JP2004199713A - System including ferroelectric memory - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the probability with which great damage to a system is caused by software error of a ferroelectric memory. <P>SOLUTION: A first memory block 122 including a plurality of memory cells each having a ferroelectric capacitor and a switching transistor has a first area 124 for storing normal data and a second area 125 for storing error correction information for correcting errors with the normal data. In a first mode, the normal data are accessed without a correction process including error checks that use the error correction information, and writing to the first memory block 122 is inhibited. In a second mode, writing to the first memory block 122 is permitted and error checks for the data are made using the normal data and the error correction information. When false data are detected through the error checks, a correction process is carried out by which the false data are corrected to rewrite the data. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a system including a nonvolatile semiconductor memory using a ferroelectric material, and more particularly, to a highly reliable ferroelectric device having a greatly reduced probability of causing a serious failure in a system function due to a malfunction of the memory. A system that includes body memory.

  A memory using a ferroelectric, for example, a ferroelectric random access memory (FERAM) is a non-volatile memory that stores data in a polarization direction of the ferroelectric. In a ferroelectric memory, for example, one memory cell is configured by one ferroelectric capacitor and one switching transistor, and reading of stored information is performed by a voltage that aligns the polarization of the ferroelectric capacitor in one direction. Is applied to the capacitor, and at this time, it is determined whether or not the polarization has been reversed. An example of such a ferroelectric memory is described in, for example, the digest of the IEEE International Solid-State Circuits Conference, 1994, pp. 268 to 269 (1994 IEEE International Solid-State Circuits Conference, DIGEST OF TECHNICAL PAPERS, pp. 294-214). 268-269).

  On the other hand, in a normal operation, there is a method in which the plate potential of the ferroelectric capacitor is fixed to, for example, a power supply voltage and used as a dynamic random access memory (DRAM). However, when the power is off, the information on the stored potential is converted into information on the polarization direction of the ferroelectric. Thus, information can be retained even after the power is turned off. An example of such a ferroelectric memory is described in, for example, 1990 Symposium on VLSI Technology, DIGEST OF TECHNICAL PAPERS, pp. 15-16, digest of the 1990 SSI Technology Symposium. It is described in.

Digest of the IEEE International Solid-State Circuits Conference, pp. 268-269, 1994 (1994 IEEE International Solid-State Circuits Conference, DIGEST OF TECHNICAL PAPERS, pp.268-269) 1990 Symposium on VLSI Technology, DIGEST OF TECHNICAL PAPERS, pp.15-16

  It is generally known that in a normal DRAM, stored information is inverted by a false signal charge generated by radiation and a malfunction may occur. Such a storage information inversion phenomenon is considered to occur similarly in a method of operating a ferroelectric memory as a normal DRAM. Also, in the ferroelectric memory method for detecting the polarization direction described above, the polarization information is temporarily lost when the information is read because the polarization is aligned in one direction. Therefore, it is necessary to rewrite the polarization based on the information read before the information reading operation is completed. If information is erroneously read out due to noise or the like, rewriting of the polarization is erroneously performed. In the following, such an error, that is, an error that is caused by erroneous inversion of stored information due to radiation, noise, or the like, although the function of the memory cell itself is not impaired, is called a soft error.

Soft errors in ferroelectric memories can cause significant problems compared to DRAMs. It is for the following reasons.
When a malfunction occurs in information stored in a storage device such as a DRAM and the system is stopped, the system can be at least restored by restarting. However, information stored in a nonvolatile memory such as a ferroelectric memory is often information that is used repeatedly, such as an OS (operating system) of a system. In particular, by storing the system OS and application programs that are used repeatedly in a ferroelectric memory in a portable device, a large-sized non-volatile storage medium such as a hard disk is not required, and a compact system can be constructed. Can be. Further, since the CPU can access the ferroelectric memory at a higher speed than the hard disk, the startup time of the portable device can be significantly reduced.

