Semiconductor memory circuit.

0001This invention relates to a semiconductor memory circuit, and more particularly to a static type semiconductor memory circuit.

0002A semiconductor memory circuit, for example, a static type semiconductor memory circuit basically comprises many memory cells, for storing data, arranged in a matrix, word decoders for specifying the address of memory cells in the matrix from the word direction and column decoders for specifying the address of memory cells from the column direction. These word decoders and column decoders are respectively arranged along one side of the memory cell matrix in a vertical direction and along one side of the memory cell matrix in a lateral direction, and can enable the data stored in one memory cell, specified in both vertical and lateral directions, to be read. In reality, the memory cells are divided into two groups:- a right memory cell group and a left memory cell group, and word decoders are arranged centrally between the right and left memory cell groups. This is because if the word decoders were arranged in the vertical direction on one side of the memory cell matrix a voltage drop between one end and the other of polysilicon electrodes extending in the lateral direction across the matrix from the decoders would be of considerable size. Therefore, the word decoders are located centrally between the right and left memory cell groups in order to reduce a voltage drop along such an electrode by half, and to share equally the voltage drop between the right and left memory cell groups, thereby improving memory functions.

0003However, a semiconductor memory circuit as mentioned above has inevitable disadvantages. The first disadvantage is high power consumption, and the second disadvantage is that output operation speed cannot be made very high even at the expense of such disadvantageous high power consumption. This latter disadvantage arises as follows. Namely, bit current flows to all memory cells storing data "L" (low level data) among memory cells associated with one selected word decoder, as a result of the structure of the semiconductor memory circuit, without regard to whether or not the relevant memory cells are also selected in the bit direction. Therefore, if that bit current, which is mostly unwanted and used in vain, can be at least reduced by half, this would be a measure going a considerable way towards removing such disadvantage above.

0004According to the present invention there is provided a semiconductor memory circuit having right and left memory cell groups and word decoders,corresponding to respective rows of the memory cell groups,located between the right and left memory cell groups, characterised in that the semiconductor memory circuit further comprises a respective right memory cell group selection and drive gate and a respective left memory cell group selection and drive gate provided for each word decoder, and discriminating means, for discriminating to which of the right and left memory cell groups a memory cell to be accessed belongs, either the right or the left memory cell group selection and drive gate being closed in dependence upon the discrimination result provided by the discrimination means.

0005An embodiment 'of the present invention can provide a semiconductor memory circuit which can provide for a reduced bit current and thereby reduced power consumption.

0006An embodiment of the present invention can provide a semiconductor memory circuit which can reduce power consumption by means of a simple additional construction. Further, an embodiment of the present invention can provide a semiconductor memory circuit which can provide for speed up of output operation .

0007An embodiment of the present invention provides discrimination means which discriminate to which of right and left memory cell groups a specified memory cell belongs, right and left memory cell group drive gates provided for each word decoder, one or other of the right and left memory cell group drive gates being closed in accordance with the result of discrimination of the discrimination means.

0008Reference is made, by way of example, to the accompanying drawings, in which:- <ul id="ul0001" list-style="none"><li>Figure 1 is a schematic circuit diagram for assistance in explanation of the structure and operation -of a general semiconductor memory circuit;</li><li>Figure 2 is a schematic circuit diagram illustrating an embodiment of the present invention; and</li><li>Figure 3 is a schematic diagram illustrating arrangement patterns of word decoders and memory cell groups different from that of Figure 2 to which embodiments of the present invention can also be applied.</li></ul>

0009In Figure 1, ML is a left memory cell group and MR is a right memory cell group, and word decoders are provided between the memory cell groups in correspondence to cell rows. However, in Figure 1, only word decoder. WDk of the kth row is indicated. The word decoder WDk drives in common all the bits on the word line WLk.

0010Considering the left memory cell group ML, n memory cells CL1 to CLn are connected along the word line WLk and each of those memory cells comprises a pair of FETs (1, 2) which operate in the complementary mode and a pair of transfer FETs (7, 8). To the pairs of FETs (1, 2) of the memory cells respective bit line pairs (BL1, BL1B) to (BLn, BLnB) are connected. The bit lines (BL1, BL1B) to (BLn, BLnB) respectively output complementary logical bit data. The complementary bits of this bit data on respective bit line pairs are led to respective pairs of FETs (3, 4) connected to respective column decoders CDL1 to CDLn, and input to a sense circuit 21 after being wire ORed for all FETs (3, 4) associated with all column decoders CDL1 to CDLn, and output as a memory output Mout via an output circuit 22. The FETs (5, 6) inserted in each of the bit lines BLIB, BL1 ... BLnB, BLn are load FETs.

0011The abovementioned structure is also adopted for the right memory cell group MR, where CDR1 to CDRn are column decoders, BR1, BR1B ... BRn, BRnB are bit lines, CR1 to CRn are memory cells.

0012If an address is specified for memory cell CL1 (the memory cell connected to the first bit lines (BL1, BL1B) within the left memory cell group ML), the word decoder WDk connected to that memory cell CL1 is selected from among the word decoders by a word address ADWk.

0013The word address ADWk consists of address bits AO (or <o>AO</o>), A1 (or <o>A1),</o> A2 (or <o>A2</o>), A3 (or <o>A3</o>) and A4 (or <o>A4</o>) when the number of memory cells in the column direction is 32.

