SYNCHRONOUS SEMICONDUCTOR MEMORY

Abstract: A dynamic random access memory capable of controlling the output timing of read data, which comprises a circuit (127) for counting clock pulses until the read data is transferred to a data latching circuit (121), and a circuit (129) for outputting the counted value to an external unit, so as to make it possible to control the output timing. Moreover, the dynamic random access memory is provided with a circuit (125) capable of controlling the output timing by an external input signal.

Description: The present invention relates to a synchronous semiconductor memory. More particularly, the present invention relates in a first aspect to a synchronous semiconductor memory comprising a memory cell array, address decoder means arranged to select a cell of said array to be read, a clock counter circuit arranged to be reset at the beginning of each read access to said array and a controlled output circuit, comprising a data latch, a transfer circuit arranged to transfer data, read from the array to the data latch and a controlled output buffer. The present invention also relates, in a second aspect, to a synchronous semiconductor memory comprising a memory cell array, address decoder means arranged to select a cell of said array to be read, a timer circuit arranged to be triggered at the beginning of each read access to said array and a controlled output buffer circuit. A conventional synchronous semiconductor memory is disclosed in JP-A-61-39295 and JP-A-62-275384. Synchronous semiconductor memories have a memory cell array in which memory cells are arranged in rows and columns. A decoder is coupled to an input of the memory cell array. A latch circuit and an output buffer are coupled to an output of the memory cell array. When data is read from the memory cell array, externally generated input addresses are decoded by a decoder so that one of the memory cells is selected in the memory cell array. Data stored in the thus selected memory cell is latched temporarily in the latch circuit. The data latched is read out via the output buffer. In a synchronous static random access memory (hereinafter referred to as SRAM), one pulse alone of a synchronous clock is supplied to the output buffer during a memory access time so that the read data can be accurately output in synchronism with the clock pulse. A plurality of consecutive synchronous clock pulses are input to the output buffer during a memory access time in a DRAM. If the conventional synchronization system is applied to a DRAM, it is necessary to previously determine which pulse of the synchronous clock should be selected to output the read data via the output buffer in response thereto. In EP-A-0 051 920, which discloses such a DRAM, this is done by trial and error. In a DRAM, since an input of the address and the reading or writing of the data are performed during a memory access time, there is the possibility of generating a time lag when the read data is applied to the latch circuit. Accordingly, it is impossible to accurately control the action timing of the output buffer in a DRAM which employs the conventional synchronization system. It is an object of the present invention to provide a synchronous type DRAM which selects an optimal pulse among a plurality of pulses of the synchronous clocks during a memory access time to determine the operating timing of the output buffer. It is another object of the present invention to provide a synchronous type DRAM which can perform the accurate synchronous control. A sychronous semiconductor memory according to the first aspect of the present invention is characterised by a clock count output circuit for outputting from the memory a signal representing the count value attained by the clock counter circuit determining a read access period; and counting by the clock counter circuit being terminated by a data transfer complete signal from the data latch. A sychronous semiconductor memory according to the second aspect of the present invention is characterised in that the timer circuit is arranged such that a timed read access period is set by the current state of an externally applied control signal and the output of the programmable timer circuit is applied to a control input of the output buffer circuit, the output of data from the output buffer circuit being enabled in dependence on the output of the programmable timer circuit. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a block diagram showing an arrangement of a synchronous type DRAM according to a first embodiment of the present invention; Fig. 2 is a circuit diagram showing a clock counter circuit and a delay clock number output circuit; Fig. 3 is a timing chart of Fig. 1; Fig. 4 is a block diagram showing an arrangement of a synchronous type DRAM according to a second embodiment of the present invention; Fig. 5 is a timing chart of Fig. 4; Fig. 6 is a block diagram showing an arrangement of a synchronous type DRAM according to a third embodiment of the present invention; Fig. 7 is a circuit diagram of an output clock delay control circuit in Fig. 6; and Fig. 8 is a timing chart of Fig. 6. Fig. 1 is a block diagram showing schematic arrangement of a synchronous type DRAM according to a first embodiment of the present invention. The synchronous DRAM includes an address input circuit 101 for receiving externally generated addresses A0 to An and generating X addresses and Y addresses. The address input circuit 101 comprises a latch circuit 103 for receiving the addresses A0 to An in synchronism with a synchronous clock (hereinafter referred to as CLK) and an address buffer 105 for generating the X addresses and the Y addresses in response to an output of the latch circuit 103. An output of the address buffer 105 is coupled to an X address decoder 107 and a Y address decoder 109. Outputs of the respective X address decoder 107 and the Y address decoder 109 are coupled to word lines 111 and bit lines 113 which are coupled to a memory cell array 115 in which memory cells, not shown, are arranged in matrix. The memory cells are located at the intersection of the word lines 111 and the bit lines 113. The X address decoder 107 selects one word line among a plurality of word lines 111. The Y address decoder selects one bit line among a plurality of bit lines 113. The word lines 111 are coupled to a data bus line 119 by way of an input/output circuit 117 for performing a read/write cycle. The data bus 119 is coupled to a data latch circuit 121. The data latch circuit 121 latches the data read on the data bus 119. The data latch circuit 121 supplies thereafter a latched read data S1 to an output circuit 123 and outputs a latch completion signal S2. The output circuit 123 is enabled by an output control signal  which is synchronized with the clock signal CLK and outputs the read data S1 as a read data D0. The output circuit 123 comprises an output buffer according to the present invention. The DRAM has a memory control signal generator 125, a clock counter circuit 127 and a delay clock number output circuit 129. The memory control signal generator 125 receives an externally generated clock signal CLK, an externally generated low address strobe signal RAS and an externally generated column address strobe signal CAS and generates various memory control signals S3 for controlling an internal circuit in the memory and a drive signal S4. The memory control signal generator 125 comprises a latch circuit 131 which latches a low address strobe signal RAS and the column address strobe signal CAS in response to the clock signal CLK and a signal generator 133 for receiving an output of the latch circuit 131 and generating the memory control signals S3 and the drive signal S4. The drive signal S4 is supplied to the delay clock number output circuit 129. The clock counter circuit 127 counts the number of pulses or clocks of the clock signal CLK in response to falling of the low address strobe signal RAS . The clock counter circuit 127 stops the counting operation in response to the latch completion signal S2 from the data latch circuit 121. An output of the clock counter circuit 127 is coupled to the delay clock number output circuit 129. The delay clock number output circuit 129 supplies an output of the clock counter circuit 127 to delay clock number output terminals 135, 137 and 139 in response to the drive signal S4 issued by the memory control signal generator 125. The output terminal 135 generates a first digit position of the counted number of clocks (binary digit) and the output terminal 137 generates a second digit position and the output terminal 139 generates a third digit position. Fig. 2 is a circuit diagram showing the clock counter circuit 127 and the delay clock number output circuit 129 as illustrated in Fig. 1. The clock counter circuit 127 comprises a NOR gate 201 for controlling an input of the clock signal CLK by the latch completion signal S2 and generating a drive clock signal S5 and a reset pulse generator 203 for generating a reset signal S6 in response to the low address strobe signal RAS . An output of the NOR gate 201 is coupled to an address counter 205. The address counter 205 is reset in response to the reset signal S6 and counts up the addresses in response to the drive clock signal S5. Outputs P0, P1 and P2 of the address counter 205 are coupled to inputs of the delay clock number output circuit 129. The output P0 of the address counter represents a first digit position, the output P1 represent the second digit position and the output P2 represents the third digit position respectively of