Semiconductor device improving error correction processing rate

In an exclusive OR circuit (XOR gate) constituting an ECC circuit, the drivability of P channel MOS transistors is set larger than the drivability of N channel MOS transistors. Accordingly, the speed of the logic level of an output node being set to an H level from an L level identified as a reset state is increased than the case where the drivability is set equal. Thus, the time required to output a syndrome from a plurality of stages of XOR gates can be reduced to allow execution of error correction processing at high speed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device including an error correcting circuit (ECC).

2. Description of the Background Art

In semiconductor devices such as semiconductor memory devices, bit errors caused by hardware failure are encountered. There is also known a phenomenon called “soft error”, caused by the generation of pairs of electrons and holes in the silicon substrate when radiation such as α rays and neutron rays present in nature is introduced into the chip, leading to the possibility of destroying, in the worst case, data stored in the storage node of a memory cell.

Reflecting the recent progress in semiconductor processing, i.e. development in microminiaturization, the size of the storage element per se has been reduced in contrast to the increase of the storage capacity. The capacity of the storage node storing data has become smaller. It is known that the resistance with respect to soft error becomes lower as the storage capacity of the storage node in which data is stored is reduced. The bit error caused by such soft error has become a critical problem.

There is conventionally known a semiconductor device including an ECC circuit that executes error correction processing on a bit error to address such bit errors.

For example, when error correction processing using a Hamming code is to be executed, the so-called parity bit of n bits is employed. When there is a bit error in the regular data of m bits, the bit error is identified using the parity bit. Then, the data bit is inverted, for example, and output. The number of bits “n” of the parity bit is set so that the relationship of 2n−m≧m+1 is established based on the relationship between the regular data of m bits and the parity bit of n bits.

More specifically, a predetermined combination using such parity bits indicates the position information, which is called “syndrome”, to identify the error position, i.e. the location where a bit error has occurred. In data readout, the parity bits consisting of n bits are received together with the regular data consisting of m bits to calculate a syndrome that is to be generated based on a predetermined exclusive OR operation. The location of an error bit is identified based on the syndrome that is the calculated result to modify the regular m-bit data. This general Hamming code theory is disclosed in, for example, “Industrial Mathematics for Restudy”, CQ Publishing Co., Ltd., pp. 47-53.

In general, the ECC circuit must implement a plurality of columns of an exclusive OR circuit (also referred to as “XOR gate” hereinafter) that takes an exclusive OR to calculate a syndrome. Since the number of parity bits increases in proportion to the amount of information, i.e. the number of bits, in the storage device, the number of XOR gates will be inevitably increased according to the amount of information in the storage device, leading to more columns.

Increase in the number of columns of XOR gates induces the problem that the error correction processing rate will become slower.

Japanese Patent Laying-Open Nos. 05-144291 and 2000-132995 disclose a system of improving the integration level to increase the error correction processing rate by relatively reducing the number of columns of the XOR gates.

It is to be noted that, if the operating rate of the XOR gate per se constituting the ECC circuit can be increased, the error correction processing rate can be improved.

The circuit complexity is increased in accordance with the increase in the number of XOR gates, whereby the wiring that connects respective circuits becomes longer. As a result, the rate of error correction processing is degraded.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is to provide a semiconductor device further improving the error correction processing rate.

A semiconductor device according to an aspect of the present invention includes a memory cell array storing a data group formed of a plurality of data bits and a plurality of parity bits, and an error correcting circuit executing correction of an error bit in the plurality of data bits and plurality of parity bits constituting the data group output from the memory cell array. The error correcting circuit includes an XOR circuit group obtaining a syndrome based on a matrix product of a predetermined check matrix represented in binary and a matrix formed of logic values of respective bits in the data group output from the memory cell array, and a correcting circuit correcting an error bit in the plurality of data bits and plurality of parity bits based on the syndrome output from the XOR circuit group. The XOR circuit group includes a plurality of check circuits receiving an input of the plurality of data bits and plurality of parity bits to compute each data of a plurality of bits constituting the syndrome. Each check circuit includes a plurality of XOR gates. Each XOR gate receives every 2 bits of input, and calculates an exclusive OR of the plurality of data bits and plurality of parity bits input corresponding to matrix elements of each row in a predetermined check matrix. The sum of the matrix elements of the predetermined check matrix is set to become lower than a predetermined value.

