Patent Publication Number: US-2023143405-A1

Title: Pseudo-static random access memory

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority of Japan Patent Application No. 2021-181689, filed on Nov. 8, 2021, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The disclosure is generally related to a pseudo-Static Random Access Memory (pSRAM). 
     Description of the Related Art 
     pSRAM is a semiconductor memory device that includes an interface compatible with a static random access memory (SRAM) (e.g., Patent Document 1: Japanese Patent No. JP2020-135914B2). 
       FIG.  1 ( a )  is a timing chart illustrating an example of the timing of a signal in a conventional pSRAM when a read command is input.  FIG.  1 ( b )  is a time chart showing the timing of signals in a conventional pSRAM when a write command is input. The pSRAM is illustrated herein as a clock-synchronized pSRAM that receives signals in synchronization with the clock signal, and also illustrated as an example that a pSRAM of the address data multiplexing interface type. The pSRAM of the address data multiplex interface type has address data terminals configured to input each address signal and the corresponding data signal. The pSRAM is configured to start a read or write operation when the chip select signal CS# transitions from inactive (high level) to active (low level), and to complete the read or write operation to precharge when the chip select signal CS# transitions from active (low level) to inactive (high level). 
     The chip selection signal CS# is synchronized with each of the first to third clocks of the external clock signal CLK transitioning from inactive (high level) to active (low level), via the address data terminal Enter the command (CMD), column address (RA), and row address (CA). Specifically, the command (CMD) is input at the rising edge of the first clock of the external clock signal CLK, the row address (RA) is input at the rising and falling edges of the second clock of the external clock signal CLK, and are the column address (CA) is input at the rising edge and falling edge of the third clock of the external clock signal CLK. In addition, the case where an 8-bit signal is input at each clock edge is shown as an example. 
     In  FIG.  1 ( a ) , the word line (WL) becomes active (high level) after the row address is input, and the column select line (CSL) is transitioned after the column address (CA) is input. Next, after an initial delay from the input command (CMD) and the row address (RA), the data in the memory cells corresponding to the input row address (RA) and column address (CA) are read and output as data. In addition, the word line (WL) is precharged when the chip select signal CS# transitions from active (low level) to inactive (high level). The initial delay is expressed as tCK (tCK display clock cycle)×delay number N (N is an integer greater than or equal to 1, which is the display delay number), and the case where the delay number is 8 (N=8) is shown as an example. In addition, the number of delays depends on the frequency of the external clock signal CLK, and the higher the frequency of the external clock signal CLK, the greater the delay number. 
     In  FIG.  1 ( b ) , the word line (WL) becomes active (high level) after the row address is input. Next, when the write data is input after an initial delay from the input command (CMD) and the row address (RA), the column select line (CSL) is transitioned. The length of the initial delay in the write operation is equal to the length of the initial delay in the read operation. In addition, the word line (WL) is precharged when the chip select signal CS# transitions from active (low level) to inactive (high level). 
     Since it is impossible to input or output reading or writing data and the time of each operation cannot be shortened until the initial delay in each operation has passed, there is a difficulty that the data transmission speed cannot be improved. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a pseudo-static random access memory. In the first operation, after an initial delay from a command and a row address being input, data in the memory cells corresponding to the inputted row address and a column address are input or output. The pseudo-static random access memory includes a control unit controlling the delay in the second operation less than the initial delay when a specific condition is satisfied. The second operation is executed after the first operation. 
     Based on the pseudo-static random access memory of the present invention, the speed of data transmission may be improved. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG.  1 ( a )  is a timing chart illustrating an example of the timing of a signal in a conventional pSRAM when a read command is input.  FIG.  1 ( b )  is a time chart illustrating the timing of signals in a conventional pSRAM when a write command is input. 
         FIG.  2    is a block diagram illustrating a configuration example of a pSRAM in accordance with a first embodiment of the present invention. 
         FIG.  3    is a schematic diagram illustrating an example of the relationship between the initial delay and short latency for each different clock cycle. 
         FIG.  4    is a schematic diagram illustrating an example of bit allocation of a display instruction. 
         FIG.  5 ( a )  is a schematic diagram illustrating a configuration example of a precharge signal generation unit, and  FIG.  5 ( b )  is a schematic diagram illustrating a configuration example of a delay signal generation unit. 
         FIG.  6 ( a )  is a timing chart illustrating an example of the timing of the signals in the pSRAM in the state where the initial delay is set when a read command is input, and  FIG.  6 ( b )  is a time chart illustrating an example of the timing of the signals in the pSRAM in the state where the short delay is set when a read command is input. 
         FIG.  7    is a timing chart illustrating an example of the timing of a signal when a register write command is input to the pSRAM in accordance with a second embodiment of the present invention. 
         FIG.  8    is a timing chart illustrating an example of the timing of the signals in the pSRAM in accordance with the second embodiment of the present invention. 