0014When the word decoder WDk is specified by the word address ADWk, all FETs (9, 10, 11, 12, and 13) constituting word decoder WDk become OFF and therefore word line WDk becomes of "H" (high)level so that the transfer FETs (7, 8) of the corresponding memory cells become ON. Thereby, bit currents iL1 to iLn and iR1 to iRn flow to each FET (7 or 8) on the side of that FET (1 or 2) storing data "L" in each pair of FETs (1, 2) of the memory cells CL1 to CLn and CR1 to CRn on the word line WLk, from power supply Vcc to power supply Vss. Then, the column decoder CDL1 corresponding to the relevant memory cell CL1 from among all the column decoders (CDL1 to CDLn and CDR1 to CDRn) is designated by a column address ADCL1, and the data stored in the memory cell CL1 is output through the sense circuit 21 and output circuit 22 via the bit lines (BL1, BL1B). The Column address ADCL, consists of address bits A5 (or A5), A6 (or A6), A7 (or A7) AS (or A8) and A9 (or A9) when the number of memory cells in the word direction is 32.

0015Focusing attention on the bit currents, even when an address is specified only for memory cell CL1, unwanted and useless bit currents also flow into all memory cells other than the memory cell CL1.

0016The total (unwanted) bit current becomes very heavy and is as high as indicated by the following equation.<maths id="math0001" num=""><img file="EP0032014A2_D0001.tif" /></maths>This means that power consumption becomes high and a high speed operation is considerably restricted because all memory cells must be driven at one time.

0017In an embodiment of the present invention, some additional circuits are connected to each word decoder with a view to reducing the total amount of the bit current by half. Figure 2 shows an embodiment of the present invention. In this Figure, a left memory cell group (ML) drive gate 31L and a right memory cell group (MR) drive gate 31R are provided on the left and right sides of each word decoder, and these left and right memory cell group drive gates 31R and 31L are closed alternatively.

0018The gates 31L and 31R receive respectively the lowest (last) column address bit A9 and its complementary bit <o>A9</o>, for example, which discriminates whether a memory cell specified for access is of the left or the right memory cell group (MR, ML). Since memory cell CL1 of the left memory cell group ML is designated, the address bit A9 is logic "1" i.e. "H" level. Thus, the left memory cell group drive gate 31L opens and simultaneously the right memory cell group drive gate 31R closes. Thus the right side of the word line WLk is not driven and the current indicated by <maths id="math0002" num=""><img file="EP0032014A2_D0002.tif" /></maths> iRr in the total bit current above becomes zero.

0019On the other hand, if a memory cell in the right memory cell group MR is accessed, the address bit A9 is logic "0" i.e. "L" level and the right memory_.cell group drive gate 31R opens and simultaneously the left memory cell group drive gate 31L closes. Therefore, the left part of the word line WLk is not driven and the current indicated by <maths id="math0003" num=""><img file="EP0032014A2_D0003.tif" /></maths> iLr in the total bit current becomes zero. Power consumption is substantially halved.

0020The right and left memory cell group drive gates 31R and 31L respectively comprise transistors (32, 33) and transistors (34, 35). The transistor 32 and transistor 34 are inserted in series in the word line WDk but transistor 33 and transistor 35 are connected in parallelbetween word line WDk and power supply Vss. In principle, the desired gate function can baobtained , by transistor 32 and transistor 34 alone, but it is desirable to keep the corresponding right or left word line forcibly to a specified potential (Vss) by means of transistor 33 or 35 which becomes ON simultaneously with the transistors 34 or 32 when they become ON, in order to prevent that right or left word line floating, which could result in erroneous operation when the transistor 32 or 34 becomes OFF. Namely, the transistors 32 and 33 are ON or OFF complementarily and the transistors 34 and 35 also are ON or OFF complementarily.

0021In addition, transistors 32 and 34 must become ON or OFF alternatively and therefore they require the discrimination results A9 and A9 as input signals. According to the example above, in a case in which the memory cell CL1 is specified, the transistor 34.becomes ON because A9 = "H"- and the transistor 35 becomes OFF because A9 = "L". On the other hand, the transistor 32 becomes OFF because A9 = "L" and the transistor 33 becomes ON because A9 = "H". In this case, the memory cells in the right side can be put into the not-driven condition without being placed in a floating mode.

0022Figure 3 shows examples of arrangement patterns of word decoders and memory cell groups to which the present invention can be applied.

0023In Figure 3, (a) shows an arrangement pattern corresponding to Figure 1.

0024In Figure 3, (b) shows an arrangement pattern in which two word decoders (WD_A, WD_B) are provided between a first right memory cell group MR_A and a first left memory cell group ML_A, and a second right memory cell group MR_B and a second left memory cell group ML_B, respectively.

0025As explained above, embodiments of the present invention can realise semiconductor memory circuits in which power consumption is reduced to almost one half,. that of a existing circuits and which are also suitable for high speed operation.

0026A semiconductor memory circuit embodying this invention is provided with a right memory cell group and a left memory cell group, divided to the right and left, and word decoders corresponding to respective rows of the memory cell groups which are located between the right and left memory cell groups and which specify an address in the word direction of the memory cell groups, and column decoders corresponding to respective columns and which specify an address in the bi_' direction of the memory cell groups. The semiconductor memory circuit comprises right and left memory cell group selection and drive gates provided for each word decoder and means for discriminating whether a memory cell to be accessed is of the right or left memory cell groups.

0027Thereby, the right or left memory cell group selection and drive-gates are closed alternatively in accordance with the discrimination result provided by the discrimination means.