In accordance with the semiconductor device of the present invention, the number of XOR gates constituting the check circuit can be reduced to allow a smaller layout area. As a result, increase in the number of columns of check circuits formed of a plurality of XOR gates can be suppressed to allow computation of a syndrome at high speed. In other words, the error correction processing can be executed at high speed.

A semiconductor device according to another aspect of the present invention includes a memory cell array storing a data group formed of a plurality of data bits and a plurality of parity bits, and an error correcting circuit executing correction of an error bit in the plurality of data bits and plurality of parity bits constituting the data group output from the memory cell array. The error correcting circuit includes an XOR circuit group obtaining a syndrome based on a matrix product of a check matrix and a matrix formed of logic values of respective bits in the data group output from the memory cell array, and a correcting circuit correcting an error bit in the plurality of data bits and plurality of parity bits based on the syndrome output from the XOR circuit group. The XOR circuit group includes a plurality of XOR gates. Each XOR gate receives every 2 bits of input of the plurality of data bits and plurality of parity bits. Each XOR gate includes first and second transistors for setting an output node at a first logic level and a second logic level based on a predetermined combination of the logic values of every 2 bits input. Each XOR gate has its output node set to the first logic level in a reset state. The second transistor is set to have a drivability greater than that of the first transistor.

According to the semiconductor device of the present aspect, the rate of setting the logic level of the output node to the second logic level from the first logic level identified as a reset state becomes higher as compared to the case where the same drivability is set. Thus, the time required to output a syndrome from the XOR circuit group is reduced. Error correction processing can be executed at high speed.

A semiconductor device according to a further aspect of the present invention includes a memory cell array storing a data group formed of a plurality of data bits and a plurality of parity bits, and an error correcting circuit executing correction on an error bit in the plurality of data bits and plurality of parity bits constituting the data group output from the memory cell array. The error correcting circuit includes an XOR circuit group obtaining a syndrome based on a matrix product of a check matrix and a matrix formed of logic values of respective bits in the data group output from the memory cell array, and a correcting circuit correcting an error bit in the plurality of data bits and plurality of parity bits based on the syndrome output from the XOR circuit group. The XOR circuit group includes a plurality of XOR gates. Each XOR gate receives every 2 bits of input of the plurality of data bits and plurality of parity bits. When there are 2k(k: a natural number of at least 2) inputs to an XOR gate group formed of at least a portion of the plurality of XOR gates to which the plurality of information bits and plurality of parity bits are input, the XOR gate group includes (2k−1) XOR gates calculating the exclusive OR of 2kinputs. The (2k−1) XOR gates are arranged in 2 columns.

Accordingly, an effective layout can be executed, allowing reduction in the area of the XOR gate group.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described hereinafter with reference to the drawings. In the drawings, the same or corresponding components have the same reference characters allotted, and description thereof will not be repeated.

First Embodiment

Referring toFIG. 1, a semiconductor device1according to a first embodiment of the present invention includes a memory array MA and an ECC circuit10.

Memory array MA is provided to store regular data corresponding to the information amount of 32 bits. Parity bits of 6 bits are also applied and stored in memory array MA for the purpose of executing error correction on the 32 data bits.

As shown inFIG. 1, 32 information bits, i.e. data Din0-Din31, and also 6 bits of parity bits, i.e. Pin0-Pin5to check the information bits, are applied to memory array MA (data-in, parity-in).

In a data read out operation from memory array MA, data is applied to ECC circuit10via a data bus DB for execution of error correction processing. Specifically, data bits D0-D31and parity bits P0-P5stored in memory array MA are applied to ECC circuit10(data-out, parity-out).

ECC circuit10of the first embodiment includes a correcting circuit2and an EXOR Tree circuit3.

EXOR Tree circuit3receives data bits D0-D31and parity bits P0-P5to calculate the syndrome, and outputs syndrome data S0-S5. Syndrome data S0-S5from EXOR Tree circuit3are applied to correcting circuit2.

Correcting circuit2identifies the error position in the 32 data bits D0-D31based on syndrome data S0-S5, and inverts the error bit data to provide proper data Dout0-Dout31to a module external interface that function as an interface with an external circuit. The aforementioned output to the module external interface is only a way of example, output to an internal circuit or the like that executes another predetermined function is possible.