         FIG.  9    is a block diagram illustrating a configuration example of a pSRAM in accordance with a third embodiment of the present invention. 
         FIG.  10    is a timing chart illustrating an example of the timing of the signals in the pSRAM in accordance with a third embodiment of the present invention. 
         FIG.  11 ( a )  is a schematic diagram illustrating an example of the bit allocation of the command, and  FIG.  11 ( b )  is a time chart illustrating an example of the timing of the signal when the command is input. 
         FIG.  12    is a block diagram illustrating a configuration example of a pSRAM in accordance with a fourth embodiment of the present invention. 
         FIG.  13 ( a )  is a timing chart illustrating an example of the timing of the signals in the pSRAM when no error is detected in the write command, and  FIG.  13 ( b )  is a timing chart illustrating the pSRAM when an error is detected in the write command. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. The scope of the invention is best determined by reference to the appended claims. 
     It would be understood that, in the description herein and throughout the claims that follow, although the terms “first,” “second,” etc. may be used to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the embodiments. 
     It is understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the application. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Moreover, the formation of a feature on, connected to, and/or coupled to another feature in the present disclosure that follows may include embodiments in which the features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the features, such that the features may not be in direct contact. 
     First Embodiment 
       FIG.  2    is a block diagram illustrating a configuration example of a pSRAM in accordance with a first embodiment of the present invention. The pSRAM of this embodiment is operated in the first operation. After an initial delay from the input of the command and the row address, data is input or output to/from the memory cell corresponding to the inputted row address and column address. The pSRAM of this embodiment includes a control unit  10  and a memory cell array  20 . The control unit  10  and the memory cell array  20  may each be constituted by a dedicated hardware device or logic circuit. The memory cell array  20  includes a plurality of memory cells arranged in rows and columns. 
     When a specific condition is satisfied, the control unit  10  controls the delay in the second operation posterior to the first operation to be less than the aforementioned initial delay. In this embodiment, the specific condition includes inputting a specific command in the first operation, and the specific command is configured to make the delay in the second operation less than the initial delay. In this embodiment, the specific command includes a command for instructing not to precharge the word line corresponding to the input row address. In this embodiment, the specific command includes a read command and a write command. 
     The pSRAM in this embodiment is a pseudo-static random access memory that the input signal or the output signal is synchronized with a clock signal (e.g., an external clock signal CLK). Since the command signal, the address signal, and the data signal are input or output synchronously with the clock signal (external clock signal CLK), the length of the delay from the input of the command signal and the address signal to the input or the output of the data signal may be set according to the number of clock cycles. 
     The pSRAM of the embodiment is a pseudo-static random access memory of the address data multiplex interface type, which has an address data terminal configured to input each address signal and a data signal. Since each address signal and data signal are not input through an independent terminal but through a common terminal (the address data terminal), the commands, addresses, and data for other operations are not input during operation. The delay control may be suppressed according to operations other than the first operation and/or the second operation that are input. 
     The control unit  10  includes an instruction decoder  101 , a row control unit  102 , a delay counter  103 , a column control unit  104 , a precharge signal generation unit  105 , a precharge control unit  106 , and a delay signal generation unit  107 . 
     When the chip selection signal CS# input externally changes from inactive (high level) to active (low level), the command decoder  101  is synchronized with the external clock signal CLK to decode the command input through the address data terminal CMD (as shown in  FIG.  6 ( a ) ) so as to generate internal commands. The generated internal commands include, for example, the activation signal ACT, the read signal RD, the write signal WR, and so on. Although it is not shown in  FIG.  2   , the command decoder  101  generates update signals and the like as the internal commands. 
     The row control unit  102  controls the activation/deactivation of the corresponding memory cells in the memory cell array  20  according to the activation signal ACT input from the command decoder  101 . For example, when the active (high-level) activation signal ACT is input from the command decoder  101 , the row control unit  102  utilizes the row address RA (as shown in  FIG.  6 ( a ) ) input through the address data terminal to activate the signal WL corresponding word line to be active (high level), and outputs the signal WL to the memory cell array  20 . In addition, when the activation signal ACT, which is active (high level), is input from the command decoder  101 , the row control unit  102  activates the sense amplifier signal SA, and outputs the sense amplifier signal SA to the memory cell array  20 . 
     When the precharge signal PRE, which is active (high level), is input from the precharge control unit  106 , the row control unit  102  deactivates the signal WL (low level) and outputs the signal WL to the memory cell array  20  after the operation (the chip select signal CS# is changed from active (low level) to inactive (high level)). Therefore, the activated word line is deactivated after the operation is completed, and the precharge operation of the word line is performed. On the other hand, when the inactive (low level) precharge signal PRE is input from the precharge control unit  106 , the row control unit  102  also outputs the signal WL, which is active (high level), to the word line of the memory cell array  20  after the operation is completed. The active state of the word line is also maintained without precharging the word line. 
