                 ADV 
                 Outputs 
                 Mode 
               
               
                   
                   
               
             
             
               
                   
                 0 
                 0 
                 X 
                 High-Z 
                 Single Write 
               
               
                   
                 0 
                 1 
                 X 
                 Data-out 
                 Single Read 
               
               
                   
                 1 
                 X 
                 0 
                 Data-out 
                 Burst Read 
               
               
                   
                 1 
                 X 
                 0 
                 High-Z 
                 Burst Write 
               
               
                   
                 1 
                 X 
                 1 
                 High-Z 
                 Deselected 
               
               
                   
                   
               
             
          
         
       
     
     The memory device  100  may initialize in a power-up state  128 . After initialization, the memory device  100  may be automatically transition to a ready or deselect state  130 . As long as the signal SCE remains in the logical HIGH state, the memory device  100  may remain in the deselect state  130 . The memory device  100  may transition to a single write state  132  when the signal SCE is in the logical LOW state and the signal SWE is in the logical HIGH state. The memory device  100  may transition to a signal write state  134  when the signal SCE is in the logical LOW state and the signal SWE is in the logical LOW state. 
     The memory device  100  generally remains in the single read state  132  as long as the signal SCE remains in the logical LOW state and the signal SWE remains in the logical HIGH state. The memory device  100  may transition to the deselect state  130  when the signal SCE is in the logical HIGH state and the signal ADV is in the logical HIGH state. The memory device may transition to a burst read state  136  when the signal SCE is in the logical HIGH state and the signal ADV is in the logical LOW state. 
     The memory device  100  generally remains in the burst read state  136  as long as the signal SCE remains in the logical HIGH state and the signal ADV remains in the logical LOW state. The memory device  100  may transition to the signal read state  132  when the signal SCE is in the logical LOW state and the signal SWE is in the logical HIGH state. The memory device  100  may transition to the deselect state  130  when the signals SCE and ADV are both in the logical HIGH state. 
     The memory device  100  may remain in the signal write state  134  while the signals SCE and SWE are both in the logical LOW state. The memory device  100  may transition to the deselect state  130  when the signals SCE and ADV are both in the logical LOW state. The memory device  100  may transition to the signal read state  132  when the signal SCE is in the logical LOW state and the signal SWE is in the logical HIGH state. The memory device  100  may transition to a burst write state  138  when the signal SCE is in the logical HIGH state and the signal ADV is in the logical LOW state. 
     The memory device  100  generally remains in the burst at write state  138  as long as the signal SCE remains in the logical HIGH state and the signal ADV remains in the logical LOW state. The memory device  100  may transition back to the signal write state  134  when the signals SCE and SWE are both in the logical LOW&#39;state. The memory device  100  may transition to the signal read state  132  when the signal SCE is in the logical LOW state and the signal SWE is in the logical HIGH state. 
     All read and write accesses to the memory device  100  are generally initiated as single accesses. Asserting the signal ADV may initiate a burst sequence. The burst sequence is generally controlled by the state of the signal MODE. The least significant address bits (e.g., A[ 1 : 0 ] of A[ 2 : 0 ]) represented by signal J may provide a starting location for the burst sequence. Therefore, critical word first operations may be supported since all memory locations in the memory array  116  are available to the user. Incremented sufficiently, the burst counter  112  may wrap the signal K around the four or eight words. 
     A “critical word first” may refer to any word in the memory array  116  that may be accessed at a particular time. Other burst devices include a burst sequence configured such that the data words are presented in a specific order (e.g., 1-2-3-4). The words are presented in the specific order because, for example, of the four address locations for the four data words, only address location  1  is accessible. Therefore, if word  3  is needed, a delay occurs while words  1  and  2  are first accessed before word  3  is accessed. While this type of burst sequence generator may be suitable for use in the present invention, a burst sequence generator that is configured to access any location in the memory array as an initial address (and thus, that supports critical word first operations) is preferred. The above reasoning may also apply to eight-word bursts. 
     Referring to FIG. 4, a timing diagram of single read and burst read cycles are shown in accordance with the state transition diagram shown in FIG.  3 . Read accesses are initiated when the following conditions are satisfied at clock rise (e.g., at time  140 ): (1) the signal SCE is asserted active in the logical LOW state, and (2) the signal SWE is deasserted in the logical HIGH state. The signal ADD representing an address (e.g., A) received at the inputs A[ 17 : 0 ] may be stored into the input register  114  and the signal J may be presented to the burst counter  112 . The memory device  100  is generally a 2-stage pipelined SRAM. Therefore, the read data at the address A (e.g., QA) may be presented at the interface DQ within 12 ns (50 MHz) (e.g., time delay tco) following the next clock rise of the signal CLK (e.g., at time  142 ), if the signal OE is asserted in the logical LOW state. The read data QA may then be held valid at the interface DQ during the next rising clock edge (e.g., at time  144 ) for latching into other circuits external to the memory device  100  such as a memory controller. After the first cycle of the access, the output driver  124  may be controlled by the signal OE. 
     The memory device  100  may increment through the remaining words of a 4-word burst on the rising edges of the clock signal CLK (e.g., at times  142 ,  144  and  146 ) if the signal ADV is asserted in the logical LOW state. The burst sequence (e.g., linear or interleave) is generally determined by the state of the signal MODE. Read accesses may be pipelined such that the read data (e.g., QA+1, QA+2 and QA+3) will be presented by the memory device  100  during every subsequent rising edge of the clock signal CLK (e.g., at times  146 ,  148 , and  150 ). 
     In another example, the signal ADD may be changed (e.g., B, C, and D) during three consecutive clock cycles (e.g., times  148 ,  150  and  152 ). The read cycles for the addresses B and C may be single reads. The read data (e.g., QB and QC) stored at addresses B and C may be presented at the interface DQ at subsequent times (e.g., times  152  and  154 ) respectively. The address D may be a start to another burst read sequence. The read data (e.g., DQ, DQ+1, DQ+2) may be presented during sequential clock cycles (e.g., cycles starting at times  156 ,  158 , and  160 ). 
     Wait states may be added to the read access of the memory device  100 . The wait states may be implemented by maintaining the signal SCE in the logical LOW state after the initiation of a read operation without changing the signal ADD. Doing so may initiate a second single read access to the same address location. General switching characteristics for the memory device  100  at a clock frequency of 50 MHz and 33 MHz are shown in TABLE 3. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 −50 MHz 
                 −33 Mhz 
                   