Referring toFIG. 2, correcting circuit2of the first embodiment includes a 6:32 decoder5, and a correction unit6formed of exclusive OR circuits (XOR gates)7.

6:32 decoder5receives syndrome data S0-S5from EXOR Tree circuit3to output information of 32 bits identifying the error position in data bits D0-D31.

Correction unit6includes a plurality of XOR gates7corresponding to the 32-bit data. Each XOR gate7receives a corresponding one of data bits D0-D31and a data input identifying an error position output from 6:32 decoder5to invert the error bit data. The plurality of XOR gates7output data Dout0-Dout31to the module external interface.

For example, 6:32 decoder5outputs data indicative of error data (“1”) to a corresponding XOR gate7. That corresponding XOR gate7inverts data bit D for output.

A method of setting a parity check table according to the first embodiment of the present invention will be described with reference toFIG. 3. This parity check table represents information to identify the error position in the syndrome computation that will be described afterwards.

For example, output of the syndrome result “000100” from the higher order bits identified as S5-S0indicates that parity bit P2is the bit data in error.

Similarly, the values of 6 bits represented in binary numbers for all the data bits D0-D31and parity bits P0-P51are allocated so as to differ from each other.

Based on this parity check table, a check matrix H of the following expression is provided.

Specifically, based on the parity check table set forth above, a matrix product of a check matrix H and a matrix w constituting data bits D0-D31and parity bits P0-P5can be represented.

Parity bits P0-P5are stored in memory array MA such that the above equation (1) is met with respect to data bits D0-D31.

Therefore, determination can be made that there is a bit error in the case of the following equation (2).
Hwt≠0t(2)

EXOR Tree circuit3of the first embodiment calculates the left-hand side of equation (1) to output syndrome data S0-S5as the syndrome result. Expansion of equation (1) yields the following equations:
D0⊕D4⊕D6⊕D7⊕D11⊕D13βD14⊕D17⊕D18⊕D22⊕P0⊕D24⊕D28⊕D29=S0(3)
D1⊕D5⊕D7⊕D8⊕D10⊕D12⊕D14⊕D16⊕P1⊕D18⊕D19⊕D21⊕D24⊕D26⊕D28⊕D31=S1(4)
P2⊕D2⊕D5⊕D6⊕D10⊕D13⊕D15⊕D16⊕D20⊕D22⊕D23⊕D27⊕D28⊕D31=S2(5)
D0⊕D3⊕D4⊕D7⊕D9⊕D13⊕D16⊕D17⊕P3⊕D19⊕D20⊕D21⊕D23⊕D26⊕D27⊕D30=S3(6)
D1⊕D4⊕D5⊕D8⊕P4⊕D9⊕D14⊕D15⊕D19⊕D22⊕D25⊕D27⊕D29⊕D30=S4(7)
D2⊕D3⊕D6⊕D8⊕D11⊕D12⊕D15⊕D17⊕P5⊕D18⊕D20⊕D21⊕D25⊕D29⊕D30⊕D31=S5(8)

With regards to the values allocated to respective bits inFIG. 3, the values in the vertical direction and horizontal direction are set to become smaller than a predetermined number. Specifically, the sum of the matrix elements of check matrix H is set to be smaller than a predetermined number. Here, the sum of the entire matrix elements is set to take a value (natural number) not larger than (n−2)×(m+n), where n is the number of parity bits and m is the number of data bits, by way of example.

Further, the values allocated to respective bits are set such that the values in the vertical direction become smaller than a predetermined number. Here, the value of the sum in the vertical direction, i.e. the sum in each column in check matrix H, is set to take a value (natural number) not larger than (n−2), where n is the number of parity bits. In the table ofFIG. 3, the values in the horizontal direction are set to 3 or below.

Further, the values in the horizontal direction are set so as to be smaller than a predetermined number. Here, the value of the sum in the horizontal direction, i.e. the sum in each row in check matrix H, is set to take a value (natural number) not larger than (n−2)×(m+n)/n, where n is the number of parity bits and m is the number of data bits. In the table ofFIG. 3, the values in the vertical direction are set to 16 or below.

Furthermore, the values in the vertical direction are set to take an even number. In the table ofFIG. 3, such values are “16” and “14”, which are both even numbers.