     The delay counter  103  counts the number of pulses (the number of toggles) of the external clock signal CLK input externally when the read signal RD or the write signal WR is input from the command decoder  101 . Then, when the number of pulses counted by the delay counter  103  reaches a specific delay value, the signal CAS input through the address data terminal is output to the column control unit  104 , and the signal CAS is used to designate the input column address CA (shown in  FIG.  6 ( a ) ). 
     The delay counter  103  is configured to set the counted delay value according to the signal NLTNCY input from the delay signal generation unit  107 . For example, when the signal NLTNCY is active (high level), the delay counter  103  determines that the delay is an initial delay, and sets the counted delay value as a value corresponding to the initial delay. In addition, when the signal NLTNCY is inactive (low level), the delay counter  103  determines that the delay is a delay less than the initial delay, and sets the counted delay value to a value corresponding to the short delay (a value smaller than the value corresponding to the initial delay). 
     An example of the relationship between the value corresponding to the initial delay and the value corresponding to the short delay is shown in  FIG.  3   , and the value corresponding to the initial delay and the value corresponding to the short delay are set at each clock cycle (tCK). In addition, in each clock cycle, the value of the initial delay (the number of clock cycles of the external clock signal CLK) is set to be a value larger than the value of the short delay (the number of the clock cycles of the external clock signal CLK). Furthermore, the value corresponding to the initial delay and the value corresponding to the short delay are set to be larger as the clock period is less (the frequency of the external clock signal CLK is higher). In addition, information showing such a relationship may be stored in a mode register or the like as the information of the operation mode of the pSRAM (e.g., burst length, etc.), and referenced by the delay counter  103 . 
     In addition, when the delay counter  103  has an initial delay (when the signal NLTNCY is active (high level)), the external clock starts counting the number of pulses of the clock from that the command CMD and the row address RA are input (the third clock in the example shown in  FIG.  6 ( a ) ). Furthermore, when the delay counter  103  has a short delay (when the signal NLTNCY is inactive (low level)), the external clock starts counting the number of pulses of the clock from that the command CMD and the column address CA are input (the fourth clock in the example shown in  FIG.  6 ( a ) ). 
     When the signal CAS is input from the delay counter  103 , the column control unit  104  activates the signal of the column select line corresponding to the column address CA designated by the signal CAS to be active (make it active) and outputs the signal of the column select line to the memory cell array  20 . 
     The precharge signal generation unit  105 , according to the command CMD input through the address data terminal, generates a precharge enable signal PREEN indicating whether to precharge the word line corresponding to the row address RA input through the address data terminal, and outputs the precharge enable signal PREEN to the precharge control unit  106 . Specifically, when the precharge signal generating section  105  is at the first external clock signal CLK in the operation (shown as 1stCLK in  FIG.  6 ( a ) ) and the value of ADQ 5  bits of the 8-bit signals ADQ[ 7 : 0 ] is inputted at the rising edge of the first external clock signal CLK, a precharge enable signal PREEN is generated and provided to the precharge control unit  106 . 
       FIG.  4    is a schematic diagram illustrating an example of the bit allocation of the command CMD. The command CMD consists of 3 bits of ADQ 7  bits, ADQ 6  bits, and ADQ 5  bits among the 8-bit signals ADQ[ 7 : 0 ] inputted at the rising edge of the first external clock signal CLK during operation. The ADQ 7  bit is a bit used to specify the command type (read command or write command). When the value of ADQ 7  is 1, the command CMD is a read command. When the value of ADQ 7  is 0, the command CMD is a write command. ADQ 6  is configured to specify the processing object (a memory cell or a register) of the command. When the value of ADQ 6  is 1, the command is performed on a register. When the value of ADQ 6  is 0, the command is performed on a memory cell. ADQ 5  is configured to indicate whether to precharge the word line corresponding to the row address RA input through the address data terminal. When the value of ADQS is 1, the command CMD instructs not to precharge the word line, and when the value of ADQS is 0, the command CMD instructs to precharge the word line. 
       FIG.  5 ( a )  illustrates the configuration of the precharge signal generation unit  105 . The precharge signal generator  105  includes a shift register  105   a  and an inverter  105   b . The shift register  105   a  latches ADQS of the  8 -bit signal ADQ[ 7 : 0 ] when the first external clock signal CLK(1stCLK) is input as a clock signal during operation. The inverter  105   b  logically inverts the signal output from the shift register  105   a , and outputs the logically inverted signal to the precharge control unit  106  and the delay signal generation unit  107  as the precharge enable signal PREEN. In addition, a reset signal enable generated by, for example, another circuit in the control unit  10  is applied to the /CLR terminal of the shift register  105   a.    
