SEMICONDUCTOR MEMORY WITH IMPROVED WRITE FUNCTION

Abstract: A static memory which can operate with a small power consumption and produces the same data written into a selected memory cell, is disclosed. The memory comprises a memory array (609), a sense amplifier (903), a latch circuit (610′) for holding data from the sense amplifier and a data-in buffer (603) for generating a buffered input signal (D′ IN ) and is featured by a circuit for directly writting the buffered input signal to the latch circuit in a write mode.

Description: BACKGROUND OF THE INVENTION: The present invention relates to a memory circuit and more particularly to a static memory circuit of asynchronous type. Asynchronous type static memory are known and widely used as high speed memories. The asynchronous type static memory operates without receiving any clock signal from the outside the memory, but it generates timing signals for controlling the respective sections in accordance with change in address signals and a write enable signal. However, in the prior art the respective peripheral circuits of the memory are controlled in the same manner both in a read operation and in a write operation, and a sense amplifier is also enabled in a write operation even though a function of the sense amplifier is not necessary in a write operation, resulting in a large power consumption. Also, it is frequently required to output the same data written into a selected memory cell to an output terminal. For that end, after the write operation, the same word line is selected again to perform a read operation on the same memory cell which was selected in the previous write operation. Thus, the reading of the written data cannot be performed at a high speed and such read operation also consumes a large power. A memory circuit as mentioned above and including all features of the preamble of claim 1 is disclosed in EP-A-0 186 906 and described with respect to figs. 1-14. SUMMARY OF THE INVENTION: It is an object of the present invention to provide a memory circuit which can operate in a write mode with a reduced power consumption. It is another object of the present invention to provide a memory circuit which can output the same data as that written into a selected memory cell at a high speed. These objects are achieved by a memory device as defined in claim 1; the dependent claims are related to further developments of the invention. According to the present invention, the internal data signal is directly written into the latch circuit, and therefore the same data as the internal data signal can be outputted to the output terminal in the same write cycle. In the present invention, in a write mode, the sense amplifier is not enabled to reduce a power consumption. Also, a signal for selecting a word line is also generated when a write enable signal is enabled even any address signal is not changed. BRIEF DESCRIPTION OF THE DRAWINGS: Fig. 1 is a schematic block diagram of a memory circuit according to a prior art; Fig. 2 is a schematic circuit diagram of a memory cell matrix; Fig. 3 is a schematic circuit diagram of a memory cell; Fig. 4 is a schematic block diagram of an address buffer; Fig. 5 is a schematic block diagram of a chip select input buffer; Fig. 6 is a schematic block diagram of a data input buffer; Fig. 7 is a schematic block diagram of a write control signal buffer; Fig. 8 is a schematic block diagram of a word line driver; Fig. 9 is a schematic block diagram of a data latch circuit; Fig. 10 is a schematic block diagram of a clock signal generator; Fig. 11 is a schematic block diagram of a data input driver; Fig. 12 is a schematic block diagram of a switching circuit; Fig. 13 is a wave-form diagram of a read operation of the circuit in Fig. 1; Fig. 14 is a wave-form diagram of a write operation of the circuit in Fig. 1; Fig. 15 is a schematic block diagram of a memory according to the present invention; Fig. 16 is a schematic block diagram of a clock generator employed in the memory of Fig. 15; Fig. 17 is a schematic block diagram of a data latch section of the memory of Fig. 15; Fig. 18 is a wave-form diagram showing an operation of the memory of Fig. 15; Fig. 19 is a block diagram of a data latch section as another example employed in the memory of Fig. 15; Fig. 20 is a diagram of a column selection circuit as another example, employed in the memory of Fig. 15; and Fig. 21 is a schematic block diagram of a switching circuit, as another example, employed in the memory of Fig. 15. DETAILED DESCRIPTION OF THE INVENTION: Referring to Fig. 1, a memory circuit according to a prior art is explained. A memory cell matrix 609 includes a plurality of static type memory cells in a known way. Fig. 2 shows a major structure of the matrix 609. As shown in Fig. 2, a plurality pairs of bit lines (BL, BL ) are arranged in parallel in column direction while a plurality of word lines Wi are arranged in row direction. Each of bit lines is connected to a power voltage terminal Vcc through a P channel load transistor Q₉₁. In the drawings, P channel transistors are indicated by the mark with a circle while N-channel transistors are indicated by the simple mark without the circle. A plurality of memory cells 901 are arranged at the intersections of the word lines and the bit line pairs in a known way. Each of the memory cells 901 is shown in Fig. 3 and composed of a pair of flip-flop transistors Q₉₈, a pair of resistors R₉₁ and a pair of transfer gate transistors Q₉₇. A column selection gate 902 is provided between each pair of bit lines BL, BL and a pair or data bus lines DB, DB . Each of the gate 902 includes a pair of CMOS switches each composed of an N channel transistor Q₉₂ and a P channel transistor Q₉₃ which are controlled by a column selection signal such as Yo generated from a column decoder 606. A sense amplifier 903 includes a differential amplifier composed of transistors Q₉₄ to Q₉₆ and a CMOS inverter INV from which a read signal PB is produced. In the memory of Fig. 1, although a plurality of address input buffers are employed, one address input buffer 601 for an address input signal Ai is respresentatively shown. The address input buffer 601 is structured as shown in Fig. 4, in which three inverters IVI-IV3 are used to generate true and complementary address signals Ai' and Ai' and a delay circuit 701 and logic gates G1-G3 form a detection circuit for detecting change in the address signal Ai to generate a detection signal  i . Other address buffers have the same structure as 601 but receive other address signals in a known way. A row decoder 605 receives a plurality of row address signals such as Ai', Ai' from the address buffers and generates row selection signals such as Xi of the same number as the word lines. Among the row selection signals, only one is selected with others non-selected. A word line drive circuit 607 includes a plurality of drive units each of which is structured by an AND gate receiving the row selection signal Xi and a drive timing signal  x , an output of the AND gate being coupled to the word line Wi as shown in Fig. 8. A chip select signal buffer 602 receives a chip selection signal CS and generates an internal chip selection signal CS ; and a timing signal  CS when the chip selection signal CS is changed from a high inactive level to a low active level. The structure of the buffer 602 is illustrated in Fig. 5 in which inverters IV4 and IV5 are used to invert the signal CS and a delay circuit 701, an inverter IV6 and an AND gate G4 form a detection circuit of change of CS . A data-in buffer 603 buffers an input data D IN to generate a buffered data signal D' IN , and structured as shown in Fig. 6. A write enable signal WE is input to a buffer 604 and an internal write signal WE' is generated therefrom. The buffer 604 is structured by three inverters as shown in Fig. 7. A data write circuit (D IN ) 608 receives the buffered data signal D' IN and the internal write signal WE' to generate true and complementary write data signals to DB and DB when WE' is high in level. The circuit 608 is structured as shown in Fig. 11 in which two CMOS inverters are employed in a driver stage 801. A data latch circuit 610 incorporates the read signal RB through a CMOS transfer circuit 802 when a control signal  L is a high level and holds the signal RB by a flip-flop 803. The state of the flip-flop 803 is inputted to an inverter and outputted as a latched signal LB. A switch circuit 611 is composed of two CMOS transfer gates S11 and S12 which are controlled by a control signal  SW and its inverted signal generated by an inverter IVC, as shown in Fig. 12. When the control signal  SW is high in level, the signal RB is outputted as a selected signal SB via the transfer gate S11 while the signal  SW is at a low level, the latched signal LB is outputted via the transfer gate S12 as the signal SB. A data output circuit 612 produces an output signal D OUT in accordance with the signal SB when CS' is at a low level and WE' is at a low level. A clock generator 613 have an OR gate receiving the signal WE', the plurality of detection signals  i from all the address buffers and the signal  CS and a plurality of buffers B11 to B14 from which control signals  X ,  S ,  L and  SW are generated with predetermined delay times. A read operation of the memory of Fig. 1 is explained with reference to Fig. 13. Under the low level of CS i.e. the active state of the memory, at least one address signal Ai is changed at a time t₁. Thereafter, the buffered signals A'i, A'i are changed in level. Also, the detection signal  i of a high level is generated. The row and