Referring toFIG. 4A, a parity circuit PC0constituting EXOR Tree circuit3is formed of a plurality of XOR gates XO according to equation (3) to output syndrome data S0identified as an exclusive OR, i.e. syndrome result. Specifically, this corresponds to the result of computing the exclusive OR of data corresponding to “1” in the vertical direction (row direction in check matrix H) in the parity check table ofFIG. 3.

Referring toFIG. 4B, a parity circuit PC1constituting EXOR Tree circuit3is formed of a plurality of XOR gates XO according to equation (4) to output syndrome data S1identified as an exclusive OR, i.e. syndrome result. Specifically, this corresponds to the result of computing an exclusive OR of data corresponding to “1” in the vertical direction in the parity check table ofFIG. 3.

Referring toFIG. 5A, a parity circuit PC2constituting EXOR Tree circuit3is formed of a plurality of XOR gates XO according to equation (5) to output syndrome data S2identified as an exclusive OR, i.e. syndrome result. Specifically, this corresponds to the result of computing an exclusive OR of data corresponding to “1” in the vertical direction of the parity check table ofFIG. 3.

Referring toFIG. 5B, a parity circuit PC3constituting EXOR Tree circuit3is formed of a plurality of XOR gates XO according to equation (6) to output syndrome data S3identified as an exclusive OR, i.e. syndrome result. Specifically, this corresponds to the result of computing an exclusive OR of data corresponding to “1” in the vertical direction in the parity check table ofFIG. 3.

Referring toFIG. 6A, a parity circuit PC4constituting EXOR Tree circuit3is formed of a plurality of XOR gates XO according to equation (7) to output syndrome data S4identified as an exclusive OR, i.e., syndrome result. Specifically, this corresponds to the result of computing an exclusive OR of the data corresponding to “1” in the vertical direction in the parity check table ofFIG. 3.

Referring toFIG. 6B, a parity circuit PC5constituting EXOR Tree circuit3is formed of a plurality of XOR gates XO according to equation (8) to output syndrome data S5identified as an exclusive OR, i.e. syndrome result. Specifically, this corresponds to the result of computing an exclusive OR of data corresponding to “1” in the vertical direction in the parity check table ofFIG. 3.

By setting the sum of each row and each column of the matrix elements in check matrix H to be smaller than a predetermined number, the number of XOR gates constituting parity circuit PC can be reduced. Accordingly, the layout area can be reduced. As a result, increase in the number of columns of the parity circuits PC formed of a plurality of XOR gates can be suppressed to allow the syndrome to be computed at high speed. In other words, error correction processing can be executed at high speed.

Referring toFIG. 7, XOR gate XO includes logic units20and21. XOR gate XO receives the inputs of an input signal Da and an inverted signal thereof Dan, and inputs of an input signal Db and an inverted signal thereof Dbn to compute an exclusive OR and generate an output signal y and an inverted signal thereof yn.

Logic unit20receives the input signals of Da, Dan, Db and Dbn to generate an output signal y.

Logic unit21receives the input signals of Da, Dan, Db and Dbn to output an output signal yn that is an inverted version of output signal y.

Logic unit20includes transistors PT1-PT4and transistors NT1-NT4. Transistors PT1and P2are connected in series between a power supply voltage VCC and a node NO to receive input signals Da and Dbn at their gates. Transistors PT3and PT4are connected in series between power supply voltage VCC and output node NO to receive input signals Db and Dan at their gates. Transistors PT1and PT2are connected parallel to transistors PT3and PT4.

Transistors NT1and NT2are connected in series between output node NO and ground voltage GND to receive input signals Dan and Dbn at their gates. Transistors NT3and NT4are connected in series between output node NO and ground voltage GND to receive input signals Da and Db at their gates. Transistors NT1and NT2are connected parallel to transistors NT3and NT4.

Logic unit21includes transistors PT5-PT8and transistors NT5-NT8. Transistors PT5and PT6are connected in series between power supply voltage VCC and an output node N1to receive input signals Da and Db at their gates. Transistors PT7and PT8are arranged between power supply voltage VCC and output node N1to receive input signals Dan and Dbn at their gates. Transistors PT5and PT6are connected parallel to transistors PT7and PT8.