     The precharge control unit  106  controls the precharge of the word line corresponding to the input row address when the precharge enable signal PREEN indicates that precharge is being performed (when the precharge enable signal PREEN is active (high level)). In this embodiment, when the chip selection signal CS# is active (low level), the precharge control unit  106  generates the precharge signal PRE according to the precharge enable signal PREEN input from the precharge signal generation unit  105 , and provides the generated precharge signal PRE to the row control unit  102 . For example, when the precharge enable signal PREEN is active (high level) (when precharging is performed), the precharge control unit  106  makes the precharge signal PRE active (high level), and provides the precharge signal PRE to the row control unit  102 . In addition, when the precharge enable signal PREEN is inactive (low level) (when precharging is not performed), the precharge control unit  106  deactivates the precharge signal PRE (low level), and outputs the precharge signal PRE to the row control unit  102 . 
     The delay signal generation unit  107  controls the delay in the second operation less than the initial delay when the precharge enable signal PREEN indicates that precharging is not performed (when the precharge enable signal PREEN is inactive (low level)). set to short delay). In this embodiment, the delay signal generation unit  107  generates the signal NLTNCY based on the precharge enable signal PREEN input from the precharge signal generation unit  105 , and outputs the generated signal NLTNCY to the delay counter  103 . In addition, in the present embodiment, the delay signal generation unit  107  is an example of the “delay control unit” of the present invention. 
       FIG.  5 ( b )  illustrates the configuration of the delay signal generation unit  107 . The delay signal generation unit  107  includes a shift register  107   a  and inverters  107   b ,  107   c  and  107   d.  When the chip select signal CS# logically inverted by the inverter  107   c  is input as the clock signal, the shift register  107   a  outputs the precharge enable signal PREEN which is logically inverted by the inverter  107   b . The inverter  107   d  logically inverts the signal output from the shift register  107   a , and outputs the logically inverted signal to the delay counter  103  as the signal NLTNCY. In addition, a reset signal enable generated by, for example, another circuit in the control unit  10  is applied to the /CLR terminal of the shift register  107   a.    
     The control unit  10  configured as described above is arranged in a state in which the word line accessed in the previous operation (first operation) is activated (in a state in which the precharge enable signal PREEN is inactive (low level)). Next, when the word line is accessed in the following operation (second operation), the row address RA in the address input in the following operation is deactivated. By only activating the column address CA, the delay in the following operation is less than the initial delay. 
       FIG.  6 ( a )  is a timing chart illustrating an example of the timing of the signals in the pSRAM in the state where the initial delay is set when a read command is input, and FIG.  6 ( b ) is a timing chart of an example of the timing of the signals in the pSRAM in the state where the short delay is set when a read command is input. 
     First, the operation shown in  FIG.  6 ( a )  will be described. It is assumed that the precharge enable signal PREEN is active (high level) (precharge is performed) and the signal NLTNCY is active (high level) (delay in the read operation is the initial delay) before the read operation (the first operation) starts. 
     By activating the chip selection signal CS# (low level), after the read operation starts, when the command CMD is input in synchronization with the external clock signal CLK, the precharge signal generating unit  105  generates a precharge enable signal PREEN according to the command CMD. In addition, it is assumed that the command CMD is a read command without precharge (the value of ADQ 5  is 1), the precharge signal generation unit  105  deactivates the precharge enable signal PREEN (low level) and provides the precharge enable signal PREEN to the precharge control unit  106  and the delay signal generation unit  107 . 
     On the other hand, the shift register  107   a  of the delay signal generation unit  107  is in a state where the signal input to the clock terminal (the logic inversion signal of the chip select signal CS#) is already active (high level). The logic inversion signal of the precharge enable signal PREEN output from the precharge signal generation unit  105  is not latched. Therefore, the delay signal generation unit  107  outputs the active (high level) signal NLTNCY to the delay counter  103 . 
     When the active (high level) signal NLTNCY is input, the delay counter  103  determines that the delay in the read operation is an initial delay, and counts the number of pulses of the external clock signal CLK until it reaches a value corresponding to the initial delay. In addition, the row control unit  102  activates the word line corresponding to the input row address RA during the initial delay period. 
     Next, when the number of pulses of the external clock signal CLK reaches the value corresponding to the initial delay, the delay counter  103  outputs the signal CAS to the column control unit  104 , and the data in the memory cells corresponding to the input row address RA and the input column address CA may be read. In addition, the read operation is ended by deactivating the chip select signal CS# (high level). Moreover, in this read operation, since a read command without precharge (the value of ADQ 5  is 1) is input, the precharge of the word line corresponding to the input row address RA is not performed, so that the word line is maintained in the activated state. 
     Next, the operation shown in  FIG.  6 ( b )  will be described. It is assumed that the read operation (second operation) shown in  FIG.  6 ( b )  is performed after the read operation shown in  FIG.  6 ( a ) . Before the read operation starts, the precharge enable signal PREEN is inactive (low level), and the signal NLTNCY is active (high level). In addition, since the input of the row address RA becomes inactive in the subsequent read operation, the row address RA is omitted from the inputted row addresses. 
