Memory device with reduced power dissipation

Abstract: A memory device comprises an array (1) of memory cells (2,DMC0-DMCn) arranged in rows (R1-Rn) and columns (C1-Cm,DC), a plurality of gates (RD1-RDn) for transmitting respective selection outputs (RS1-RSn) of a row decoder (3) to respective rows (RS), a dummy column (DC) of dummy memory cells (DMC0-DMCn) substantially indentical to the memory cells, precharge means (P4,P5,P6,P7) for precharging the columns and the dummy column at a precharge potential (VDD) when no row is selected, and programming means (N7,N8,7) for setting selected columns at respective programming potentials. The device comprises dummy memory cell preset means (P3) for presetting the dummy memory cells in a first logic state when no row is selected; dummy column programming means (N9,N10) for setting the dummy column at a prescribed programming potential corresponding to a second logic state opposite to the first logic state; first detector means (DET1) for detecting that the dummy column has discharged from the precharge potential to the prescribed programming potential and for consequently enabling said plurality of gates. Each of said gates has an input coupled to a respective dummy memory cell so that the gate is disabled as soon as the respective dummy memory cell has switched from said first logic state to said second logic state.

Description: The present invention pertains to the field of integrated circuits, particularly to integrated memory circuits. More particularly, the invention relates to integrated Random Access Memories (RAMs), for example static RAMs. Conventionally, a memory device such as a static RAM comprises four main circuit blocks: a bidimensional array of readable and writable memory cells arranged in rows and columns, a row decoder circuit block for selecting the desired rows inside the array, a column decoder circuit block for selecting the desired columns of the array, and read/write circuits. In general, the memory cell array comprises n rows and m columns; usually, an information word of p bits is stored in the memory cells belonging to a same row. When the memory device is operated in active mode (for reading or programming), the row decoder circuits select one row of the array (i.e., m memory cells are simultaneously selected) and the column decoder circuit allows to select the desired columns among the m columns of the array: this means, m memory cells are selected, but only a subset p of them is actually accessed. In active mode the current consumption of the memory array is therefore given by: Iddr = m x Cb x ΔVr for the read operation, and Iddw = p x Cb x ΔVw + (m - p) x Cb ΔVr for the write operation, where Cb is the capacitance of each column, ΔVw is the voltage swing of the columns to which the memory cells to programmed belong, and ΔVr is the voltage swing of the remaining columns. The most common approach to reduce the current consumption consists in reducing the column capacitance Cb. Another approach which can be used, also in combination with the previous one, consists in partitioning the memory cell array into multiple blocks, selectively activatable; in this way the number m of cells connected to each row is reduced. Still another technique provides for limiting the voltage swing ΔVr by controlling the width of the time pulse during which the row is selected; this technique however is used only for the read operation. In view of the state of the art described, it is an object of the present invention to provide a memory device having a Low current consumption when it is accessed in write mode. According to the present invention, said object is attained by means of a memory device comprising an array of memory cells arranged in rows and columns, a plurality of gates for transmitting respective selection outputs of a row decoder to respective rows, a dummy column of dummy memory cells substantially indentical to the memory cells, precharge means for precharging the columns and the dummy column at a precharge potential when no row is selected, and programming means for setting selected columns at respective programming potentials, characterized in that it comprises: dummy memory cell preset means for presetting the dummy memory cells in a first logic state when no row is selected; dummy column programming means for setting the dummy column at a prescribed programming potential corresponding to a second logic state opposite to the first Logic state; first detector means for detecting that the dummy column has discharged from the precharge potential to the prescribed programming potential and for consequently enabling said plurality of gates; and in that each of said gates has an input coupled to a respective dummy memory cell so that the gate is disabled as soon as the respective dummy memory cell has switched from said