  In a system including such a ferroelectric memory, if a soft error occurs once in the ferroelectric memory, erroneous information is rewritten, and a serious failure occurs in the function of the system. There is. In such a case, in order to restore the system, it is necessary to connect to an external nonvolatile storage medium such as a hard disk and rewrite data such as an OS into the ferroelectric memory. For a device, it is quite inconvenient for the function of the system to stop until a nonvolatile storage medium such as a hard disk is obtained and connected.

  In the DRAM, as a method of avoiding the above-described soft error, there is a method of providing an error correction circuit (ECC circuit) and automatically detecting and correcting the soft error. In large systems such as large computers, the ECC circuit can be provided on a separate chip from the main unit, but in small systems such as mobile devices and personal computers, the DRAM chip itself has an error correction function to maintain compactness. It is desirable to have The 1987 IEEE International Solid-State Circuits Conference, Digest of Technical Papers, pp. 22-23, has an error correction function. An example of a DRAM chip is shown.

FIG. 8 is a diagram showing a conventional example. FIG. 1A shows the basic configuration of a DRAM equipped with an ECC circuit, and FIG. 1B shows a write / read operation flowchart.
As shown in FIG. 1A, a DRAM 80 includes a memory cell array 81 and a peripheral circuit section 84. There are two types of data stored in the memory cell array 81: an information storage bit 82 for storing information and a parity bit 83. The peripheral circuit section 84 includes an ECC circuit 85.

  As shown in the flowchart of FIG. 9B, at the time of writing information (step 91), after generating parity bit data (step 92), the information storage bit and the parity bit are written to the DRAM 80 (step 93). . At the time of reading information (step 95), first, a plurality of information storage bits and their corresponding parity bits are read (step 96). The ECC circuit 85 determines from an operation based on these data whether any bit has an error and, if an error has occurred, which bit has an error. After correcting the error (step 97), the data is sent from the DRAM to the CPU (step 98). As a result, a DRAM free from malfunction for the CPU can be realized.

However, in a conventional DRAM configuration equipped with an ECC circuit, (1) a parity bit must be generated each time a write is performed, and the write speed is reduced. (2) ECC is performed by reading a parity bit together with storage information each time a read is performed. Since the error correction must be performed by performing the check operation, the read speed is reduced, and (3) the chip size is increased by the area of the ECC circuit, thereby increasing the chip price. .
Most of the DRAMs currently on the market are equipped with ECC circuits because of the balance between the damage to the system and the frequency of occurrence of soft errors when the DRAM causes a soft error, and the degree of the above-mentioned adverse effects when the ECC circuit is installed. I didn't.

On the other hand, in the case of a ferroelectric memory, it is expected that damage to the system when a soft error occurs is expected to be large for the above-described reason, and when an ECC circuit is mounted to prevent the soft error. However, there is a problem that the operation speed is reduced and the chip price is increased.
SUMMARY OF THE INVENTION An object of the present invention is to solve the above-described problem, greatly reduce the probability that a system error is caused by a soft error in a ferroelectric memory, and do not cause a decrease in operating speed or an increase in chip price. An object is to provide a system including a ferroelectric memory.

The system of the present invention achieves the above object by:
(A) In a system having a first mode and a second mode, a first memory block including a plurality of memory cells each having a ferroelectric capacitor and a switching transistor;
A CPU connected to the first memory block, wherein the first memory block stores a first area for storing normal data and error correction information for correcting an error of the normal data. In the first mode, the CPU accesses the normal data without performing a correction process including an error check using the error correction information, and simultaneously accesses the first memory block. A write operation is prohibited, a write operation to the first memory block is permitted in the second mode, and the system performs an error check on data using the normal data and the error correction information, and When error data is detected by the check, a correction process of correcting the error data and writing it back is performed. It is (claim 1).

(B) The correction processing is performed by the CPU according to a correction processing program.
(C) The system generates a trigger signal based on a predetermined condition and starts the correction processing (claim 3).
(D) In the second mode, a memory cell to which correction data is written is the same as a memory cell from which the error data has been read (claim 4).
(E) The system is a portable device (claim 5).