     By activating the chip selection signal CS# (low level), after the read operation starts, the precharge signal generation unit  105  generates a precharge enable signal PREEN according to the command CMD when the command CMD is input in synchronization with the external clock signal CLK. It is assumed that the command CMD is a read command with precharge (the value of ADQ 5  is 0). The precharge signal generation unit  105  enables the precharge enable signal PREEN and outputs the precharge enable signal PREEN to the precharge control unit  106  and the delay signal generation unit  107 . 
     On the other hand, the shift register  107   a  of the delay signal generation unit  107  latches the logic inversion signal of the precharge enable signal PREEN when the chip selection signal CS# is active (low level). In addition, when the chip selection signal CS# is active (low level), the precharge enable signal PREEN is at the low level, the logic inversion signal of the precharge enable signal PREEN is at the high level, and the delay signal generation unit  107  outputs the inactive signal NLTNCY(low level) to the delay counter  103 . 
     When the inactive (low level) signal NLTNCY is input, the delay counter  103  determines that the delay in the read operation is a short delay, and counts the number of pulses of the external clock signal CLK until the value corresponding to the short delay is reached. 
     Next, when the number of pulses of the external clock signal CLK reaches the value corresponding to the short delay, the delay counter  103  outputs the signal CAS to the column control unit  104 , and the data in the memory cell of the input column address CA corresponding to the read operation (second operation) may be read. In addition, the read operation is ended by deactivating the chip select signal CS# (high level). In addition, in the following read operation (the second operation), since the read command with precharge (the value of ADQ 5  is 0) is input, the word line corresponding to the row address RA input in the first operation is precharged. 
     Therefore, when a specific command (a read command or a write command without precharge) is input in the previous read operation (the first operation), the following read operation (the second operation) may be delayed less than the initial delay. Therefore, since the time of the subsequent read operation may be shortened, the data transfer speed in the subsequent read operation can be improved. 
     Second Embodiment 
     In the pSRAM of this embodiment, when a specific command is input in the first operation, the delay in the second operation is less than the initial delay, which is the same as in the first implementation. However, the specific command includes a register write command, which is different from the first embodiment. 
     As shown in  FIG.  7   , the register write command includes a command, an address, and data written to a mode register (MR). The MR data includes more than one bit information indicating whether to precharge. In addition, the register write command is as described in the first embodiment that ADQ 6  of the 8-bit signal ADQ[ 7 : 0 ] is set to 1 at the first rising edge of the first external clock signal CLK during operation. 
     The precharge signal generation unit  105  of the control unit  10  obtains the information about the precharge for writing the mode register when the register write command is input. Next, the precharge signal generation unit  105  activates the precharge enable signal PREEN (high level) when the information indicating to precharge is written to the mode register, and outputs the precharge enable signal PREEN to the precharge control unit  106  and the delay signal generation unit  107 . In addition, when the precharge signal generation unit  105  writes the information indicating not to precharge to the mode register, the precharge signal generation unit  105  deactivates the precharge enable signal PREEN (low level) and outputs the precharge enable signal PREEN to the precharge control unit  106  and the delay signal generation unit  107 . 
     In the case where the register write command is written in the first operation, the delay signal generation unit  107  of the control unit  10  latches the precharge enable signal PREEN and outputs the latched signal to the delay counter  103  as the signal NLTNCY when the second operation (a read or write operation) following the first operation is completed (the rising edge of the chip select signal CS#). 
       FIG.  8    is a timing chart showing an example of the timing of the signals in the pSRAM of the present embodiment. In addition, as in the first embodiment, it is assumed that the same column address is accessed in each operation. 
     In the sequence before the timing t 1 , the precharge enable signal PREEN is active (high level) (precharge is performed), and the signal NLTNCY is active (high level) (the delay is the initial delay in the read operation) will be described as an example. 
     When a register write command without precharge is input at the timing t 1 , the precharge signal generation unit  105  deactivates the precharge enable signal PREEN (low level) and outputs the precharge enable signal PREEN to the precharge control unit  106  and the delay signal generation unit  107  after referring to the information about precharge written in the mode register. In addition, since it is not the timing of latching the precharge enable signal PREEN, the delay signal generating unit  107  outputs the active signal NLTNCY (high level) to the delay counter  103 . 
     When the read operation or the write operation starts at the timing t 2 , the delay counter  103  sets the delay in the operation as the initial delay according to the active signal NLTNCY (high level). Then, the delay signal generation unit  107  latches the inactive (low level) precharge enable signal PREEN by deactivating the chip selection signal CS# (high level) and outputs the inactive signal NLTNCY(low level) to the delay counter  103  when the read or write operation completes. 
     When the read operation or the write operation starts at the timing t 3 , the delay counter  103  sets the delay in operation to be a short delay according to the inactive signal NLTNCY (low level). Therefore, the period for the read or write operation becomes less than the previous read or write operation. 