first logic state to said second logic state. The memory device of the present invention has a Lower current consumption than conventional devices: in fact, when the memory device has to be programmed, the desired row remains selected for only a short time strictly necessary to the memory cells to switch the desired state; the dummy memory cells are used to monitor the switching time of the memory cells. The unselected columns are thus not completely discharged. These and other features and advantages of the present invention will be made apparent by the following detailed description of a particular embodiment thereof, illustrated by way of a non-limiting example in the accompanying drawings, wherein: Figure 1 is a schematic block diagram of a memory device; Figure 2 is a more detailed circuit diagram of the memory device of Figure 1, showing the features of the present invention; Figure 3 shows the circuit diagram of a memory cell; Figure 4 is a schematic circuit diagram of a precharge and programming circuit for a memory cell column of the memory device shown in Figure 2; Figure 5 is a schematic circuit diagram of a precharge and programming circuit for a dummy column of the memory device of Figure 2; and Figure 6 is a time diagram of some signals of the circuit of Figure 2. Figure 1 schematically shows the main circuit blocks of a memory device according to the present invention, such as for example a static RAM (SRAM). The memory device comprises a bidimensional array 1 of readable and programmable memory cells 2; conventionally, within the array 1 the memory cells 2 are arranged in n rows R1-Rn and m columns C1-Cm. A row decoder circuit 3 is supplied with row address signals X1-Xk and allows to select one row R1-Rn of the array, depending on the specific Logic value of signals X1-Xk. Similarly, a column decoder circuit 4 is supplied with column address signals Y1-Yh and allows to select a desired group of p columns of the array, according to the logic value of signals Y1-Yh. In the schematic diagram of Figure 1 block 4 is also intended to include the sensing circuits necessary for reading the selected memory cells 2 and for outputting read data Q1-Qp; block 4 also include programming circuits for programming the memory cells of the selected columns according to the logic states data signals D1-Dp. A control circuit 5 is provided for controlling blocks 3 and 4 in accordance to the particular operation mode of the memory device. Figure 2 shows in more detail the structure of the memory array 1 in the case the memory device is a static RAM. Each column C1-Cm comprises a pair of complementary bit lines BL, BL'. The memory cells 2 are, in this particular case, six transistor memory cells (6T memory cells). The known circuit structure of the memory cells 2 is shown in Figure 3. The memory cell 2 conventionally comprises two inverters I1, I2, for example of the CMOS type. Inverter I1 comprises a P-channel pull-up MOSFET P1 and an N-channel pull-down MOSFET N1 connected in series between a voltage supply VDD and ground. Inverter I2 similarly comprises a P-channel pull-up MOSFET P2 and an N-channel pull-down MOSFET N2 connected in series between VDD and ground. An output node A of inverter I1 is connected to the input node of inverter I2 (i.e., to the gates of P2 and N2); similarly, an output node B of inverter I2 is connected to an input node of inverter I1 (i.e., to the gates of P1 and N1). The memory cell further comprises a first and a second selection transistors N3, N4, formed for example by N-channel MOSFETds: N3 couples node A to a first input/output node O of the memory ceLL 1, while N4 couples node B to a second, Logically complemented input/output node O' of the memory cell. Node O is connected to a first bit Line BL of the pair of complementary bit lines of a generic column Ci of the memory array, and node O' is connected to a second bit Line BL' of said pair. The gates of the selection transistors N3 and N4 are both connected to a generic row Rk of the memory array. Referring now again to Figure 2, the memory array 1 comprises, in addition to rows R1-Rn and columns C1-Cm, a dummy row DR and a dummy column DC of memory cells. The dummy column DC comprises (n+1) dummy memory cells DMC0-DMCn, each of which similarly to the memory cells 2 comprise two cross-coupled inverters I3, I4 (for example, of the CMOS type), and two selection transistors N5, N6 for coupLing the output nodes A, B of the inverters to a pair of complementary dummy bit Lines DBL, DBL' of the dummy column; the gates of the selection transistors N5 and N6 of dummy memory cell DMCO are connected to the dummy row DR, the gates of the selection transistors N5, N6 of dummy memory cells DMC1-DMCn are connected to a respective one of rows R1-Rn of the memory array. Each dummy memory cell DMC0-DMC1 further comprises a