the counted address number (binary number). The delay clock number output circuit 129 comprises tri-state inverters 207, 209 and 211 which are respectively closably controlled by the drive signal S4 and delay clock number output terminals 135, 137 and 139 are respectively coupled to outputs of the tri-state inverters 207, 209 and 211. Fig. 3 is a timing chart of Fig. 1. The operations of the arrangements in Figs. 1 and 2 will be described with reference to Fig. 3. When the row address strobe signal RAS falls at the time t1 to thereby start access (reading data), a one-shot pulse of the reset signal S6 is supplied from the reset pulse generator 203 in the clock counter circuit 127. The address counter 205 is reset in response to the reset signal S6 so that the outputs P0, P1 and P2 go LOW. Since no read data is transferred from the data bus 119 to the data latch circuit 121 until the time t7, the latch completion signal S2 which has been supplied from the data latch circuit 121 is held LOW. Accordingly, the NOR gate 201 in the clock counter circuit 127 receives the clock signal CLK and supplies a drive clock signal S5 to the address counter 205 until the time t7, whereby the address counter 205 counts up the binary digits (refer to outputs P0, P1 and P2) in synchronism with the rising of the drive clock signal S5 at the times t2, t3, t4 and t5. The externally generated addresses A0 to An are fetched to the address input circuit 101 during those times. The X address and the Y address are generated from the address input circuit 101 and are respectively supplied to the X address decoder 107 and the Y address decoder 109. The X address decoder 107 decodes the X address and the Y address decoder decodes the Y address and respectively select a memory cell in the memory cell array 115. The data stored in the selected memory cell is transferred to the data latch circuit 121 via the data bus 119 at the time t6. At the time t7 when a certain time lapses after the time t6, the latch completion signal S2 supplied from the data latch circuit 121 goes HIGH so that the output of the clock signal CLK from the NOR gate 201 in the clock counter circuit 127 is stopped. Accordingly, the levels of the signals from the outputs P0, P1 and P2 are fixed. The drive signal S4 which is supplied from the memory control signal generator 125 goes HIGH at the time t8 so that the tri-state inverters 207, 209 and 211 in the delay clock number output circuit 129 are ON and the fixed levels of the signals of the outputs P0, P1 and P2 of the address counter appear on the output terminals 135, 137 and 139 of the delay clock number output circuit 129 by way of the tri-state inverters 207, 209 and 211. It is possible to easily judge which clock pulse among the plurality of consecutive pulses of the clock signal CLK, which is applied during a memory access time can generate the output of the output circuit 123 in the optimal clock synchronous operation when the output circuit 123 is enabled by measuring the output terminals 135, 137, 139, etc. of the delay clock number output circuit 129 with a tester and the like. Generally in the DRAM, the data transferring time ranging from the access start time to the latching time by the data latch circuit 121 varies due to the manufacturing variation. The data transferring time can be judged by the numbers of delay clocks which are supplied from the output terminals 135, 137 and 139 of the delay clock number output circuit 129. The output control signal  which is synchronous with the clock signal CLK can be generated based on the result of judgement. The output circuit 123 is activated by the output control signal  to thereby generate the read data D0, whereby the read data D0 can be accurately generated in synchronism with the clock signal CLK. Fig. 4 is a block diagram showing a schematic arrangement of the synchronous type DRAM according to a second embodiment of the present invention in which the elements which are common to those in Fig. 1 are denoted at the same numerals. The DRAM of the second embodiment is same as that of the first embodiment except that a delay circuit 401 is coupled between the output of the data latch circuit 121 and the input of the clock counter circuit 127. The delay circuit 401 receives the latch completion signal S2 supplied by the data latch circuit 121 and generates a latch completion delay signal S7 which is delayed by a given delay time Td. The delay circuit 401 supplies the latch completion delay signal S7 to the clock counter circuit 127. The delay circuit 401 of the second embodiment comprises multiple stages of inverters. Fig. 5 is a timing chart of Fig. 4 with reference to which the operation of the DRAM of the second embodiment will be described hereinafter. Since the read data is not transferred to the data