     When a register write command with precharge is input at the timing t 4 , the precharge signal generation unit  105  makes the precharge enable signal PREEN active (high level) and outputs the precharge enable signal PREEN to the precharge control unit  106  and the delay signal generation unit  107  after referring to the information about precharge of the mode register. In addition, since it is not the timing of latching the precharge enable signal PREEN, the delay signal generating unit  107  outputs the inactive signal NLTNCY (low level) to the delay counter  103 . 
     When the read or write operation begins at the timing t 5 , the delay counter  103  sets the delay in operation to be a short delay according to the inactive signal NLTNCY (low level). Then, when the read or write operation is completed by deactivating the chip selection signal CS# (high level), the delay signal generation unit  107  latches the active precharge enable signal PREEN and outputs the active signal NLTNCY (high level) to the delay counter  103 . 
     When the read or write operation starts at the timing t 6 , the delay counter  103  sets the delay in operation as the initial delay according to the active signal NLTNCY (high level). 
     Therefore, according to the register write command input in a specific operation (first operation), the delay in the subsequent operation (second operation) may be set as an initial delay or a short delay. 
     Third Embodiment 
     In the present embodiment, when the control unit  10  satisfies the specific condition, the delay in the second operation is set to be less than the initial delay, which is the same as in the above-described embodiments. However, the specific condition includes inputting the same row address RA in the first operation and the second operation, which is different from the above-mentioned embodiments. 
     The control unit  10  is configured to control the delay in each successive operation posterior to the second operation to be less than the initial delay during the period in which the same row address RA is input in each successive operation when a specific condition is satisfied. 
       FIG.  9    is a block diagram showing a configuration example of a pSRAM according to this embodiment. The control unit  10  includes: an instruction decoder  101 ; a row control unit  102 ; a delay counter  103 ; a column control unit  104 ; a row address [n] latch unit  108 , latching the row address RA input in the n-th (n is greater than 1) operation; a column address [n−1] latch unit  109  latching the row address RA input in the (n−1)-th operation; a comparator  110 , and an inverter  111 . The configuration of the command decoder  101 , the row control unit  102 , the delay counter  103 , and the column control unit  104  is the same as that in the first embodiment. 
     The row address [n] latch section  108  latches the row address RA input in the operation during every operation. Next, the row address [n] latch unit  108  outputs a signal RA[n] indicating the value of the latched row address RA to the row address [n−1] latch unit  109 , the comparator  110 , and the memory cell array  20 . 
     The row address [n−1] latch unit  109  latches the input signal RA[n] and the signal RA[n−1] input as the signal RA[n] in the previous operation every time when the row address [n−1] latch unit  109  receives the input signal RA[n] from the column address [n] latch unit  108 . Next, the row address [n−1] latch unit  109  outputs the signal RA[n−1] to the comparator  110 . 
     The comparator  110  compares the value of the signal RA[n] input from the column address [n] latch  108  with the value of the signal RA[n−1] input from the column address [n−1] latch  109 . Then, when the two values are equal, the comparator  110  makes the signal RACMP indicating the comparison result active (high level) and outputs the signal RACMP to the inverter  111 , so that the precharge signal PRE is inactive (low level) and output to the row control unit  102 . On the other hand, when the two values are different, the comparator  110  deactivates the signal RACMP (low level) indicating the comparison result and outputs the signal RACMP to the inverter  111 , so that the precharge signal PRE is active (high level) and output to the row control unit  102 . 
     The inverter  111  logically inverts the signal RACMP output from the comparator  110 , and outputs the logically inverted signal to the delay counter  103  as the signal NLTNCY. That is, the inverter  111  controls the delay in the n-th operation less than the initial delay (set as a short delay) by outputting the inactive (low level) signal NLTNCY to the delay counter  103  when the signal RACMP indicating the comparison result that the row address (the value of the signal RA[n]) input in the n-th operation and the row address (signal RA) input in the n−1-th operation are the same row address (when the signal RACMP is active (high level)). In addition, in this embodiment, the inverter  111  is an example of the “delay control unit” of the present invention. 
       FIG.  10    is a timing chart illustrating an example of the timing of the signals in the pSRAM of the present embodiment. It is assumed that the precharge signal PRE is inactive (low level)), and the signal NLTNCY is active (high level) in a case where the row address RA having a value of 0 is accessed in the operation before the timing t 11  and the word line of the row address RA is activated. 
     When the next operation starts at the timing t 11  and the row address RA having a value of 1 is input, the row address [n] latch unit  108  outputs a signal RA[n] having a value of 1 to the row address [n−1] latch unit  109 , the comparator  110 , and the memory cell array  20 . On the other hand, when the signal RA[n] having the value of 1 is input, the row address [n−1] latch unit  109  outputs the signal [n−1] having the value of 0 to the comparator  110 . 