pre-set P-channel MOSFET P3 connected to node A and to the voltage supply VDD, and with gate connected to the dummy row DR, in the case of DMCO, and to a respective one of the rows R1-Rn in the case of DMC1-DMCn. The dummy row DR comprises the dummy memory cell DMCO and m memory cells 2. As visible in Figure 2, the row decoder circuit 3 comprises, in a per-se known way, a predecoder 6 supplied by the row address signals X1-Xk and having n output signals RS1-RSn individually activatable to select one of the n rows R1-Rn of the array. The row decoder circuit 3 further comprises n row drivers RD1-RDn, each one controlling the selection of a respective row R1-Rn and each one substantially comprised of an AND gate with a first input supplied with a respective one of signals RS1-RSn, a second input supplied with an enable signal EN generated by the control circuit 5, and a third input supplied with a respective signal RES1-RESn taken from node A of the dummy cell DMC1-DMCn belonging to said respective row. The row decoder circuit 3 also comprises a dummy row driver DRD substantially formed by an AND gate with a first input supplied with signal EN and a second input connected to VDD. The control circuit 5 is supplied in a per-se known way with a clock signal CK, and is further supplied with a signal SBW generated by a first level detector DET1 detecting the voltage level of the dummy bit Line DBL of the dummy column DC, and with a signal ENDW generated by a second level detector DET2 detecting the voltage Level of node A of the dummy cell DMCO belonging to the dummy row DR. The control circuit 5 generates, in addition to signal EN, a precharge signal PRE and a write signal WE supplied to the column decoder circuit 4. Signals PRE and WE are also supplied to a write and precharge circuit 4' for the dummy column DC. Figure 4 schematically shows the circuit structure of a write and precharge circuit of the column decoder circuit 4. m of such circuits are provided, each one associated to a respective column Ci of the memory array. Signal PRE controls first switching means, formed for example by P-channel MOSFETs P4, P5, for selectively connecting the complementary bit Lines BL and BL' of the column to VDD. Second switching means are also provided, for example represented by N-channel MOSFETs N7, N8, for selectively connecting the bit lines BL, BL' of the column Ci to a data signal Dj and, respectively, to the logic complement thereof; data signal Dj is one of the p data signals D1-Dp shown in Figure 1. N5 and N6 are controlled by an AND gate 7 supplied with signal WE and with a column selection signal CSi, generated by a column predecoder (not shown because per-se known) which is supplied with the columns address signals Y1-Yh; signal CSi is activated to select the respective column Ci. The column decoder circuit 4 further comprises, in addition to the m write and precharge circuits of Figure 4 and the predecoder circuit, sensing circuits for reading the information stored in the selected memory cells. Figure 5 schematically shows the structure of the dummy write and precharge circuit 4' for the dummy column DC. Similarly to the circuit of Figure 4, first switching means formed by P-channel MOSFETs P6, P7 controlled by signal PRE, are provided to selectively connect the complementary dummy bit Lines DBL, DBL' to VDD; second switching means, formed by N-channel MOSFETs N9, N10, and controlled by signal WE, are provided for selectively connecting bit Line DBL to ground, and bit Line DBL' to VDD. The operation of the circuits previously described will be now explained with the help of the timing diagram of Figure 6. Before the start of a write operation of the memory device, signal PRE is activated ("0") and signal WE is deactivated ("0"); signal EN is also deactivated ("0"), so to keep all the rows R1-Rn of the array deactivated (i.e., at the ground potential); in this way, transistors N3 and N4 of all the memory cells 2 are off and nodes A and B of the memory cells are disconnected from the bit Lines BL, BL'. Transistors P4 and P5 in the write and precharge circuits, as well as transistors P6 and P7 in the dummy write and precharge circuit 4', are activated, and all the bit lines BL, BL' and the dummy bit Lines DBL, DBL' are connected to (and precharged at) VDD. Transistors N5 and N6 of the dummy memory cells DMC0-DMCn are off, and nodes A and B of the dummy cells are disconnected from the dummy bit Lines DBL, DBL'; however, the pre-set transistors P3 are on, and nodes A of the dummy memory cells are connected to VDD: in this way, the dummy memory cells are preset in the state A="1", B="0". At the rising edge of clock signal CK (Figure 6) signal PRE is deactivated ("1"), and signal WE is activated ("1"); P4 and P5 in the write and precharge circuits of the columns are turned off, so that all the bit