(F) the plurality of memory cells store data according to a polarization direction of the ferroelectric capacitor;
The data is read from a corresponding memory cell by detecting the polarization direction (claim 6).
(G) The first memory block is used to store an OS program of the system (claim 7).
(H) The error correction information of the second area is a parity bit of normal data of the first area (claim 8).

According to the system including the ferroelectric memory of the present invention, there is an effect that a system can be obtained due to a malfunction of the ferroelectric memory. Further, the number of memory chips in the system can be reduced, and a system suitable for a low-cost and compact portable device can be obtained. Further, as compared with the case of using the ECC circuit, there is an effect that the problem of a decrease in operating speed and an increase in cost due to an increase in chip area can be avoided.
Further, by adopting the method of generating the error correction processing start command as in the present invention, a user-friendly and highly reliable system can be obtained.

Further, when the method of storing the error correction processing program as in the present invention is adopted, the risk of the error correction processing program itself being broken can be avoided, and a highly reliable system can be obtained.
Further, by adopting the method of setting the rewrite prohibited area according to the present invention, a configuration in which the rewrite prohibited area is variable can be easily realized, and a user-friendly system can be obtained.
In addition, when the configuration of the rewrite prohibited area according to the present invention is adopted, rewrite prohibition and permission control become easy.
Further, by employing the control circuit of the present invention for permitting writing to the rewriting prohibited area, a malfunction in which writing is erroneously performed on the prohibited area can be greatly reduced, and a highly reliable system can be obtained. Further, the control circuit can be easily arranged for each memory mat.

The outline of the embodiment of the present invention is as follows.
First, the system of the present invention has at least a CPU and a ferroelectric memory.
The CPU can access a storage area of a program for performing data error correction processing.
The storage area (memory cell array) of the ferroelectric memory is divided into a rewrite prohibited area and a rewrite permitted area. The OS and application programs are stored in the rewrite prohibited area, and the rewrite permitted area is used as a work area. The rewrite prohibited area has an information storage bit area and a parity bit area. The information storage bit area is used for normal information storage, and the parity bit area is used for storing information (parity bit) for recognizing and correcting a soft error in the information storage bit information when it occurs. . The rewrite permission area is composed of only information storage bits. Means for normally prohibiting writing to the rewriting prohibited area and temporarily permitting it, for example, a control circuit is provided in the peripheral circuit section (see FIG. 1).

In the system of the present invention, an error correction process start command causes the CPU to perform an error correction process on the data in the rewrite-inhibited area (see FIG. 2).
The error correction processing start command is automatically generated by an internal circuit of the system when the power of the system of the present invention is turned on, for example. Alternatively, a user can turn on a switch provided in the system of the present invention to generate an error correction processing start command (see FIG. 3).
The storage area for the error correction processing program is provided in a ROM section in the CPU. Alternatively, it is provided twice in the rewrite prohibited area in the ferroelectric memory (see FIG. 4).
An address storage unit for defining the range of the rewrite prohibited area is provided in the peripheral circuit unit or the rewrite prohibited area in the memory cell array (see FIG. 5).
The above-mentioned rewrite prohibited area is configured, for example, in units of two memory mats that face each other across the sense amplifier row (see FIG. 6).

  The control circuit gives, for example, a write command to one arbitrary address of two memory mats facing each other across the sense amplifier row among the memory mats in the rewrite prohibited area (the write command is not accepted). ), The write operation to the other memory mat is permitted only for a certain period (see FIG. 7).

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is an embodiment showing the basic configuration of the system of the present invention.
The system (100) of this embodiment has at least a CPU (110) and a ferroelectric memory (120). The storage area (memory cell array 121) of the ferroelectric memory (120) is divided into a rewrite prohibited area (122) and a rewrite permitted area (123). The rewrite prohibited area (122) is used repeatedly in the system, such as the OS and application programs of the system, and is used to store data that has little chance of being rewritten. The rewrite permitted area (123) is a temporary storage that has many opportunities for rewrite. It is used as an area, that is, a work area. In the rewrite prohibited area (122), a parity bit area is provided together with a normal information storage bit area. The parity bit area has information for restoring the information in the information storage bit area when the information is inverted due to a soft error. For example, a parity bit consisting of eight memory cells is provided for an information storage bit consisting of 120 memory cells. At this time, one of the eight memory cells of the parity bit is “0” when the number of “1” information is even, and “0” when the number of information is “odd” among the information of the 127 memory cells excluding this memory cell. 1 'is stored. This memory cell has information indicating that an error has occurred in any one of the 128 memory cells.

