Semiconductor memory device

Abstract: A semiconductor memory device comprises column decoders (12) of the number greater than the number of column addresses of a memory cell array and logical circuits (13, 14) of the same number as that of the column addresses. A column gate of a column is controlled by means of a logical OR between outputs from a plurality of column decoders for decoding different column addresses. As a result, even a column located at an end of the memory cell array can be accessed by means of a logical OR between outputs from a column decoder corresponding to the column and another column decoder.

Description: The present invention relates generally to a random access semiconductor memory (RAM) device, and more particularly to an improvement of column selection of a memory cell array. FIG. 6 shows part of a structure of a memory core portion of a conventional general purpose DRAM. In the memory core portion, memory cells 10 are arranged in a matrix, forming a memory cell array. A sense amplifier 11 is connected to a pair of bit lines in each column of the cell array. The pair of bit lines (21, 22; 23, 24; ...) in each column is connected to a pair of common input/output data lines via a corresponding pair of column selection gates. Column decoders 12 are connected to the respective columns. The pair of column selection gates (31, 32; 33, 34; ...) in each column is controlled by an output of a corresponding column decoder 12. As a result, each column can be selected by an output of the corresponding column decoder both in writing and reading operations. A conventional random access semiconductor memory, such as the aforementioned DRAM, has an architecture in which the number of column decoders is the same as that of column addresses, so that the same number of columns can be selected in both reading and writing operations. A computer system in recent years tends to be formed as a so-called "system on chip", wherein a memory portion and a data processing function portion are mounted on the same semiconductor chip. When the "system on chip" is improved such that the memory itself has the data processing function portion or that an exclusive memory for a limited purpose is to be realized, a new architecture different from the above-described conventional architecture may be introduced with regard to selection of a column of the memory cell array, in which case, the number of columns selected in a reading operation can be different from the number of columns selected in a writing operation. FIG. 7 shows a structure of a memory core portion in a case where an architecture, in which the numbers of selected columns are different in the reading and writing operations, is applied to a conventional DRAM having column decoders of a number which coincides with the number of column addresses. The memory core portion has two pairs of input/output data lines (41, 42 and 43, 44) so that data of two bits can be simultaneously read. More specifically, the pair of bit lines of an odd column is connected to a first pair of input/output data lines (41 and 42) via a corresponding pair of column selection gates, while the pair of bit lines of an even column is connected to a second input/output data lines (43 and 44) via a corresponding pair of column selection gates. The memory core portion further includes a gate circuit comprising a two-input OR gate 13 and a two-input AND gate 14, which corresponds to a pair of column selection gates (31, 32; 33, 34; ...) in each column. An output of the corresponding column decoder 12 is input to one input of each of the two-input OR gates 13 and an output of the corresponding two-input AND gate 14 is input to the other input of the gate 13. A write enable signal (/WE) is input to one input of each of the two-input AND gates 14 and an output of a column decoder (for example, "i+1"th) adjacent to the corresponding column decoder 12 ("i"th) is input to the other input of the gate 14. In the above structure, during a writing operation, the signal /WE is "L" level, with the result that the output of the AND gate 14 in each column becomes "L" level. When an output of the column decoder 12 becomes "H" level, an output of the two-input OR gate 13 becomes "H" level, thereby turning on a pair of column selection gates, so that the corresponding column is selected. As a result, data is input to the selected column via the pair of input/output data lines (41, 42; or 43, 44), so that 1 bit data can be written in the memory cell of the column. During a reading operation, the signal /WE is "H" level. An output from the gate of each column is "H" level, when an input from a column decoder 12 adjacent to the corresponding column decoder of the column decoder row is "H" level. Therefore, one column can be selected not only when an output of the corresponding column decoder (e.g., i) becomes "H" level and an output of the corresponding two-input OR gate 13 becomes "H" level, so that the corresponding pair of column selection gates 31 and 32 can be turned on, but also when an output of the adjacent column decoder (i+1) becomes "H" level and an output of the corresponding two-input OR gate 13 becomes "H" level, thereby turning on the corresponding pair of column selection gates 31 and 32. In other words, in a reading operation, when an output of a column decoder 12 becomes "H" level, the corresponding column and a column corresponding to an adjacent column decoder are simultaneously selected. Hence, data of two bits are