latch circuit 121 during the period ranging from the time t11 when the RAS falls to start access to the time t15, the latch completion signal S2, which is supplied by the data latch circuit 121, is LOW. Accordingly, the clock counter circuit 127 is synchronous with the rising of the drive clock signal S5 at the times t12, t13, t14 and t16 and starts to count the pulses. During the period, a memory cell is selected and the data stored in the thus selected memory cell is transferred to the data latch circuit 121 via the data bus 119 at the time t15. When the read data is transferred to the data latch circuit 121, the latch completion signal S2 supplied by the data latch circuit 121 goes HIGH. The latch completion signal S2 is delayed for the prescribed delay time Td in the delay circuit 401 and supplied by the delay circuit 401 as the latch completion delay signal S7 which becomes HIGH at the time t17. The clock counter circuit 127 stops to count the pulses in response to the latch completion delay signal S7. Thereafter, the drive signal S4, which is supplied by the memory control signal generator 125, goes HIGH at the time t18. The levels of the signals of the outputs P0, P1 and P2 of the address counter 205 of the stopped clock counter circuit 127 appear on the output terminals 135, 137 and 139 of the delay clock number output circuit 129 in response to the drive signal S4. Since the delay circuit 401 is provided in the output of the data latch circuit 129 according to the second embodiment, a margin can be obtained considering the variation in manufacture and the worst condition in the operation. Accordingly, since the optimal delay clock number can be selected considering the margin compared with that of the first embodiment, it is possible to generate the read data synchronizing with the clock signal CLK with high accuracy. Fig. 6 is a block diagram showing a schematic arrangement of the synchronous type DRAM according to a third embodiment of the present invention in which the elements which are common to those in the first embodiment are denoted at the same numerals. The DRAM of the third embodiment has a data latch circuit 601, instead of the data latch circuit 121 in the first embodiment, which latches the data read from the memory cell from the data bus 119 and supplies the thus read and latched data S1 to the output circuit 123. The DRAM of the third embodiment has external input terminals 603 and 605 and an output clock delay control circuit 607 instead of the clock counter circuit 127 and the delay clock number output circuit 129 in the first embodiment. The external input terminals 603 and 605 receive respectively externally generated input signals SEL0 and SEL1 for designating the delay clocks and are coupled to an output clock delay control circuit 607. The output clock delay control circuit 607 receives the clock signal CLK, the low address strobe signal RAS , the drive signal S4 from the memory control signal generator 125 and the externally generated input signals SEL0 and SEL1 to thereby generate the output control signal a for operating the output circuit 123. Fig. 7 is a circuit diagram showing an arrangement of the output clock delay control circuit 607 as illustrated in Fig. 6. The output clock delay control circuit 607 comprises a shift register portion 609 and a logic circuit 611. The shift register portion 609 comprises an inverter 613 for inverting the drive signal S4 which falls in synchronism with the fall of the row address strobe signal RAS , a reset pulse generator 615 which generates a reset signal S8 having a one-shot pulse which is synchronous with the rise and the fall of the row address strobe signal RAS , five flip-flops (hereinafter referred to as FFs) 617, 619, 621, 623 and 625 for shifting the output of the inverter 613 in synchronism with the clock signal CLK. The output of the inverter 613 is coupled to the input of the first FF 617. The output of the first FF 617 is coupled to the input of the second FF 619. The output of the second FF 619 is coupled to the input of the third FF 621. The output of the third FF 621 is coupled to the input of the fourth FF 623. The output of the fourth FF 623 is coupled to the input of the fifth FF 625. The output of the reset pulse generator 615 is coupled to reset inputs of the first to fifth FFs 617 to 625. The clock signal CLK is applied to the timing inputs of the first to fifth FFs 617 to 625. The output of the third, fourth and fifth FFs 621, 623 and 625 of the shift register portion 609 are respectively supplied to the logic circuit 611. The logic circuit portion 611 is the circuit to select the timing of the fall of the output control signal a based on the combination of the output of the shift register portion 609 and the external input signals SELO and SEL1. The