  That is, if the number of "1" information of the 127 memory cells does not correspond to the information of the memory cell, it means that an error has occurred. The remaining seven memory cells of the parity bit can take 2 7, that is, 128 states. By associating this with information indicating which of the 128 memory cells has an error, the error can be repaired. In the above-described example, if an error occurs in two or more memory cells, the error cannot be repaired. Is sufficient for the amount of information that can be corrected by one error.

In order to repair an error that has occurred in a memory cell, for example, it is necessary to compare an operation result for 120 information storage bits with a result of a parity bit and determine an error location based on the result. This is performed by the CPU (110) based on the program stored in the error correction processing program storage area (111).
When it is clear that an error has occurred in the memory cell, it is necessary to rewrite the corrected data to the memory cell. A control circuit (127) for temporarily permitting writing to the rewrite prohibited area (122) is provided in the peripheral circuit section (126) of the ferroelectric memory.

FIG. 2 is a flowchart of the error correction process in the system (100) of FIG.
First, an error correction processing start command is given to the CPU (110) (step 201). In response, the CPU (110) starts operation according to the error correction processing program (step 202). First, the CPU (110) sets a plurality of information storage bit data (data of 120 memory cells in the above-described example) and a parity bit data corresponding thereto (the above-mentioned data of the 120 memory cells) in the rewrite prohibited area (122) in the ferroelectric memory. In the example, data of eight memory cells are loaded (step 203). Next, the CPU (110) checks whether there is an error in the loaded data according to the procedure instructed by the program (step 204).

If there is an error in the data (step 205: Y), the CPU (100) instructs the control circuit (127) in the ferroelectric memory to temporarily permit writing for data correction. (Step 206). Then, according to the procedure instructed by the program, the error data is corrected and written back to the ferroelectric memory (step 207). After writing back the corrected data to the memory cell, the control circuit (127) sets the memory cell to the rewrite prohibited state again (step 208).
After step 208, and if there is no error in the data (step 205: N), it is determined whether or not correction processing has been performed on all data in the rewrite-protected area. Is present (Step 209: N), the flow returns to Step 203 again. If there is no uncorrected data (step 209: Y), the CPU (110) ends the error correction processing (step 210).
The error correction process is performed on all data in the rewrite prohibited area according to the procedure described above.

  Here, an embodiment of a specific method of determining a parity bit and a method of 1-bit error correction will be described. As an example, a case is shown in which eight parity bits are added to 120 information storage bits. First, identification numbers from 1 to 128 are assigned to 120 information storage bits and 8 parity bits. However, an identification number of “2 to the power of n” (that is, 1, 2, 4, 8, 16, 32, 64, 128) is assigned to the parity bit. Note that this identification number is merely a virtual one for identifying each bit, and does not indicate a storage address in the ferroelectric memory. For example, eight parity bits may be stored at consecutive addresses.

Next, data of seven parity bits of the identification number "2 to the power of n" (n is 0 to 6) is determined by the following procedure. That is, when the number of bits whose data is “1” is even among 63 information storage bits in which the (n + 1) th digit is 1 instead of 0 when the identification number is expressed in binary, The parity bit data of “2 to the nth power” is set to “0”, and is set to “1” for an odd number.
The data of the remaining one parity bit (identification number 128, that is, 2 to the seventh power) is determined as follows. That is, when the number of bits whose data is “1” among the 120 information storage bits and the 7 parity bits defined above is an even number, the parity bit data of the identification number 128 is set to “0” and the odd number In this case, it is set to '1'.