read through the two pairs of input/output data lines (41, 42; 43, 44). The above system, in which two columns are accessed by means of a logical OR between outputs of two adjacent column decoders of the column decoders 12 of the same number as the number of column addresses, is called a two-column access system. When this system is to be employed in a memory device, only one column can be selected by an output of the corresponding one column decoder 12 in a column located at an end of the memory cell array (or a sub-array in a case where the array is divided into a plurality of blocks). This system is thus disadvantageous in that data of two bits cannot be read simultaneously. As shown in FIG. 8, as regards a column located at an end (for example, the rightmost column in the drawing), i.e., the column corresponding to the most significant bit of the column address, although this column includes a column decoder (N), there is no column decoder adjacent thereto in the column decoder row (on the right side thereof in the drawing). Accordingly, it is impossible to arrange a logical gate so that the column can be selected when an output from the adjacent column decoder is "H" level. Hence, when the most significant column is designated in a reading operation, since only the most significant column is selected, data of only one bit can be read. To solve this problem, it is proposed to constitute a memory device such that a column at an end of the memory cell array can be accessed by means of a logical OR between outputs from the two column decoders in the columns located both ends of the memory cell array, i.e., the two column decoders at both ends of the column decoder row. In this case, however, it is necessary to arrange a long signal line for supplying an output from the column decoder at an end of the column decoder row to an input node of the logical gate of an output side of the column decoder at the other end of the column decoder row. This long signal line may cause a problem of a signal delay. In addition, since the long signal line has a great parasitic capacitance, the column decoder which supplies an output to the signal line must have a greater driving capacity than the other column decoders. Therefore, the column decoders cannot be arranged in a regular pattern. Moreover, when the aforementioned signal line is arranged, the pattern layout of the column decoder row is inevitably limited. As described above, in the conventional semiconductor memory device wherein the number of column decoders is the same as the number of columns, if an additional column is to be selected by a logical OR between outputs from a plurality of column decoders in order to access a plurality of columns simultaneously, a column located at an end of the memory cell array can be selected only by an output of the corresponding column decoder, i.e., only one column can be selected. Therefore, it is difficult to achieve a system in which a plurality of columns can be simultaneously accessed. The present invention has been made to solve the above problem of the conventional art. It is accordingly an object of the present invention to provide a semiconductor memory device, wherein when an additional column is to be selected by means of a logical OR between outputs from a plurality of column decoders in order to access a plurality of columns simultaneously, even a column located at an end of the memory cell array can be selected by a logical OR between outputs from the corresponding column decoder and another column decoder. A semiconductor memory device of the present invention comprises: a memory cell array in which a plurality of memory cells are arranged in a matrix, the memory cell array including a plurality of groups of columns; a plurality of column gates each respectively connected to one of the plurality of columns of the memory cell array; a plurality of data lines each provided in common for a selected column in each group of said columns and each connected in common to a first end of the column gate of said selected column; a column decoder row including column decoders of the number greater than the number of column addresses of the memory cell array, for decoding a column address signal which designates a column of the memory cell array; and a plurality of logical circuits, provided for columns of different column addresses in the memory cell array, for controlling the column gates of the respective columns by means of a logical OR between outputs from a predetermined plurality of column decoders for decoding different column addresses of the column decoder row. According to the present invention, the number of column decoders for decoding column addresses to designate a column of the memory cell array is greater than the number of column addresses. With the above structure, a column corresponding to one column address can be selected by means of a logical OR between outputs from a plurality of column decoders for decoding different column addresses. As a result, even a column located at an end of the memory cell array can be selected by means of a logical OR between outputs from the corresponding column decoder and another column decoder. Further, the logical circuits are controlled by control signals which vary in logical level in reading and writing operations, with the result that the number of columns selected in the reading operation can be different from the number of columns selected in the writing operation. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1 is a circuit diagram showing part of a memory core portion of a DRAM according to a first embodiment of the present invention; FIG. 2 is a circuit diagram showing a modification of the memory core shown in FIG. 1; FIG. 3 is a circuit diagram showing another modification of the memory core shown in FIG. 1; FIG. 4 is a circuit diagram showing part of a memory core portion of a DRAM according to a second embodiment of the present invention; FIG. 5 is a circuit diagram showing part of a memory core portion of a DRAM according to a third embodiment of the present invention; FIG. 6 is a circuit diagram showing part of a memory core portion of a conventional general-purpose DRAM; FIG. 7 is a circuit diagram showing part of a memory core portion proposed in a case where an architecture, in which the numbers of selected columns are different in the reading and writing operations, is applied to a conventional DRAM; and FIG. 8 is a block diagram showing an overall column decoder group shown in FIG. 7. An embodiment of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a circuit diagram showing part of a memory core portion of a DRAM according to a first embodiment of the present invention. Memory cells 10 are arranged in a matrix, thereby forming a memory cell array. Each column of the memory cell array has a pair of bit lines (21, 22; 23, 24; ...). Word lines WL are connected to the memory cells in the same row of the memory cell array. Sense amplifiers 11 are connected to the pairs of bit lines of the respective columns. A plurality of pairs of column gates (31, 32; 33, 34; ...) are connected to the pairs of bit lines of the respective columns. Column decoders 12 decode a column address signal for designating a column of the memory cell array. The number of the column decoders 12 is greater than the number of column addresses of the memory cell array. The row i.e. the arrangement, of the column decoders 12 of this embodiment is comprised of a plurality of column decoders (0 to N) corresponding to the respective columns of the memory cell array and an additional column decoder (N + ). The additional column decoder (N + ) is to decode a column address which designates a column located at the right end in the drawing, i.e., the column corresponding to the most significant bit of the column address. Thus, in the embodiment shown in FIG. 1, a column decoder for decoding the column address, which designates the column at an end of the memory cell array, is provided at each end of the arrangement of the row of the column decoders 12. Further, in the embodiment shown in FIG. 1, logical circuits are respectively provided for the columns of the different addresses in the memory cell array. In other words, the memory cell array includes the logical circuits of the number which coincides with the number of column address. Each logical circuit controls the column selection gate of the corresponding column by means of a logical OR between outputs of predetermined column decoders in the column decoder row (including the column decoder of the corresponding column) for decoding different column addresses. In this embodiment, the logical circuits are provided in the respective columns of the memory cell array, so as to obtain a logical OR between outputs of two adjacent column decoders in the column decoder row. For example, a logical circuit comprising a two-input OR circuit (OR gate) 13 and a two-input AND circuit (AND gate) 14 is connected to each pair of column selection gates (31, 32; 33, 34; ...). A control signal, which varies in logical level in writing and reading operations, i.e., a write enable signal (/WE), is input to one input terminal of the two-input AND gate 14. An output of a column decoder 12 (an adjacent column decoder in this embodiment), which is different from the column decoder for decoding the column address of the corresponding column, is input to the other input terminal of the two-input AND gate 14. An output of the column decoder 12 for decoding the column address of the corresponding column is input to one input terminal of the two-input OR gate 13. An output of the corresponding two-input AND gate 14 is input to the other input terminal of the two-input OR gate 13. The columns of the memory cell array are divided into a plurality of (two in this embodiment) groups. The pairs of data lines (41, 42; and 43, 44 in this embodiment) are provided for the respective groups. The plurality of pairs of data lines are connected to first ends of the pairs of column selection gates (31, 32; 33, 34; ...) of the respective groups. The memory cell array of this embodiment includes two pairs of input/output data lines (41, 42; and 43, 44), so that data of two bits can be simultaneously read. The pair of bit lines of an odd column is connected to a first pair of input/output data lines (41, 42) of the two pairs of the input/output data lines via the corresponding pair of column selection gates. The pair of bit lines of an even column is connected to a second pair of input/output data lines (43, 44) of the two pairs of input/output data lines via the corresponding pair of column selection gates. With the above structure, in a writing operation, the signal /WE becomes "L" level and outputs from the AND gates 14 of the respective columns become "L" level. When an output of the column decoder 12 of a column becomes "H" level, an output from the corresponding two-input OR gate 13 becomes "H" level, thereby turning on the corresponding