logic circuit portion 611 comprises first and second input terminals 603 and 605 for receiving the externally generated input signals SELO and SEL1, inverters 627 and 629, first, second and third NAND gates 631, 633 and 635 for receiving and selecting signals, and a fourth NAND gate 637 for generating the output control signal a in response to the outputs of the NAND gates 631, 633 and 635. The first external terminal 603 is coupled to first inputs of the first and second NAND gates 631 and 633 and is also coupled to a first input of the third NAND gate 635 by way of the inverter 629. The second external terminal 605 is coupled to a second input of the first NAND gate 631 and also coupled to second inputs of the second and third NAND gates 633 and 635 by way of the inverter 627. The output of the third FF 621 is coupled to a third input of the third NAND gate 635. The output of the fourth FF 623 is coupled to a third input of the second NAND gate 633. The output of the fifth FF 625 is coupled to a third input of the first NAND gate 631. The outputs of the first to third NAND gates 631, 633 and 635 are coupled to the first to third inputs of the fourth NAND gate 637. Fig. 8 is a timing chart showing the operation of the synchronous type DRAM in Fig. 6. The address input circuit 101 receives an X address 801 among addresses A0 to An at the time close to the time t22 and also receives a Y address 803 at the time close to the time t23. Another X address and another Y address, which are respectively supplied as analog data from the address buffer 105 independently of the clock signal CLK, are transferred to the X address decoder 107 and the Y address decoder 109. A memory cell in the memory cell array 115 is selected in response to the outputs of the X address decoder 107 and the Y address decoder 109. A data 805 stored in the selected memory cell is transferred to the data latch circuit 601 via the data bus 119 at the time t25. The low address strobe signal RAS , which is applied to the shift register portion 609 of the output clock control circuit 607, falls at the time t21 and the reset pulse generator 615 generates the reset signal S8 having one shot pulse in response to the fall of the low address strobe signal RAS . The first to fifth FFs 617 to 625 are reset slightly behind the time t21 in response to the reset signal S8 (since all the outputs of the first to fifth FFs in Fig. 8 are LOW, reset operations do not appear on the signals). The drive signal S4 falls thereafter and the output signal S4A of the inverter 613 rises to go HIGH in response to the fall of the drive signal S4. At the time t22 when the output signal S4A goes HIGH and the first clock signal CLK rises, the output signal A of the first FF 617 goes HIGH from LOW. Successively, the output signals B to E of the second to fifth FFs 619 to 625 go HIGH from LOW at the times t23 to t27. Thereafter, the low address strobe signal RAS rises at the time t28 and the reset pulse generator circuit generates the reset signal S8 having one shot pulse in response to the rise of the low address strobe signal RAS and at the same time the drive signal S4 also rises. Accordingly, the output signal A of the inverter 613 goes LOW from HIGH. The output signals A to E of the first to fifth FFs 617 to 625 go LOW from HIGH in response to the pulse of the reset signal S8. The operation of the logic circuit portion 611 of the output clock delay control circuit 607 will be described hereinafter. The operation will be described in three cases. The first case is that the external input signal SEL0 is HIGH and the external input signal SEL1 is LOW (refer to Fig. 8(a)). The second case is that the external input signals SEL0 and SEL1 are respectively LOW (refer to Fig. 8(b)). The third case is the external input signals SEL0 and SEL1 are respectively HIGH (refer to Fig. 8(c)). In the First Case The output control signal a which is the output signal of the fourth NAND gate 637 rises at the time t26 and falls at the time t28. This is explained in the Table 1 set forth hereunder in which the inputs and outputs of the first to fourth NAND gates and other signals are represented by HIGH and LOW levels. Signals level at time t26 level at time t28 SEL0 H H SEL1 L L C H H → L D L → H H → L E L H → L input of the first NAND 631 L, H, L H → L, H, L output of the first NAND 631 H H input of the second NAND 633 L → H, H, H H → L, H, H output of the second NAND 633 H → L L → H input of the third NAND 635 H, H, L H → L, H, L output of the third NAND 635 H H input of the fourth NAND 637 (outputs of the first to third NANDS 631 to 635) H, H → L, H H, L → H, H output of the fourth NAND 637 (a) L → H H → L The output circuit 123 generates the read data D0 (805) at a time delayed by a prescribed time Ta behind the time t26 in response