  Using the parity bits determined as described above, a one-bit error can be detected and corrected by the following method. That is, data of 120 information storage bits and 8 parity bits are read from the ferroelectric memory, and first, 7 parity bits of the identification number “2 to the power of n” (n is 0 to 6) are: It is checked whether or not the above-mentioned predetermined value (a value determined from the information storage bit) is reached. If the value is a predetermined value, a 7-digit binary number is formed by setting the (n + 1) th digit to 0; The 7-digit binary number formed in this manner becomes the error determination number, and indicates the identification number of the bit in which the 1-bit error has occurred (excluding 1 to 127.128). If the error determination number is 0, there is no error from identification numbers 1 to 127.

  Next, it is checked whether the remaining parity bit of the identification number 128 has the above-mentioned predetermined value. This is a predetermined value. If the error determination number is other than 0, a two-bit error has occurred. However, the location of the error at this time is unknown. Conversely, if the error determination number is 0 and the parity bit of the identification number 128 is not a predetermined value, an error has occurred in the parity bit of the identification number 128 itself. As described above, since the occurrence and error location of a 1-bit error can be known, it is possible to correct the information by inverting the data of the bit at the error location.

According to the embodiment of the configuration of the present invention shown in FIG. 1 and the flowchart of the error correction processing shown in FIG. 2, the following highly reliable and high performance system can be obtained. That is,
(A) First, when a soft error occurs in the storage area of the OS or the application program, it is possible to prevent a serious failure from occurring in the function of the system. This is because, by giving an error correction processing start command, an error part can be repaired and the function of the system can be restored. In addition, since the temporarily stored information remains in the work area in a non-volatile manner, even if the system power needs to be turned on again, there is no great trouble for the user.

(B) Second, the number of chips used in the system can be reduced and a low-cost system can be obtained as compared with the case where the OS and application programs are stored in the ROM and the work area is a DRAM. Further, since the system can be configured compactly, there is an advantage that a system suitable for a portable device can be obtained. There is a similar advantage as compared with a system in which an OS or an application program is stored in a hard disk when the system is not used, and is read from the hard disk into a DRAM or the like when the system is used. Furthermore, at the time of system startup, the OS program already exists in the ferroelectric memory that can be accessed at high speed from the CPU. Thus, there is an advantage that the startup time can be reduced.

(C) Third, the operation speed of the system does not decrease as in the conventional example shown in FIG. This is because no data check is performed during a normal read operation, and no new parity bit is generated during a normal write operation. This is because the parity bit is provided only for the data in the rewrite prohibited area.
(D) Fourth, since the error correction processing is performed using the CPU, an increase in chip area due to the mounting of the ECC circuit and an increase in chip price due to the increase can be avoided.

FIG. 3 is a flowchart showing a procedure for performing error correction processing by two types of error correction processing start command generation methods.
In the first embodiment, as shown in the flow of FIG. 7A, when the power supply of the system of the present invention is turned on (step 301), an error correction processing start command is automatically generated by the system internal circuit (step 301). Step 302), whereby the CPU executes an error correction processing program (Step 303) to perform error correction processing.
In the second embodiment, as shown in FIG. 13B, when a user turns on a switch provided in the system of the present invention (step 351), an error correction processing start command is generated (step 352). In this method, the CPU executes the error correction processing program (step 353) to perform the error correction processing.

In any of the embodiments, the error correction processing start command does not need to occur frequently, and may be given, for example, about once a day. It is clear from the following calculation that sufficiently high reliability can be obtained with such a frequency.
Semiconductor memories are usually designed so that the frequency of occurrence of soft errors is 1000 FIT or less. This is the rate at which at most one soft error occurs on an average of 10 6 times per chip. Now, suppose that an OS program is stored in ferroelectric memory chips of 10 million systems worldwide. At this time, according to the conventional system, there is a possibility that an average of 10 systems may fail in one hour. However, it is assumed that one error causes the OS program to stop operating.

On the other hand, when the system of the present invention is operated for 10 hours a day and an error correction processing start command is given once a day, the following is performed.
The worst case where the OS program is stored in the entire ferroelectric memory chip is calculated. It is assumed that the storage area is composed of a set of 1000 information storage bits and parity bits. If it is assumed that only one parity bit error can be repaired, a malfunction occurs in the system of the present invention when two or more soft errors are present in any of the above 1000 sets at the time of giving the error correction processing start command. Is generated.