column decoders 605 and 606 generate the high level of selection signals Xi and Yj, respectively. Thereafter the signal  X ,  S ,  SW and  L are sequentially generated by the generator 613, as shown in Fig. 13. The word line Wi is selected in response to the rise of  X so that the states of the memory cells coupled to the selected word line Wi are generated to the respective pairs of bit lines (BL, BL ) and the level of the bit lines are subjected to change in level. While only one of the column selection circuits 902 is enabled so that the state of the bit line pair coupled to the enabled transfer gate 902 is transmitted to the sense amplifier 903 via DB and DB and the read signal RB is developed in response to the signal  S . After the signal RB is developed, the signal  SW is raised and the signal RB is transmitted as the signal SB via the switch circuit 611. Thereafter, the circuit 612 produces the output signal D OUT in accordance with the signal SB. While, the signal  L is raised after the state of the signal RB and the latch circuit 610 holds the signal RB by the flip-flop 803 and the latch signal LB of the same level as the RB is produced. Accordingly, in this instance, the read signal RB is transmitted to as both of the signals SB and LB. In the above memory, the word line Wi is selected during the period that the signal  X is at the high level and after the above period, the output signal D OUT is maintained by the state of the latch circuit 610, and the signals  S ,  L ,  X and  SW are deactivated. Accordingly, power consumption is reduced after the deactivation of the signals  S ,  L ,  X and  SW although the D OUT is maintained. Next, a write operation of the memory of Fig. 1 is explained with reference to Fig. 14. Under the active level of the signal CS (low level) and the predetermined fixed state of the address signals, the write enable signal WE is changed from the inactive (high) level to the active (low) level at a time point t₁. Thereafter the internal write signal WE' is raised and the clock generator 603 generates the control signal  X ,  S ,  SW and  L are raised in the order as shown in Fig. 14. While, after change in WE , the input data signal D IN is determined. In response to the rise of WE', the driver 801 in the circuit 608 is activated so that the state of the signals DB and DB are determined by the buffered data signal D' IN and the signals DB and DB are written into the selected memory cell 901 and thereafter the state (LB) of the latch circuit 610 is also determined in response to the rise of  L according to D IN . Thus, the write operation of D IN to the selected memory cell 901 is accomplished and the write enable signal WE is returned to the inactive level at a time point t₂. Throughout the write operation, the selected word line Wi is maintained at the high level and DC current flows from Vcc to the ground via one of the transistors Q₉₁ and the selected memory cell 901 in the respective bit line pairs and the sense amplifier 903 also consumes DC current in this period since the amplifier 903 is also enabled in response to  S . The current flowing through the sense amplifier 903 is unnecessary current in view of the function of writing. When the data written into the selected memory cell 901 is to be read, the selected word line Wi is once deactivated to the low level and then the same word line Wi is selected again to produce the data written as the output signal D OUT . Therefore, it has been necessary to perform the selection of the word line twice, resulting in a low speed operation and a large power consumption. A memory circuit according to one embodiment of the present invention is explained with reference to Figs. 15 to 17. The present embodiment is achieved by replacing the clock generator 613 and the latch circuit 610 in Figs. 1, 10 and 9 with a clock generator 613' and a latch circuit 610' as shown in Fig. 15. The latch circuit 610' receives the signal WE' and the buffered input signal D' IN in addition to  L and RB. The detailed structure of the latch circuit 610' is Fig. 17. Namely, the latch circuit 610' further comprises a delay circuit having an input supplied with D' IN and a CMOS transfer gate 302 coupled between the output of the delay circuit 301 and the input of the latch circuit 803, in addition to the transfer gate 802 controlled by  L and the flip-flop 803 as compared to the latch circuit 610. The transfer gate 302 is enabled to transmit D' IN to the flip-flop where WE' is at the high level. The detailed structure of the clock generator 613' is shown in Fig. 16. The internal write enable signal WE' is not applied to an OR gate receiving the signals  i and  CS but applied to an OR gate G11 receiving the output of the OR gate G10. The signal  X for enabling the word line driver 607 is generated from