               
             
          
           
               
                 Parameter 
                 Description 
                 Min. 
                 Max. 
                 Min. 
                 Max. 
                 Unit 
               
               
                   
               
               
                 Clock Times 
                   
                   
                   
                   
                   
                   
               
               
                 t CYC   
                 Clock Cycle Time 
                   
                 20 
                   
                 30 
                 ns 
               
               
                 t CH   
                 Clock HIGH 
                 7 
                   
                 10 
                   
                 ns 
               
               
                 t CL   
                 Clock LOW 
                 7 
                   
                 10 
                   
                 ns 
               
               
                 Access Times 
               
               
                 t CO   
                 Clock to Data Valid 
                   
                 12.0 
                   
                 15.0 
               
               
                 t CO1   
                 Read Initiated to First Data Output Valid 
                   
                 32 
                   
                 45 
                 ns 
               
               
                 t EOV   
                 {overscore (OE)} LOW to Output Valid [6]   
                   
                 12.0 
                   
                 15.0 
                 ns 
               
               
                 Set-up Times 
               
               
                 t AS   
                 Address Set-Up Before CLK Rise 
                 4 
                   
                 5 
                   
                 ns 
               
               
                 t SWES   
                 {overscore (SWE)} Set-Up Before CLK Rise 
                 4 
                   
                 5 
                   
                 ns 
               
               
                 t DS   
                 Data Input Set-Up Before CLK Rise 
                 4 
                   
                 5 
                   
                 ns 
               
               
                 t CES   
                 Chip enable Set-Up 
                 4 
                   
                 5 
                   
                 ns 
               
               
                 Hold Times 
               
               
                 t AH   
                 Address Hold After CLK Rise 
                 1 
                   
                 1 
                   
                 ns 
               
               
                 t SWEH   
                 {overscore (SWE)} Hold After CLK Rise 
                 1 
                   