Transistors NT5and NT6are connected in series between output node N1and ground voltage GND to receive input signals Db and Dan at their gates. Transistors NT7and NT8are connected in series between output node N1and ground voltage GND to receive input signals Da and Dbn at their gates. Transistors NT5and NT6are connected parallel to transistors NT7and NT8.

Transistors PT1-PT8correspond to P channel MOS transistors. Transistors NT1-NT8correspond to N channel MOS transistors.

An operation of XOR gate XO ofFIG. 7will be described hereinafter. In the present embodiment, it is assumed that the high voltage level of “H” is set when data signals Da and Db take the binary logic level of “1”, and the low voltage level of “L” is set when data signals Da and Db take the binary logic level of “0”.

For example, when data signals Da and Db are both “1” or “0”, transistors NT3and NT4or transistors NT1and NT2are turned on in logic unit20. Therefore, output node N0is set to the “L” level. In other words, output signal y is “0”. Similarly, transistors PT7and PT8or transistors PT5and PT6are turned on in logic unit21. Therefore, output node N1is set at the H level. In other words, output signal yn is “1”.

When the logic levels of data signals Da and Db differ such as “1” and “0”, transistors PT3and PT4or transistors PT1and PT2are turned on in logic unit20. Therefore, output node NO is set at the H level. In other words, output signal y is “1”. Similarly, transistors NT7and NT8or transistors NT5and NT5are turned on in logic unit21. Therefore, output node N1is set at the “L” level. In other words, output signal yn is “0”. It is assumed that input signals Da and Db are both applied with “1” or “0” in the initial state (reset state), as will be described afterwards. Therefore, output nodes N0and N1are set to the “L” level and “H” level, respectively.

XOR gate XO according to the first embodiment of the present invention has its drivability adjusted by controlling the size of the P channel MOS transistors and N channel MOS transistors.

Specifically, in logic unit20, the size wp of P channel MOS transistors PT1-PT4is set 4 times the size wn of N channel MOS transistors NT1-NT4. When the size of P channel MOS transistors and the size of N channel MOS transistors are set at 2:1, respective transistors have the same level of drivability. Therefore, the drivability of the P channel MOS transistor is larger than that of the N channel MOS transistor here.

In the aforementioned reset state, output node N0has its logic level set to “L”. When the logic levels of input signals Da and Db do not match each other under this state, the logic level of output node N0is set to “H”. Since the drivability of the P channel MOS transistor is set high as compared to that of the N channel MOS transistor in the present embodiment, the speed of output node N0being set to the logic level of “H” is faster than that of the case in which the same drivability is set.

In logic unit21, the size wp of P channel MOS transistors PT5-PT8is set equal to the size wn of N channel MOS transistors NT5-NT8. The transistors will have the same level of drivability when the size of the P channel MOS transistor and the size of the N channel MOS transistor is set as 2:1, as mentioned above. Therefore, the drivability of the N channel MOS transistor is larger than that of P channel MOS transistor here.

In a reset state, output node N1is set to the logic level of “H”. When the logic levels of input signals Da and Db do not match each other under such a state, the logic level of output node N1is set to “L”. Since the drivability of the N channel MOS transistor is set larger as compared to that of the P channel MOS transistor, the speed of output node N1being set to the logic level of L becomes faster than the case where the same drivability is set.

XOR gate XO of the present invention is designed to be driven at high speed when the logic levels of output nodes N0and N1make a transition from the “L” level and the “H” level corresponding to a reset state.

Therefore, the speed of setting syndrome data S0-S5identified as the syndrome result to “1” in each parity circuit PC formed of a plurality of XOR gates becomes faster than that of the case where the XOR gates are set at the same drivability. In other words, a syndrome is calculated speedily at parity circuit PC.

Output of data Dout0-Dout31through XOR gates of the present embodiment will be described with reference toFIG. 8.

Semiconductor device1operates in synchronization with a system clock CLK. In the present embodiment, data readout is executed to output data bits D0-D31onto data bus DB. Then, data bits D0-D31are applied to ECC circuit10together with parity bits P0-P5for the execution of error correction processing. By advancing the drive from “L” and “H” levels corresponding to the reset state through usage of the XOR gates of the present embodiment, the time required to output a syndrome can be reduced than in the conventional error correction processing indicated by the dotted line inFIG. 8. The output rate of syndrome data S0-S5is improved to execute error correction processing at high speed, whereby the output rate of data Dout0-Dout31provided to the module external interface can be improved.