to the output control signal a. In the Second Case The output control signal a rises at the time t24 and falls at the time t28 which is described in the Table 2. Signals level at time t24 level at time t28 SEL0 L L SEL1 L L C L → H H → L D L H → L E L H → L input of the first NAND 631 L, L, L H → L, L, L output of the first NAND 631 H H input of the second NAND 633 L, L, H H → L, L, H output of the second NAND 633 H H input of the third NAND 635 L → H, H, H H → L, H, H output of the third NAND 635 H → L L → H input of the fourth NAND 637 H, H, H → L H, H, L → H output of the fourth NAND 637 (a) L → H H → L The output circuit 123 tends to generate the read data DO (805) in response to the output control signal a but it generates the read data DO after a delay time Tb which is longer than the prescribed delay time Ta at the time T24. In the Third Case The output control signal a rises at the time t27 and falls at the time t28 which is explained in the Table 3. Signals level at time t27 level at time t28 SEL0 H H SEL1 H H C H H → L D H H → L E L → H H → L input of the first NAND 631 L → H, H, H H → L, H, H output of the first NAND 631 H → L L → H input of the second NAND 633 H, H, L H → L, H, L output of the second NAND 633 H H input of the third NAND 635 H, L, L H, L, L output of the third NAND 635 H H input of the fourth NAND 637 H → L, H, H L → H, H, H output of the fourth NAND 637 (a) L → H H → L The output circuit 123 generates the read data D0 (805) at the time delayed by the prescribed delay time Ta behind the time t27 in response to the output control signal a. In the second case as illustrated in Fig. 8(b), since the read data D0 has not been supplied to the data latch circuit 601 at the rise time of the output control signal a, it takes the time Tb ranging from the time t26 when the clock signal CLK rises to the time when the output circuit 123 generates the data. Whereupon, in the first and third cases as illustrated in Figs. 8(a) and 8(c), since the read data D0 has been latched in the data latch circuit 601 when the output control signal a rises, the output circuit 123 can generate the read data D0 at the time delayed by the delay time Ta behind the rise time t26 or t27. When the first to third cases are compared with one another, the time ranging from the fall time of the low address strobe signal RAS to the access start time is short but the time from the rise time of the clock signal CLK to the access start time is long in the second case as illustrated in Fig. 8(b). The time ranging from the rise time t21 of the low address strobe signal RAS to the access start time is long in the third case as illustrated in Fig. 8(c). Accordingly, optimal output control signal a is generated in the first case as illustrated in Fig. 8(a). Consequently, the read data D0 is generated from the output circuit 123 in response to the output control signal a which is supplied by the output clock delay control circuit 607 when the external input signal SEL0 is set HIGH and the external input signal SEL1 is set LOW so that the optimum synchronous operation can be attained. In case the speed of the read data which is transferred from the memory cell array 115 to the data latch circuit 601 is fast, due to the manufacturing variation of the DRAM having the same arrangement of the third embodiment, there is a possibility that the optimal synchronizing operation can be attained in the second case as illustrated in Fig. 8(b). Since the output control signal a can be arbitrarily set in response to the external input signals SEL0 and SEL1 and the output clock delay control circuit 607, the optimal synchronous operation can be attained according to the third case even if there are manufacturing variation and the difference in the usage thereof. The present invention is not limited to the embodiments set forth above but can be modified variously. An example of the modification will be described hereinafter. The clock counter circuit 127 generates the output when the clock signal CLK rises at the time t8 immediately after the read data is transferred to the data latch circuit 121 as illustrated in Fig. 3. However, the clock counter circuit 127 can generate the output when the clock signal CLK falls at the time t8. The clock counter circuit 127 generates the output when the clock signal CLK rises at the time t18 which is immediately after the generation of the latch completion delay signal S7 as illustrated in Fig. 5. However, it is also possible that the clock counter circuit 127 generates the output when the clock signal CLK falls at the time t8. INDUSTRIAL UTILIZATION As described above, the DRAM according to the present invention can be applied to the synchronous type DRAM which can output the read data from the memory in synchronism with the optimal clock pulse.