                 1 
                   
                 ns 
               
               
                 t DH   
                 Data Input Hold After CLK Rise 
                 1 
                   
                 1 
                   
                 ns 
               
               
                 t CEH   
                 Chip enable Hold After CLK Rise 
                 1 
                   
                 1 
                   
                 ns 
               
               
                 Output Times 
               
               
                 t DOH   
                 Data Output Hold After CLK Rise 
                 2 
                   
                 2 
                   
                 ns 
               
               
                 t CHZ   
                 Clock to High-Z [6,7,8]   
                 2 
                   
                 2 
                   
                 ns 
               
               
                 t CLZ   
                 Clock to Low-Z [6,7,8]   
                 1 
                   
                 1 
                   
                 ns 
               
               
                 t EOHZ   
                 {overscore (OE)} HIGH to Output High-Z [6,8]   
                   
                 5 
                   
                 5 
                 ns 
               
               
                 t EOLZ   
                 {overscore (OE)} LOW to Output Low-Z [6,8]   
                 0 
                   
                 0 
                   
                 ns 
               
               
                   
               
               
                 Notes:  
               
               
                   [5] Unless otherwise noted, test conditions assume signal transition time of 5 ns or less, timing reference levels of 0.8 V, input pulse levels of 0 to 1.6 V, and output loading of the specified IOL/IOH and load capacitance. Shown in (a) and (b) of AC test loads.  
               
               
                   [6] tCHZ, tCLZ, tOEV, tEOLZ, and tEOHZ are specified with a load capacitance of 5 pF as in part (b) of AC Test Loads. Transition is measured ±200 mV from steady-state voltage.  
               
               
                   [7] At any given voltage and temperature t CHZ  is less than t CLZ  and t CHZ  less than t CO .  
               
               
                   [8] Tested initially and after any design or process change that may affect these parameters.  
               
             
          
         
       
     
     Referring to FIG. 5, a timing diagram of single write and burst write cycles are shown in accordance with one embodiment of the present invention. Write accesses maybe initiated when the following conditions are satisfied: (1) the signal SCE is asserted active in the logical LOW state, and (2) the signal SWE is asserted active in the logical LOW state. The signal ADD representing the address A and presented to the input A[ 17 : 0 ] is generally loaded into the input register  114  (e.g., at time  162 ). The signal J, representing the signal ADD at the input A[ 1 : 0 ] may also be loaded into the burst counter  112 . The write data (e.g., D(A)) presented at the interface DQ may be latched into the data-in register  118  and then written into the memory array  116 . 
     To continue the single write access as a burst write access, the burst counter  112  may be incremented and on the following three clock rises (e.g., at times  164 ,  166 , and  168 ). The write data (e.g., D(A+1), D(A+2), and D(A+3)) presented to the interface DQ may be written into the memory array  116 . A synchronous self-timed write mechanism between the data-in register  118  and the memory array  116  may been provided to simplify the write operations. 
     Another example of two consecutive single write accesses followed by a burst write access are also shown. The signal ADD may present the addresses B, C, and D during three consecutive clock cycles (e.g., cycles starting at times  170 ,  172  and  174 ). The write accesses to the addresses B and C may cause the write data (e.g., D(B) and D(C)) to be stored in the memory array  116 . At another time (e.g., time  174 ), the signal ADD may present the address D as the first address of another burst write sequence. Consequently, the write data (e.g., D(D) and D(D+1)) may be latched into the data-in register  118  at consecutive rising clock edges (e.g., at times  174  and  176 ). At the next rising clock edge (e.g., at time  178 ) the signal ADV may be in the logical HIGH state transitioning the memory device  100  back to the deselected state  130  after only a two-cycle burst write operation. 
     Because the memory device  100  may be a common I/O device, the signal OE may be deasserted to the logical HIGH state before presenting write data to the interface DQ. Doing so will generally three-state the output driver  124 . As a safety precaution, the output driver  124  may be automatically three-stated whenever a write cycle is detected, regardless of the state of the signal OE. 
     Referring to FIG. 6, a timing diagram of a burst read access immediately followed by a burst write access is shown. The burst read access generally starts with a single read access (e.g., starting at time  180 ) at the address A. The read data QA may be presented at the interface DQ after a delay from the next rising clock (e.g., at time  182 ). During the next three consecutive clock cycles (e.g., cycles starting at times  184 ,  186  and  188 ) the burst read access presents the read data QA+1, QA+2 and QA+3. 
     After the last read data QA+3 is presented at the interface DQ, (e.g., at time  190 ), then the signal OE may be , deasserted in the logical HIGH state to three-state the output driver  124 . The signal ADD may then present the address B to the interface A[ 17 : 0 ] and the associated write data (e.g., D(B)) may be presented at the interface DQ at the next rising clock edge (e.g., at time  192 ). During the next rising clock edge (e.g., at time is  194 ) additional write data (e.g., D(B+1)) may be received at the interface DQ. The signal ADV may be in the logical HIGH state at the next rising clock edge (e.g., at time  196 ) thus causing the memory device  100  to transition to the deselected state  130  after a two-cycle burst write. 
     Referring to FIG. 7, a timing diagram of a burst write access immediately followed by a burst read access is shown. During the first clock edge and each subsequent rising clock edge (e.g., at times  198 ,  200 ,  202 , and  204 ) write data (e.g., D(A), D(A+1) , D(A+2) , and D(A+3)) may be presented at the interface DQ as part of the burst write sequence. On the next rising clock edge (e.g., at time  206 ) the signal ADD may present the address B for a burst read sequence. The read DATA (e.g., QB) may be presented at the interface DQ starting during the nest clock cycle (e.g., starting at times  208 ). The signal SCE may then transition the memory device  100  to the deselect state  130  thus ending the burst read sequence after only one data item has been resented. 
     All read and write access to the memory device  100  are generally initiated as single accesses. Keeping the signal SCE in the logical LOW state after a first read may continue the access into a burst in a preferred embodiment. The memory device  100  generally has an option to select a number of words that can be burst. The signals B 3  and L 3  may be used to determine a burst size. A burst type as a function of the signals B 3 , L 3  and MODE is shown in TABLE 4. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 B3 
                 L3 
                 MODE 
                 Type 
                 Address Used 
               