     The comparator  110  compares the value of the signal RA[n] with the value of the signal RA[n−1]. Because the two values are different, the signal RACMP is inactive (low level) and output to the inverter  111  so that the precharge signal PRE is active (high level) and output to the row control unit  102 . At this time, the row control unit  102  precharges the word line corresponding to the row address RA input in the previous operation (the word line corresponding to the row address RA having a value of 0), and activates the word line corresponding to the row address RA having a value of 1. 
     In addition, the delay counter  103  sets the delay in operation to be the initial delay according to the active (high level) signal NLTNCY. In addition, after the operation is completed, the word line corresponding to the row address RA having a value of 1 is maintained in the active state. 
     When the next operation starts at the timing t 12  and the row address RA having a value of 1 is input, the row address [n] latch  108  outputs a signal RA[n] having a value of 1 to the row address [n−1] latch unit  109 , the comparator  110 , and the memory cell array  20 . On the other hand, when the signal RA[n] having the value of 1 is input, the row address [n−1] latch unit  109  outputs the signal [n−1] having the value of 1 to the comparator  110 . 
     The comparator  110  compares the value of the signal RA[n] with the value of the signal RA[n−1]. Since the two values are equal, the signal RACMP is activated (high level) and output to the inverter  111 , so that the pre-charge signal PRE is inactive (low level) and output to the row control unit  102 . 
     In addition, the delay counter  103  sets the delay in the operation to a short delay according to the inactive (low level) signal NLTNCY, and the period of the operation becomes less than that of the previous operation. 
     When the next operation starts at the timing t 13  and a row address RA having a value of 1 is input, since the value of the input row address RA is equal to the value of row address RA input in the previous operation, the same operation as the previous operation is performed, and the delay in operation is also set to a short delay. 
     The next operation starts at the timing t 14 . When the row address RA having the value of 2 is input, the row address [n] latch unit  108  outputs the signal RA[n] having a value of 2 to the row address [n−1] latch unit  109 , the comparator  110 , and the memory cell array  20 . On the other hand, when the signal RA[n] having the value of 2 is input, the row address [n−1] latch unit  109  outputs the signal [n−1] having the value of 1 to the comparator  110 . 
     The comparator  110  compares the value of the signal RA[n] with the value of the signal RA[n−1]. Since the two values are different, the signal RACMP is inactive (low level) and output to the inverter  111 , so that the precharge signal PRE is active (high level) and output to the row control unit  102 . At this time, the row control unit  102  precharges the word line corresponding to the row address RA input in the previous operation (the word line corresponding to the row address RA having a value of 1), and activates the word line corresponding to the row address RA having a value of 2. 
     In addition, the delay counter  103  sets the delay in operation to be the initial delay according to the active (high level) signal NLTNCY. In addition, after the operation is completed, the word line corresponding to the row address RA having the value of 2 is maintained in the active state. 
     As described above, in the subsequent operation (second operation), when accessing the same row address RA as the previous operation (first operation), the delay in the subsequent operation may be set to be less than the initial delay. In addition, in each successive operation after the subsequent operation (second operation), while the same row address RA as the previous operation (first operation) is input, the delay of each successive operation after the subsequent operation may be set to be less than the initial delay. 
     Since it is configured to maintain the word line in the active state after the operation is completed, the command CMD to precharge the word line is set.  FIG.  11 ( a )  shows an example of the data structure of the command, the command CMD consists of 3 bits that are ADQ 7 , ADQ 6 , and ADQ 4  of the 8-bit signal ADQ[ 7 : 0 ] inputted at the first rising edge of the external clock signal CLK during operation. The contents of ADQ  7  and ADQ  6  are the same as those in the first embodiment. 
     ADQ 4  is a bit configured to indicate whether to precharging the word line corresponding to the row address RA input in the previous operation. For example, when the value of ADQ 4  is 1, it is precharged. In addition, when a command CMD for precharging a word line is input, as shown in  FIG.  11 ( b ) , only a command and an address are input, and data is not input. 
     Fourth Embodiment 
     In the pSRAM of this embodiment, the control unit  10  sets the delay in the second operation to be less than the initial delay when certain conditions are satisfied, which is the same as in the above-mentioned embodiments. However, the specific conditions includes an error is detected in the input written data in the first operation, which is different from in the above-mentioned implementations. 
       FIG.  12    is a block diagram illustrating a configuration example of a pSRAM in accordance with this embodiment. The control unit  10  includes an instruction decoder  101 , a row control unit  102 , a delay counter  103 , a column control unit  104 , a precharge control unit  106 , a write data latch unit  112 , an error detection unit  113 , and an inverter  114 . The configurations of the command decoder  101 , the row control unit  102 , the delay counter  103 , and the column control unit  104  are the same as those of the first embodiment. 