Lines BL and BL' of the array are disconnected from VDD. The same holds true for the dummy column: P6 and P7 in the respective write and precharge circuit are turned off and the dummy bit Lines DBL, DBL' are disconnected from VDD. Subsequently control circuit 5 activates signal WE to start the write operation. In the write and precharge circuits associated to the selected columns (CSi="1") N7 and N8 are turned on; according to the logic value of Dj, the bit lines BL, BL' of a generic selected column Ci are either kept at VDD or are discharged: for example, if in column Ci Dj="0", the bit Line BL of said column is discharged. If instead Dj="1", it is the bit line BL' that is discharged. The bit lines of the unselected columns (CSi="0") remain charged at VDD. The activation of signal WE always causes the dummy bit Line DBL of the dummy column DC to be discharged. The discharge time of DBL is substantially identical to the discharge time of a generic bit Line BL or BL' of the array. When the potential of DBL falls below the threshold of level detector DET1, signal SBW is activated to inform control circuit 5 that the bit Lines of the array to be discharged have reached the desired voltage. Control circuit 5 then activates signal EN ("1"), so that the potential of the addressed row Rq (Figure 6) of the memory array (i.e. the row for which the respective selection signal RS1-RSn is activated) is driven high by the respective driver RD1-RDn; also, activation of signal EN causes the selection of the dummy row DR. Selection of the addressed row Rq causes the activation of transistors N3 and N4 in aLL the memory cells 2 and transistors N5, N6 in the dummy cell DMC belonging to said row; similarly, selection of the dummy row causes the activation of transistors N5, N6 of the dummy memory cell DMCO belonging to the dummy row. Since during the precharge phase, before the rising edge of CK, the dummy memory cells DMC0-DMCn are all preset in the condition A="1", B="0" (by means of the presetting transistors P3), and since the dummy write and precharge circuit 4' is designed such that the potential of bit line DBL is set to "0", after the selection of the addressed row R1-Rn and of the dummy row DR, the data stored in the dummy memory cells DMC of the dummy column DC must change from A="1", B="0" to A="0", B="1". At the same time, those memory cells 2 of the selected row and belonging to the selected columns which store a datum opposite to that which is desired to be programmed must change their state; in the unselected columns, one of the two complementary bit Lines BL or BL' will always be discharged. The dummy memory cell DMC1-DMCn belonging to the selected row serves to monitor the change of the state of the memory cells 2 which are to be programmed: as soon as the potential of node A of the dummy cell falls below the threshold voltage of the NAND gate of the respective row driver circuit, it is assumed (since the dummy memory cell is identical to the other memory cells 2 of the row) that also the other memory cells 2 of the row have changed their programming state; the row selection signal R1-Rn is thus deactivated, and the transistors N3, N4 of all the memory cells 2 of the row are turned off. In this way, the bit lines BL or BL' of the unselected columns are not completely discharged, because the row remains selected only for a time sufficient to cause the memory cells to be programmed to change their state. The change of state of the dummy memory cell DMCO of the dummy row cause the Level detector DET2 to activate signal ENDW, which in turn causes the control circuit 5 to deactivate signals EN and WE and to activate signal PRE. By providing the dummy, column DC physically adjacent to the row driver circuits RD1-RDn of the row decoder circuit, the deselection of the row after the change of state of the respective dummy memory cell is almost instantaneous; signal SBW shall instead propagate a higher distance from the dummy row to the control circuit, so that a suitable delay is provided between the activation of one of signals RES1-RESn and the deactivation of signal EN by control circuit 5. In this way, as shown by curves 100 and 100' in Figure 6, the voltage swing of unselected bit lines (curve 100) is much Less that the voltage swing of unselected bit Lines in the prior art circuits (curve 100'), because the time interval during which the accessed row Rq remains activated is shorter. In alternative to the embodiment of Figure 2, it should be possible not to provide the dummy row; signal ENDW, necessary for resetting the control circuit, could in this case be provided by OR-ing together all the signals RES1-RESn; this is however disadvantageous, and sometimes impracticable, in the case of Large memories, wherein the number of signals RES1-RESn to be OR-ed together can be of several thousands.