Claims: A synchronous semiconductor memory comprising a memory cell array (115), address decoder means (101,107,109) arranged to select a cell of said array to be read, a clock counter circuit (127) arranged to be reset at the beginning of each read access to said array (115) and a controlled output circuit (117,119,121,123), comprising a data latch (121), a transfer circuit (117) arranged to transfer data, read from the array to the data latch (121) and a controlled output buffer (123), characterised by a clock count output circuit (129) for outputting from the memory a signal (P0,P1,P2) representing the count value attained by the clock counter circuit (127) determining a read access period; and counting by the clock counter circuit (127) being terminated by a data transfer complete signal (S2) from the data latch (121). A memory according to claim 1, including a delay (401) coupled between the data latch (121) and the clock counter circuit (127) to delay the transfer complete signal (S2). A memory according to claim 1 or 2, wherein the address decoder means (101,107,109) comprises a column decoder (107) and a row decoder (109). A memory according to claim 3, wherein the address decoder means (101,107,109) comprises an address latch circuit (103) for accepting an address signal in response to the clock signal and an address buffer (105) for generating column and row address signals for application to the column and row decoders (107,109) from an address in the address latch circuit (103). A memory according to any preceding claim, including a control signal generator for generating a clock count output enable signal (S4), wherein the clock count output circuit (129) is responsive to the clock count output enable signal (S4) to output a signal (P0,P1,P2) representing the current clock count value. A memory according to claim 5, wherein the clock count output circuit (129) comprises a tri-state inverter for each bit of the signal (P0,P1,P2) representing the current clock count value. A memory according to any preceding claim, wherein the clock count circuit (127) comprises a gate circuit (201) for receiving the clock signal and the data transfer complete signal (S2) and outputting the clock signal in dependence on the state of the data transfer complete signal (S2), a reset signal generator (203) for generating a reset signal, and an address counter (205) coupled to the reset signal generator (203) and the gate circuit (201) for counting clock signal pulses output by the gate circuit (201) and being reset by the reset signal from the reset signal generator (203). A synchronous semiconductor memory comprising a memory cell array (115), address decoder means (101,107,109) arranged to select a cell of said array to be read, a timer circuit (607) arranged to be triggered at the beginning of each read access to said array (115) and a controlled output buffer circuit (123) receiving the data output from said memory array (115), characterised in that the timer circuit (607) is arranged such that a timed read access period is set by the current state of an externally applied control signal (SEL0,SEL1) and the output of the programmable timer circuit (607) is applied to a control input of the output buffer circuit, the output of data from the output buffer circuit being enabled in dependence on the output of the programmable timer circuit (607). A memory according to claim 8, wherein the programmable timer circuit (127) comprises a delay circuit (609) arranged to generate a plurality of delayed signals (C,D,E), delayed by different amounts, and a selector circuit (611) arranged to select one of the delayed signals (C,D,E) in dependence on said control signal (SEL0,SEL1). A memory according to claim 9, wherein the delay circuit (609) a comprises a shift-register. A memory according to claim 9 or 10, wherein the selector circuit (611) comprises a plurality of gates (627,629,631,633,635,637).

Inventors: TAKASUGI ATSUSHI, TAKASUGI, ATSUSHI, TAKASUGI, ATSUSHI, OKI ELECTRIC INDUSTRY CO., LTD.
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY COMPANY, LIMITED
Application year: 1991
Main classification: G11C7
IPC codes: G11C7, G11C11