               
                   
               
             
             
               
                 0 
                 0 
                 X 
                 Single Reads 
                   
               
               
                 0 
                 1 
                 1 
                 Burst of 4 Interleaved 
                 A[1:0] 
               
               
                 1 
                 0 
                 1 
                 Burst of 8 Interleaved 
                 A[2:0] 
               
               
                 0 
                 1 
                 0 
                 Burst of 4 Linear 
                 A[1:0] 
               
               
                 1 
                 0 
                 0 
                 Burst of 8 Linear 
                 A[2:0] 
               
               
                 1 
                 1 
                 X 
                 Continuous Burst 
                 A11 
               
               
                   
               
             
          
         
       
     
     If a four-word burst is selected, the burst sequence is generally controlled by the sate of the signal MODE and a starting address is controlled by the least two significant address bits of the signal ADD. If an eight-word burst is selected, the least three significant bits of the signal ADD are generally used for the starting address. Therefore, critical word first operations may be supported since all locations in the memory array  116  may be available to a user. Incremented sufficiently, the burst counter  112  generally will wrap around. 
     Referring to FIG. 8, an alternative state transition diagram is shown. The two characters on each state transition line refer to the signals SCE and SWE from left to right. The memory device  100  may initialize in a strap register state  214 where signals such as B 3  and L 3  are sampled. The memory device  100  may then be automatically transition to a reset state  216 . Once reset has completed, the memory device  100  may automatically transition to a deselect state  218 . As long as the signal SCE remains in the logical HIGH state, the memory device  100  may remain in the deselect state  218 . 
     The memory device  100  may transition to a single write state  220  when the signal SCE is in the logical LOW state and the signal SWE is in the logical HIGH state. From the single read state  220  the memory device  100  may transition back to the deselect state  218  is the signal SCE is in the logical HIGH state. From the single read state  220 , the memory device  100  may transition to a burst read state  222  if the signal SCE remains in the logical LOW state after one read. The memory device  100  may remain in the burst read state  222  as long as the signal SCE remains in the logical LOW state. The memory device  100  may transition from the burst read state  222  to the deselect state  218  if the signal SCE returns to the logical HIGH state. 
     The memory device  100  may transition to a single write state  224  when the signal SCE is in the logical LOW state and the signal SWE is in the logical LOW state. From the single write state  224  the memory device  100  may transition back to the deselect state  218  is the signal SCE is in the logical HIGH state. From the single write state  224 , the memory device  100  may transition to a burst write state  226  if the signal SCE remains in the logical LOW state after one write. The memory device  100  may remain in the burst write state  226  as long as the signal SCE remains in the logical LOW state. The memory device  100  may transition from the burst write state. 226  to the deselect state  218  if the signal SCE returns to the logical HIGH state. 
     Referring to FIG. 9, a timing diagram of single read and burst read cycles are shown in accordance with the state transition diagram shown in FIG.  8 . Read accesses are initiated when the following conditions are satisfied at clock rise (e.g., at time  228 ): (1) the signal SCE is asserted active in the logical LOW state, and (2) the signal SWE is deasserted in the logical HIGH state. The signal ADD representing an address (e.g., A) received at the inputs A[ 17 : 0 ] may be stored into the input register  114  and the signal J may be presented to the burst counter  112 . The width of the signal J. The read data at the address A (e.g., QA) may be presented at the interface DQ within the time delay tco following the next clock rise of the signal CLK (e.g., at time  230 ), if the signal OE is asserted in the logical LOW state. The read data QA may then be held valid at the interface DQ during the next rising clock edge (e.g., at time  232 ) for latching into other circuits external to the memory device  100 . After the first cycle of the access, the output driver  124  may be controlled by the signal OE. 