XOR gate7according to the present embodiment shown inFIG. 7outputs a signal y and an inverted signal thereof yn based on four inputs, i.e. input signal Da and corresponding inverted signal Dan, and also input signal Db and corresponding inverted signal Dbn. Although it is necessary to provide an inverter to generate an inverted signal for input signals Da and Db, it is to be noted that inverted signal yn is generated parallel to output signal y. Therefore, an additional inverter for signal inversion at the succeeding columns of XOR gates XO is not required. For example, when parity circuit PC is implemented with a plurality of XOR gates as shown inFIGS. 4-6, inverters22and23shown inFIG. 7must be provided to generate inverted signals of the input signal at the first column of XOR gate XO. However, inverters22and23do not have to be provided for the succeeding columns of XOR gates XO. This means that the number of circuits from input of a signal to output is reduced. In other words, the load is alleviated such that parity circuit PC operating at high speed can be realized.

In the present embodiment, the number of inputs for parity circuits PC constituting EXOR Tree circuit3is set to an even number. Specifically, the number of inputs of each of parity circuits PC0-PC5ofFIGS. 4-6is “14” or “16”, i.e. an even number, as shown in the parity check table. Therefore, if parity circuits PC0-PC5of the first embodiment has data “0” or “1” applied to the input terminals to which respective data bits D0-D31and parity bits P0-P5are input at the reset column, syndromes S0-S5are all set to “0” corresponding to a reset state. If the number of inputs is set to an odd number, syndromes S0-S5will not be set to “0” corresponding to a reset state unless “0” is applied to the input terminals to which data bits D0-D31and parity bits P0-P5are applied at the reset column. By setting the number of inputs to an even number, the degree of freedom in design for a reset state can be improved.

An effective layout of XOR gates will be described hereinafter with reference toFIG. 9.

FIG. 9(a) corresponds to the layout of XOR gates XO in the unit of 22, i.e. 4, inputs as one unit. It is appreciated that 3 XOR gates XO are employed to form an XOR gate group to calculate an exclusive OR of 2 columns.

FIG. 9(b) corresponds to the layout of XOR gates XO with 23, i.e. 8, inputs as one unit. It is appreciated that 7 XOR gates XO are employed to form an XOR gate group calculating an exclusive OR of 2 columns.

FIG. 9(c) corresponds to the layout of XOR gates XO with 24, i.e. 16, inputs as one unit. It is appreciated that 15 XOR gates XO are employed to form an XOR gate group calculating an exclusive OR of 2 columns.

In other words, (2k−1) XOR gates XO are employed when there are 2k(k: natural number of at least 2) inputs to form an XOR gate group calculating an exclusive OR of two columns. By such a layout of 2-column configuration, the layout efficiency can be improved to suppress increase in the layout area of the XOR gate group.

The layout scheme set forth above is advantageous in that, when there are two XOR gate groups with 8 inputs, for example, as one unit XOR gate groups can be arranged and formed with high area efficiency as shown inFIG. 9(d) by combining the two XOR gate groups so as to be arranged in an inverted manner with respect to each other to reduce the area occupied by the two XOR gate groups. In the case where XOR gate XO takes a rectangular configuration, by way of example, a rectangular layout of high area efficiency can be achieved by combining the two XOR gate groups.

FIG. 10is a diagram to describe a layout of parity circuits PC0-PC5executed according to the scheme described with reference toFIG. 9.

Referring toFIG. 10, parity circuits PC0and PC1are arranged at the right side region ofFIG. 10. By the combining arrangement of XOR gate groups constituting parity circuits PC0and PC1, an efficient layout is achieved so as to reduce the area of parity circuits PC0and PC1.

At the center region ofFIG. 10, parity circuits PC2and PC3are arranged. By the combining arrangement of XOR gate groups constituting parity circuits PC2and PC3, an effective layout is achieved so as to reduce the area of parity circuits PC2and PC3.

At the left side region ofFIG. 10, parity circuits PC4and PC5are arranged. By the combining arrangement of XOR gate groups constituting parity circuits PC4and PC5, an effective layout is achieved so as to reduce the area of parity circuits PC4and PC5.