     In a case where the chip selection signal SC# is active (low level), when the signal ERR in an active (high level) state indicating that an error is detected in the write data WRDATA is input from the error detection unit  113 , the precharge control unit  106  deactivates the precharge signal PRE (low level) and output the precharge signal PRE to the row control unit  102 . On the other hand, the precharge control unit  106  activates the precharge signal PRE (high level) and outputs the precharge signal PRE to the row control unit  102  when the signal ERR in an inactive (low level) state is input from the error detection unit  113  (when no error is detected in the write data WRDATA). 
     The write data latch unit  112  latches the write data input through the address data terminal, and outputs the latched write data WRDATA to the error detection unit  113  and the memory cell array  20  during each write operation. In this embodiment, the write data input externally is input in a state having check data for error detection (e.g., the parity code or the cyclic redundancy check (CRC) code, etc.). 
     When the write data WRDATA and the check data are input from the write data latch unit  112 , the error detection unit  113  performs error detection on the write data WRDATA by using the check data. When the error detection unit  113  detects that the written data WRDATA contains an error, the signal EER is activated (high level) and output to the inverter  114  and the precharge control unit  106 , and output to the outside through the error terminal as well. On the other hand, when the error detection unit  113  detects that the written data WRDATA does not contain an error, the signal EER is deactivated (low level) and output to the inverter  114  and the precharge control unit  106 , and output to the outside through the error terminal as well. 
     The inverter  114  logically inverts the signal EER output from the error detection unit  113 , and outputs the logically inverted signal to the delay counter  103  as the signal NLTNCY. That is, the inverter  114  controls the delay in the second operation less than the initial delay (set as a short delay) by outputting the inactive (low level) signal NLTNCY to the delay counter  103  when the error detection unit  113  detects that the written data contains an error (when the signal ERR is active (high level)). In addition, in this embodiment, the inverter  114  is an example of the “delay control unit” of the present invention. 
       FIG.  13 ( a )  is a timing chart illustrating an example of the timing of the signals in the pSRAM when no error is detected in a write command, and  FIG.  13 ( b )  is a timing diagram of an example of the timing of the signals in the pSRAM when an error detected in a write command. In addition, it is assumed that the same row address is accessed in each operation. 
     Referring to  FIG.  13 ( a ) , when the write data WRDATA and the check data are input, the error detection unit  113  detects whether the write data WRDATA contains an error by using the check data. Next, when the error detection unit  113  detects that the write data WRDATA does not contain an error, the signal ERR is deactivated (low level) and output to the inverter  114  and the precharge control unit  106 , and the signal ERR is output to the outside through the error terminal as well. At this time, the inverter  114  activates the signal NLTNCY (high level) and outputs the signal NLTNCY to the delay counter  103 . Therefore, the delay in the next operation is set to be the initial delay. In addition, the precharge control unit  106  activates the precharge signal PRE (high level) and outputs the precharge signal PRE to the row control unit  102  to precharge the word line after the writing operation is completed. 
     Referring to  FIG.  13 ( b ) , when the error detection unit  113  detects that the write data WRDATA contains an error, the precharge signal ERR is activated (high level) and output to the inverter  114  and the precharge control unit  106 , and output to the outside through the error terminal as well. At this time, the inverter  114  deactivates the signal NLTNCY (low level) and outputs the signal NLTNCY to the delay counter  103 . Therefore, the delay in the next operation is set to be a short delay. In addition, the precharge control unit  106  deactivates the signal PRE (low level) and outputs the signal PRE to the row control unit  102  to maintain the active state of the word line even after the writing operation is completed. 
     As described above, when an error is detected in the write data WRDATA input in the write operation, the delay in the subsequent operation may be set to be less than the initial delay. 
     In addition, it is configured as similar to the third embodiment that the active state of the word line is maintained and a command CMD for precharging the word line is set after the operation of writing data WRDATA to detect an error is completed. The command CMD is configured in the same configuration as shown in  FIG.  11   . 
     The various embodiments described above are described to facilitate the understanding of the present invention, and the above description is not intended to limit the present invention. Therefore, the elements disclosed in the above-mentioned embodiments are intended to include all design changes or equivalents that fall within the technical scope of the present invention. 
     In the above embodiments, although the specific command includes a read or write command and a command for instructing whether to precharge a word line as an example, the present invention is not limited thereto. For example, the specific command may only be either a read command or a write command. At this time, in a case where the same row address RA is input in the first operation and the second operation, when a read command or a write command is input in the first operation, the delay in the second operation may be controlled to be less than the initial delay. In addition, the specific command may only be a command for indicating whether to precharge the word line. Furthermore, the specific command may be an arbitrary command configured to set the delay in the subsequent operation (second operation) to a short delay. 
     Although some embodiments of the present disclosure and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. For example, it will be readily understood by those skilled in the art that many of the features, functions, processes, and materials described herein may be varied while remaining within the scope of the present disclosure. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.