     The memory device  100  may be incremented through a 4-word burst, an 8-word burst, or a continuous burst if the signal SCE remains asserted in the logical LOW state. The example shown in as FIG. 9 illustrates a 4-word burst. Here, the signal SCE is in the logical LOW state on the next three rising edges of the clock signal CLK (e.g., at times  230 ,  232  and  234 ). The burst sequence (e.g., linear or interleave) is generally determined by the state of the signal MODE. Read accesses may be pipelined such that the read data (e.g., QA+1, QA+2 and QA+3) will flow out of the memory device  100  following every rising edge of the clock signal CLK (e.g., at times  234 ,  236 , and  238 ). 
     In another example, the signal ADD may be changed (e.g., B) one clock cycle (e.g., time  238 ) following entering the deselect state  218  at the time  236 . The read cycle for the address B may start as another single read. The read data (e.g., QB) stored at addresses B may be presented at the interface DQ following a next clock edge at a later time (e.g., time  240 ). The data QB may be presented at the interface DQ through the next rising clock edge (e.g., at a time  242 ). If the signal SCE is deasserted to the logical HIGH state at the time  240 , then the read cycle may end as a single read. 
     In yet another example, the signal ADD may be changed (e.g., C) for another rising clock edge (e.g., at time  244 ). As before, the read cycle for the address C may start as a single read. The read data (e.g., QC) stored at the address C may be presented at the interface DW following the a next clock edge (e.g., at a time  246 ) and then held through a subsequent clock edge (e.g., at a time  248 ). However, the signal SCE may be in the logical LOW state at the times  246  and  248  resulting in another burst read. 
     Referring to FIG. 10, a timing diagram of single write and burst write cycles are shown in accordance with a preferred embodiment of the present invention. Write accesses may be initiated when the following conditions are satisfied: (1) the signal SCE is asserted active in the logical LOW state, and (2) the signal SWE is asserted active in the logical LOW state. The signal ADD representing the address A and presented to the input A[ 17 : 0 ] is generally loaded into the input register  114  (e.g., at time  250 ). The signal J, representing the signal ADD at the input A[ 1 : 0 ] may also be loaded into the burst counter  112 . The write data (e.g., D(A)) presented at the interface DQ may be latched into the data-in register  118  and then written into the memory array  116 . 
     The signal SCE may remain in the logical LOW state to continue the single write access as a burst write access. In particular, the burst counter  112  may be incremented and on the following three clock rises (e.g., at times  252 ,  254 , and  256 ). The write data (e.g., D(A+1), D(A+2), and D(A+3)) presented to the interface DQ during the times  252 ,  254 , and  256  may be written into the memory array  116 . A synchronous self-timed write mechanism l between the data-in register  118  and the memory array  116  may been provided to simplify the write operations. On a subsequent clock rise (e.g., at a time  258 ), the signal SCE may be in the logical HIGH state to end the burst by transitioning the memory device  100  to the deselect state  218 . 
     Another example of a single write accesses is also shown. The signal ADD may present the address B one rising clock edge (e.g., at a time  260 ) after the memory device  100  enters the deselect state  218 . The write access to the address B may cause the write data (e.g., D(B)) present at the interface DQ at the time  260  to be stored in the memory array  116 . The signal SCE may return to the logical HIGH state for a next clock rise (e.g., at a time  262 ) to end the single write. Another single write cycle may be initiated on a next rising clock (e.g., at a time  264 ). At the time  264  the signal ADD may present the address D and the write data (e.g., D(D)) may be presented at the interface DQ. The single write cycle for address D may be ended on a next clock rise (e.g., at a time  266 ) if the signal SCE is in the logical HIGH state. 