Claims: A memory device comprising an array (1) of memory cells (2,DMC0-DMCn) arranged in rows (R1-Rn) and columns (C1-Cm,DC), a plurality of gates (RD1-RDn) for transmitting respective selection outputs (RS1-RSn) of a row decoder (3) to respective rows, a dummy column (DC) of dummy memory cells (DMCO-DMCn) substantially indentical to the memory cells, precharge means (P4,P5,P6,P7) for precharging the columns and the dummy column at a precharge potential (VDD) when no row is selected, and programming means (N7,N8,7) for setting selected columns at respective programming potentials, characterized in that it comprises: dummy memory cell preset means (P3) for presetting the dummy memory cells in a first Logic state when no row is selected; dummy column programming means (N9,N10) for setting the dummy column at a prescribed programming potential corresponding to a second logic state opposite to the first Logic state; first detector means (DET1) for detecting that the dummy column has discharged from the precharge potential to the prescribed programming potential and for consequently enabling said plurality of gates; and in that each of said gates has an input coupled to a respective dummy memory cell so that the gate is disabled as soon as the respective dummy memory cell has switched from said first logic state to said second logic state. A memory device according to claim 1, characterized by comprising a dummy row (DR) containing one dummy memory cell (DMC0) of the dummy column (DC) and which is always selected when said plurality of gates (RD1-RDn) is enabled, and second level detector means (DET2) for detecting that said one dummy memory cell (DMC0) has switched from the first to the second Logic state and for consequently disabling said plurality of gates. A memory device according to claim 2, characterized by comprising control means (5) activating said programming means (N7,N8,7) and dummy column programming means (N9,N10) in response to a clock signal (CK), said control means being supplied by the first and second detector means (DET1,DET2) and supplying a common enable signal (EN) to said plurality of gates (RD1-RDn) and to a dummy row gate (DRD) for enabling/disabling said plurality of gates and said dummy row gate. A memory device according to claim 3, characterized in that each of said memory cell (2) and dummy memory cell (DMCO-DMCn) comprises a bistable element (I1,I2;I3,I4) with a first (A) and a second nodes (B) storing complementary data, first (N3;N5) and second selection means (N4;N6) for respectively coupling the first (A) and second nodes (B) to a first (BL;DBL) and a second complementary bit Line (BL';DBL') of a respective column (C1-Cm) or, respectively, of the dummy column (DC), said first (N3;N5) and second selection means (N4;N6) being commonly controlled by a respective row (DR,R1-Rn). A memory device according to claim 4, characterized in that each gate (RD1-RDn) of said plurality has an input coupled to the first node (A) of the respective dummy memory cell (DMC1-DMCn), and said second detector means (DET2) are coupled to the first node (A) of the dummy memory cell (DMC0) of said dummy row (DR). A memory device according to claim 5, characterized in that said bistable element comprises two cross-coupled inverters (I1,I2;I3,I4), the first node (A) being an output of a first inverter (I1;I3) and the second node (B) being an output of a second inverter (I2;I4). A memory device according to claim 6, characterized in that said inverters (I1,I2,I3,I4) are CMOS inverters. A memory device according to claim 7, characterized in that said first (N3,N5) and second selection means (N4,N6) comprise each a transistor with first and second terminals connected between the first node (A), or respectively the second node (B), of the bistable element (I1,I2;I3,I4) and the first (BL,DBL), or respectively the second complementary bit Lines (BL',DBL'), and a control terminal connected to the respective row (R1-Rn,DR). A method for programming a memory device comprising a matrix of memory cells (2) arranged in rows (R1-Rn) and columns (C1-Cm), and a dummy column (DC) of dummy memory cells (DMCO-DMCn) substantially identical to the memory cells (2), the method providing for: a) precharging the columns to a precharge potential (VDD) when no row is selected; b) setting selected columns to respective programming potentials corresponding to data to be programmed into the memory cells, before selecting a row; c) selecting a row for transferring said data to be programmed to memory cells belonging to the selected row and to the selected columns, characterized by providing for: during phase a), precharging the dummy column to said precharge potential, and simultaneously presetting the dummy memory cells in a first Logic state; during phase b), setting the dummy column to a respective programming potential corresponding to a second Logic state opposite to the first Logic state; waiting until the dummy column has discharged to said programming potential and then performing phase c); as soon as the dummy memory cell associated to the selected row has switched from the first to the second Logic state, deselecting the selected row.

Inventors: BARONI ANDREA, RIMONDI DANILO, TALIERCIO MICHELE, TORELLI COSIMO, BARONI, ANDREA, RIMONDI, DANILO, TALIERCIO, MICHELE, TORELLI, COSIMO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1997
Main classification: G11C7
IPC codes: G11C11, G11C7