pair of column selection gates. As a result, data is input to the selected column through the first pair of input/output data lines (41, 42), so that data of one bit can be written in the memory cell of the selected column. In a reading operation, the signal /WE is "H" level. An output from the gate of each column is "H" level, when an input from a column decoder 12 adjacent to the corresponding column decoder of the column decoder row is "H" level. Therefore, one column can be selected not only when an output of the corresponding column decoder (e.g., 0) becomes "H" level and an output of the corresponding two-input OR gate 13 becomes "H" level, thereby turning on the corresponding pair of column selection gates 31 and 32, but also when an output of the adjacent column decoder (N + ) becomes "H" and an output of the corresponding two-input OR gate 13 becomes "H" level, thereby turning on the corresponding pair of column selection gates 31 and 32. In other words, in a reading operation, when an output of a column decoder 12 becomes "H" level, the corresponding column and a column corresponding to an adjacent column decoder are simultaneously selected. Hence, data of two bits are read through the two pairs of input/output data lines (41, 42; 43, 44). In the DRAM of the embodiment as described above, the number of column decoders 12 for decoding a column address to designate a column of the memory cell array is greater than the number of column addresses. Hence, a column corresponding to one column address can be selected by means of a logical OR between outputs from two adjacent column decoders 12 for decoding different column addresses. In this manner, even a column located at an end of the memory cell array can be accessed by means of a logical OR between outputs from the corresponding column decoder 12 and another column decoder 12 adjacent thereto. Further, the logical circuits are controlled by control signals which vary in logical level in reading and writing operations, so that the number of columns selected in the reading operation can be different from the number of columns selected in the writing operation. In the above embodiment, the additional column decoder (N + ) is provided to decode a column address which designates the rightmost column in the drawing (the column corresponding to the most significant bit of the column address) of the memory cell array. However, the embodiment can be modified as shown in FIG. 2. A memory core portion shown in FIG. 2 comprises an additional column decoder (0 + ) for decoding a column address which designates the leftmost column in the drawing (the column corresponding to the least significant bit of the column address) of the memory cell array. In FIG. 2, the same elements as shown in FIG. 1 are identified with the same reference numerals as used in FIG. 1. In the above embodiment, a column is selected by means of a logical OR between outputs from two adjacent column decoders in the column decoder row. However, the embodiment can be modified as shown in FIG. 3, in which a column can be selected by means of a logical OR between outputs from two columns, not adjacent to each other, in the column decoder row. The memory core portion shown in FIG. 3 is the same as that shown in FIG. 1, except for the arrangement of column decoders and the connection of logical circuits. Hence, the same elements as shown in FIG. 1 are identified with the same reference numerals as used in FIG. 1. In this modification, two additional column decoders 0 + and 1 + are provided at an end (the right end in FIG. 3) of the row of a plurality of column decoders 0 to N corresponding to the respective columns of the memory cell array. The two additional column decoders 0 + and 1 + are to decode column addresses decoded by column decoders O and 1, respectively. Each of logical circuits (13, 14) is connected to a pair of two alternately located column decoders 12 so as to obtain a logical OR between outputs of the two column decoders 12. FIG. 4 shows part of a memory core portion of a DRAM according to a second embodiment of the present invention. The memory core portion shown in FIG. 4 is the same as that shown in FIG. 1, except that the memory cell array is divided into a plurality of blocks (sub-arrays) 51 and a column decoder row 52, common to two sub-arrays, is formed between the regions of the two sub-arrays. The relationship between a sub-array 51 and a column decoder row 52 is the same as that in the memory core section shown in FIG. 1, 2 or 3. Therefore, a detailed structure of the second embodiment is not shown in the drawings. FIG. 5 shows part of a memory core portion of a DRAM according to a third embodiment of the present invention. The memory core portion shown in FIG. 4 is the same as those shown in FIGS. 1 to 3, except that one column address is assigned to eight columns so that data of 8 bit can be written simultaneously and data of 16 bit can be read simultaneously. Hence, the same elements as shown in FIG. 1 are identified with the same reference numerals as used in FIG. 1. As shown in FIG. 5, one logical circuit (13, 14) and one column decoder 12 are assigned to eight columns. The third embodiment realizes a multi-bit DRAM having an architecture in which data of one byte can be written in a writing operation and data of 2 bytes can be read in a reading operation. With such a DRAM, the following functions can be achieved. (1) A function of successively writing code data of two bytes into two different