     Referring to FIG. 11, a timing diagram of a burst read access immediately followed by a burst write access is shown. The burst read access generally starts with a single read access (e.g., starting at time  268 ) at the address A. During the next five consecutive clock cycles (e.g., cycles starting at times  270 ,  272 ,  274 ,  276 , and  278 ) the signal SCE may be asserted in the logical LOW state to signal a burst read. The read data QA may be presented at the interface DQ after a delay from the rising clock at time  270 . During the next four consecutive clock cycles starting at the times  272 ,  274 ,  276 , and  278 , the burst read access presents the read data QA+1, QA+2, QA+3, and QA respectively. Since the total length of the burst sequence is six clock cycles (one single read followed by five burst reads) and the burst length programmed at four, the burst counter  112  may wrap around causing the data QA to be presented as the fifth data item. The data QA+1 may not be presented a second time because the memory device  100  may transition to the deselect state  218  at the time  280 . 
     The signal SCE may be deasserted to the logical HIGH state after the time  280  to transition the memory device  100  to the deselect state  218 . After the read data QA is presented at the interface DQ for the second time past the rising clock edge at the time  280 , then the signal OE may be deasserted in the logical HIGH state to three-state the output driver  124 . The signal ADD may then present the address B to the interface A[ 17 : 0 ] and the associated write data (e.g., D(B)) may be presented at the interface DQ at the next rising clock edge (e.g., at time  282 ). The signals SCE and SWE may be asserted in theological LOW state at the time  282  to transition the memory device  100  to the single write state  224 . During the next rising clock edge (e.g., at time  284 ) the signal SCE may be deasserted to the logical HIGH state to end the single write. 
     Referring to FIG. 12, a timing diagram of a burst write access immediately followed by a burst read access is shown. During the first clock edge and each subsequent rising clock edge (e.g., at times  286 ,  288 ,  290 , and  292 ) write data (e.g., D(A), D(A+1), D(A+2), and D(A+3)) may be presented at the interface DQ as part of the burst write sequence. On the next rising clock edge (e.g., at time  294 ) the signal SCE may be deasserted to the logical LOW state to transition the memory device  100  to the deselect state  218 . On the next rising clock edge (e.g., at time  296 ), the signal ADD may present the address B for a burst read sequence. The read DATA (e.g., QB) may be presented at the interface DQ starting during the next clock cycle (e.g., starting at times  298 ) and held through the subsequent rising clock (e.g., at time  300 ). The signal SCE may be deasserted in the logical HIGH state during the rising clock edge at the time  300 . Here, the memory device  100  is generally transitioned to the deselect state  218  thus ending the burst read sequence after only one data item has been presented. 
     Power consumed by the memory device  100  is generally dependent upon a clock frequency of the signal CLK and a condition of the memory device  100 . While the memory device  100  is in a normal operating condition, the power consumed may by clock frequency dependent. The memory device  100  in the operating condition may consume approximately 20 miliamps (mA) of current at a clock frequency of approximately 33 MHz. The power consumption of the memory device  100  may increase to approximately 25 mA at a clock frequency of approximately 50 MHz. The power consumption may increase into a range of approximately 35 to 50 mA for other operating conditions. The memory device  100  in a standby condition may consume approximately 20 microamps (μA) at the clock frequency of approximately 33 MHz and approximately 25 μA at the clock frequency of approximately 50 MHz. Other power consumption levels may be obtainable at other clock frequencies and operating conditions using a variety of semiconductor technologies and design techniques. 
     The various signals of the present invention are generally “on” (e.g., a digital HIGH, or 1) or “off” (e.g., a digital LOW, or 0). However, the particular polarities of the on (e.g., asserted) and off (e.g., deasserted) states of the signals may be adjusted (e.g., reversed) accordingly to meet the design criteria of a particular implementation. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.