In the present embodiment, the input data and parity bits are divided into a plurality of subgroups in the parity check table ofFIG. 3, each subgroup corresponding to a predetermined number of input data and predetermined number of parity bits. Specifically, subgroup SG0includes parity bit P2and data bits D0-D8. Subgroup SG1includes parity bit P4and data bits D9-D17. Subgroup SG2includes parity bits P1, P3and P5, and data bits D18-D22. Subgroup SG3includes parity bit P0and data bits D23-D31.

Each of parity circuits PC0-PC5is arranged corresponding to the plurality of subgroups such that the data bits and parity bits included in each of the divided subgroup are in proximity to each other.

For example, the top region R1of each of parity circuits PC0-PC5is applied with data bits and parity bits corresponding to subgroup SG0. The second region R2is applied with data bits and parity bits corresponding to subgroup SG1. The third region R3has XOR gate groups formed to output syndrome data S0-S5identified as the syndrome result. The fourth region R4is applied with data bits and parity bits corresponding to subgroup SG2. The fifth region R5is applied with data bits and parity bits corresponding to subgroup SG3.

Since data applied to each predetermined region is arranged in close proximity in each parity circuit, the routing of wiring such as the data input line can be suppressed to reduce the wiring length. Thus, the load can be further reduced to allow execution of error correction processing at higher rate.

Second Embodiment

A parity check table according to a second embodiment of the present invention will be described with reference toFIGS. 11A and 11B.

The second embodiment corresponds to the case where data bits of 64 bits and 7 parity bits are stored in memory array MA.

A parity check table is set according to the scheme described with reference toFIG. 3. In the second embodiment, the values of 7 bits represented in binary numbers are allocated with respect to respective data bits and parity bits such that the values of 7 bits differ. Further, the sum of matrix elements in each row and each column of check matrix H is set so as to become lower than a predetermined value. The sum in the horizontal direction (column direction in check matrix H) is set to 4 or below. The sum in the vertical direction (row direction in check matrix H) is set to be 28 or below.

Referring toFIG. 12A, a parity circuit PC6# constituting EXOR Tree circuit3is formed of a plurality of XOR gates XO to output syndrome data S6identified as an exclusive OR, i.e. the syndrome result. Specifically, this corresponds to a result computing an exclusive OR of data corresponding to “1” in the vertical direction in the parity check table ofFIGS. 11A and 11B(row direction in check matrix H).

Referring toFIG. 12B, a parity circuit PC5# constituting EXOR Tree circuit3is formed of a plurality of XOR gates XO to output syndrome data S5identified as an exclusive OR, i.e. the syndrome result. Specifically, this corresponds to the result of computing an exclusive OR of data corresponding to “1” in the vertical direction in the parity check table ofFIGS. 11A and 11B.

Referring toFIG. 13A, a parity circuit PC4# constituting EXOR Tree circuit3is formed of a plurality of XOR gates XO to output syndrome data S4identified as an exclusive OR, i.e. syndrome result. Specifically, this corresponds to the result of computing an exclusive OR of data corresponding to “1” in the vertical direction in the parity check table ofFIGS. 11A and 11B.

Referring toFIG. 13B, a parity circuit PC3# constituting EXOR Tree circuit3is formed of a plurality of XOR gates XO to output syndrome data S3identified as an exclusive OR, i.e. the syndrome result. Specifically, this corresponds to the result of computing an exclusive OR of data corresponding to “1” in the vertical direction in the parity check table ofFIGS. 11A and 11B.

Referring toFIG. 14A, a parity circuit PC2# constituting EXOR Tree circuit3is formed of a plurality of XOR gates XO to output syndrome data S2identified as an exclusive OR, i.e. syndrome result. Specifically, this corresponds to the result of computing an exclusive OR of data corresponding to “1” in the vertical direction in the parity check table ofFIGS. 11A and 11B.

Referring toFIG. 14B, a parity circuit PC1# constituting EXOR Tree circuit3is formed of a plurality of XOR gates XO to output syndrome data S1identified as an exclusive OR, i.e. the syndrome result. Specifically, this corresponds to the result of computing an exclusive OR of data corresponding to “1” in the vertical direction in the parity check table ofFIGS. 11A and 11B.