the OR gate G11. A read operation of the circuit of Fig. 15 is the same as the memory of Fig. 1. A write operation of the present embodiment is explained with reference to Fig. 18. The write enable signal WE is changed from the inactive (high) level to the active (low) level at a time point t₁. Then, the signal WE' is raised and the state of the input data signal D IN is determined. In response to the rise of WE', the signal  X is generated from the OR gate G11 in the clock generator 613' at a time point t₂. In this instance,  S ,  L and  SW are all low in level. At a time t₃, the buffered input signal D' IN is generated by the buffer 603 and the circuit 608 determines the levels of DB and D B in accordance with the level of D' IN and the state of the bit lines BL, BL are determined at a time t₄ according to the D' IN . Thus, the state of the selected memory cell 901 is determined in accordance with D' IN at a time t₅. While, at the time t₄, the delay circuit 301 in the latch circuit 610' produces at its output node 31 a delayed signal of D' IN and the output 31 of the delay circuit 301 is transferred to the flip-flop 803 via the transfer gate 302 enabled by WE' and held thereby. Namely, the same data written into the selected memory cell 901 is also stored in the flip-flop 803 in the latch circuit 610'. Thus, the write operation to the selected memory cell is accomplished at a time t₆. In the above embodiment, the delay time of the delay circuit is set at such value that the write to the selected memory cell and the determination of the state of the flip-flop 803 are accomplished simultaneously. According to the above embodiment, the same data written to the selected memory cell is simultaneously latched by the latch circuit 610' and produced as the signal LB. Therefore, the data latched by the latch circuit 610' is produced as the output signal D OUT because the switch circuit 611 transmits the signal LB in response to the low level of  SW . Since, the signal  S is maintained at the low level in the write operation, the sense amplifier 903 is not enabled to avoid DC current flowing in the sense amplifier 903, resulting in a low power consumption in the write operation. Moreover, the write data is outputted as D OUT in the write operation, it is not necessary to drive the selected word line again to read the written data. Figs. 19, 20 and 21 show another example of the latch circuit 610", column selection circuit 902' and the switch circuit 611'. In the modified circuits 610", 902' and 611', transistors Q101 - Q106 have a small value of threshold voltage such as 0.3 volt and they are employed as transfer gates in place of the CMOS transfer gates in the previous embodiment. The operations and functions of those circuits 610", 902' and 611' are the same as the previously explained circuits 610', 902 and 611, respectively.

Claims: A memory circuit comprising a memory cell array (609) having a plurality of memory cells (901); means (601,605,606,607) for selecting at least one memory cell in said memory cell array; at least one sense amplifier (903) for amplifying a signal from the selected memory cell to generate a read signal (RB) at its output end; means for receiving an input signal (DIN); an input buffer (603) for generating an internal data signal (DIN') at its output end in response to said input signal; means (608) for writing said internal data signal to the selected memory cell; a latch circuit (803) for operatively storing the read signal (RB); an output terminal (DOUT); and means (612) for determing the level of said output terminal in accordance with data appearing at the output (LB) of said latch circuit (803), wherein said latch circuit (803) is coupled to the output end (RB) of said sense amplifier through a first switch circuit (802), characterized in that said latch circuit (803) is coupled to the output end (DIN') of said input buffer (603) through a second switch circuit (302) and that the first switch circuit (802) is rendered conductive in a read mode ( L )and the second switch circuit is rendered conductive in a write mode (WE') thereby to directly determine the level of said output terminal (DOUT) by the internal data signal (DIN') while the internal data signal is under writing operation to the selected memory cell (Fig. 17). The memory circuit according to claim 1, further comprising means for generating a detection signal (i) when at least one address signal is changed. The memory according to claim 1, further comprising means for receiving a write enable signal (WE) and control means for inhibiting an operation of said sense amplifier (903) when said write enable signal is present.

Inventors: KOBAYASHI, YASUO C/O NEC CORPORATION
Applicants: NEC CORPORATION
Application year: 1987
Main classification: G11C7
IPC codes: G11C11, G11C7