columns, revising data of one byte for code conversion, and then simultaneously reading code data of two bytes. With this function, code data of one byte can be converted to code date of two bytes. (2) A function of successively writing data of two bytes, including a flag bit, into two different columns, revising data of one byte including the flag bit and then simultaneously reading code data of two bytes. With this function, the number of bits, which are accessed in the data revision, can be reduced. (3) A function of successively writing data of two bytes, consisting of RGB data for forming a colored image and luminance data, into two different columns, revising data of one byte including the luminance data for adjusting the tone of the colored image, and then simultaneously reading code data of two bytes. With this function, the number of bits, which are accessed in the data revision, can be reduced. (4) A function of successively writing the same data of one byte into two different columns and simultaneously reading the two pieces of one byte data. With this function, the reliability of read data can be checked by detecting whether the two pieces of one byte data coincide with each other. (5) A function of successively writing data of two field images, obtained by interlaced scanning, into two different columns and simultaneously reading the data of the two field images. With this function, it is possible to obtain data of one frame image of a non-interlaced style or a correlation between the two field images. In the above-described embodiments, the number of columns selected in a reading operation is greater than the number of columns selected in a writing operation. However, the former can be less than the latter. The arrangement of the column decoders and the interconnection between the column decoders and the logical circuits in the DRAM of each of the above embodiments can be variously modified. Further, the present invention can be applied not only to the DRAM but also the other semiconductor memories. As has been described above, according to the present invention, there is provided a semiconductor memory device of the present invention, wherein when an additional column is to be selected by means of a logical OR between outputs from a plurality of column decoders in order to access a plurality of columns simultaneously, even a column located at an end of the memory cell array can be selected by a logical OR between outputs from the corresponding column decoder and another column decoder.

Claims: A semiconductor memory device comprising: a memory cell array in which a plurality of memory cells (10) are arranged in a matrix, the memory cell array including a plurality of groups of columns; a plurality of column gates (31 - 38) each respectively connected to one of the plurality of columns of the memory cell array; a plurality of data lines (41 - 74) each provided in common for a selected column in each group of said columns and each connected in common to a first end of the column gate of said selected column; a column decoder row including column decoders (12), for decoding a column address signal which designates a column of the memory cell array; and a plurality of logical circuits (13, 14), provided for columns of different column addresses in the memory cell array, for controlling the column gates of the respective columns by means of a logical OR between outputs from a predetermined plurality of column decoders for decoding different column addresses of the column decoder row, characterized in that the number of said column decoders is greater than the number of column addresses of the memory cell array. The semiconductor memory device according to claim 1, characterized in that at least one column decoder for decoding a column address which designates a column at an end of the memory cell array is provided at each end of the column decoder row. The semiconductor memory device according to claim 1 or 2, characterized in that each of the logical circuits carries out a logical OR between outputs of a pair of adjacent column decoders in the column decoder row. The semiconductor memory device according to claim 3, characterized in that at least one column decoder for decoding a column address which designates a column at an end of the memory cell array is connected to each of ends of the column decoder row, and each of the logical circuits carries out a logical OR between outputs of adjacent two column decoders in the column decoder row. The semiconductor memory device according to claim 1, 2, 3 or 4, characterized in that each of the logical circuits comprises: a logical AND circuit (14) for carrying out a logical AND between a control signal, which varies in logical level in reading and writing operations, and output signals from column decoders of the predetermined number, different from a column decoder for decoding a column address of the column corresponding to the logical circuit; and a logical OR circuit (13) for carrying out a logical OR between an output of the column decoder for decoding the column address of the column corresponding to the logical circuit and an output from the logical AND circuit.

Inventors: MIYANO SHINJI, SATO KATSUHIKO, YABE TOMOAKI, MIYANO, SHINJI, SATO, KATSUHIKO, YABE, TOMOAKI, MIYANO, SHINJI, INT. PROPERTY DIVISION, SATO, KATSUHIKO, INT. PROPERTY DIVISION, YABE, TOMOAKI, INT. PROPERTY DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1995
Main classification: G11C8
IPC codes: G11C11, G11C8