Referring toFIG. 15, a parity circuit PC0# constituting EXOR Tree circuit3is formed of a plurality of XOR gates XO to output syndrome data S0identified as an exclusive OR, i.e. syndrome result. Specifically, this corresponds to the result of computing an exclusive OR of data corresponding to “1” in the vertical direction in the parity check table ofFIGS. 11A and 11B.

FIG. 16is a diagram to describe a layout of parity circuits PC0#-PC6# set forth above executed according to the scheme described with reference toFIG. 9.

Referring toFIG. 16, parity circuits PC5# and PC6# are arranged at the rightmost region ofFIG. 16. An effective layout is achieved such that the area of parity circuits PC5# and PC6# is reduced by arranging in combination XOR gate groups constituting parity circuits PC5# and PC6#.

At the second region from the right inFIG. 16, parity circuits PC4# and PC3# are arranged. An effective layout is achieved such that the area of parity circuits PC4# and PC3# is reduced by arranging in combination X gate groups constituting parity circuits PC4# and PC3#.

At the third region from the right inFIG. 16, parity circuits PC2# and PC1# are arranged. An effective layout is achieved such that the area of parity circuits PC2# and PC1# is reduced by arranging in combination X gate groups constituting parity circuits PC2# and PC1#.

In the fourth region from the right, i.e. the leftmost region inFIG. 16, parity circuit PC0# is arranged.

In the present embodiment, the input data and parity bits are divided into a plurality of subgroups in the parity check table ofFIGS. 11A and 11B, each subgroup corresponding to a predetermined number of data bits and predetermined number of parity bits. Specifically, subgroup SG0# includes data bits D56-D63with respect to parity circuits PC3#-PC6#, and data bits D58-D63with respect to parity circuits PC0#-PC2#. Subgroup SG1# includes parity bits P4-P6and data bits D38-D55with respect to parity circuits PC3#-PC6#, and parity bits P4-P6and data bits D38-D57with respect to parity circuits PC0#-PC2#. Subgroup SG2# includes parity bits P2and P3and data bits D19-D37with respect to all parity circuits PC. Subgroup SG3# includes parity bits P0and P1, and data bits D1-D18with respect to all parity circuits PC.

Each of parity circuits PC0#-PC5# are arranged such that data bits and parity bits included in each of the divided subgroups are in proximity to each other, corresponding to the plurality of subgroups.

For example, the top region R1# of each of parity circuits PC0-PC5is applied with data bits and parity bits corresponding to subgroup SG0#. The second region R2# is applied with data bits and parity bits corresponding to subgroup SG1#. The third region R3# has XOR gate groups formed to output syndrome data S0-S6identified as the syndrome result. The fourth region R4# is applied with data bits and parity bits corresponding to subgroup SG2#. The fifth region R5# is applied with data bits and parity bits corresponding to subgroup SG3#.

Thus, since the data input for each predetermined region is arranged in close proximity in each parity circuit, the routing of wiring such as a data input line can be suppressed to reduce the wiring length. Thus, the load can be alleviated to allow execution of error correction processing at higher rate.

FIG. 17is a diagram to describe a layout of ECC circuit10and data bus DB according to the second embodiment of the present invention.

Referring toFIG. 17, ECC circuit10includes correcting circuit2and EXOR Tree circuit3. Correcting circuit2and EXOR Tree circuit3are arranged so as to provide output at the other side with respect to the input from one side from data bus DB. Specifically, the data bits and parity bits at one side from data bus DB are applied to EXOR Tree circuit3. Then, syndrome data S from EXOR Tree circuit3is applied to correcting circuit2located at the other side.

FIG. 18is a sectional view of ECC circuit10ofFIG. 17taken along line X-X.

Referring toFIG. 18, an interconnection layer of data bus DB through which data bits and parity bits are transmitted from memory array is depicted. Specifically, data bus DB is formed at the interconnection layer located at the upper side or lower side of the substrate where correcting circuit2and EXOR Tree circuit3are formed. Data bus DB is connected to correcting circuit2and EXOR Tree circuit3provided on the substrate via a contact hole CH such that data bits and parity bits are transmitted. For example, data bus DB is arranged on parity circuits PC adjacent along the predetermined direction described with reference toFIG. 10orFIG. 16.

Furthermore, the pitch of signal lines laid out with respect to correcting circuit2and EXOR Tree circuit3are designed to be identical.