  If the frequency of soft errors is 1000 FIT, a total of 100 errors will occur in 10 million memory chips within 10 hours. Among them, the probability that two or more errors are concentrated on any of the 10 million × 1000 sets of blocks is smaller than 10 minus the sixth power. This is even lower than the frequency of occurrence every 10 6th day, or 2740. As described above, according to the system of the present invention, it is possible to extremely reduce the rate of occurrence of a functional failure of an unrecoverable system due to two or more errors. In the first example shown in FIG. 3A (an example in which an error correction processing start command is generated when the power is turned on), the error correction processing start command is automatically generated at a frequency of about once a day. There is an effect that a user-friendly system can be obtained. According to the first and second examples shown in FIGS. 3A and 3B, even when a soft error occurs in the storage area of the OS program and the function of the system is stopped, the power is turned on again. Or the user turns on a predetermined switch (first example) or the user turns on the predetermined switch (second example), the function can be recovered with almost 100% probability, and there is an effect that a highly reliable system can be obtained.

  Further, according to the system of the present invention, it is not necessary to always equip the system with a non-volatile medium such as a hard disk for storing an OS program or the like when the system is not used, and a compact system is realized. Further, the system startup time can be reduced. In any of the first and second methods shown in FIG. 3, even if an error is found in the rewrite-protected area, the error is corrected by the CPU, so that a non-volatile medium other than a ferroelectric memory (for example, a hard disk) It is not necessary to perform the operation of reading the correct OS program from the memory into the ferroelectric memory.

  FIG. 4 shows a more specific embodiment of the storage area (111) of the error correction processing program of FIG. FIG. 4A shows an embodiment in which a storage area for an error correction processing program is provided in a part of the on-chip ROM area in the CPU (110). According to this embodiment, since the ROM is used, no soft error occurs in the storage area of the error correction processing program itself, and the error correction processing can be executed without fail. is there. In FIG. 4B, a storage area for the error correction processing program is provided in the rewrite prohibited area (122) of the ferroelectric memory (120). However, in this case, there is a possibility that a soft error may occur in the storage area itself, so another duplicate program is provided for backup. According to this embodiment, since a system can be constructed using a general-purpose CPU, an inexpensive and highly reliable system can be obtained.

In the above-described embodiment, the range of the rewrite prohibited area is described as being fixed, but the range of this area may be changeable by designation.
FIG. 5 is an embodiment of the present invention showing a configuration method of an address storage unit for designating a range of a rewrite prohibited area.
In FIG. 5A, a storage unit of an address for designating the range of the rewrite prohibited area is provided in the peripheral circuit unit. The range of the rewrite prohibited area may be fixed by using a wired logic, a fuse, a ROM, or the like, or the range of the rewrite prohibited area may be changed by using a static RAM (SRAM) with a ferroelectric capacitor. You may.
FIG. 5B shows an example in which an address storage unit (129) for designating the range of the rewrite prohibited area (122) is provided in the rewrite prohibited area (122) itself. According to the embodiment of FIG. 5B, there is an effect that a configuration in which the rewrite prohibited area is variable can be easily realized.

  FIG. 6 is an embodiment of the present invention showing a more specific example of the memory array configuration in the system of the present invention, and schematically shows only some of the components. Each memory cell is composed of one ferroelectric capacitor and one transistor (only one memory cell MC is shown as a representative cell in FIG. 6). Each memory cell is arranged at the intersection of a word line WL and a bit line BT. For example, 512 memory cells are connected to one word line WL, and 256 memory cells are connected to one bit line pair. , 512 × 256 memory cells constitute one mat.

  The sense amplifier row connected to the bit line pair is arranged to have two mats, for example, the sense amplifier row (1) s is shared by the mat (1) u and the mat (1) d. The rewrite prohibited area and the rewrite permitted area are defined in the above two mat units. Defining in units of mats has the effect of simplifying the configuration of the control circuit (127) in FIG. The unit of the rewrite prohibited area, that is, the set of the information storage bit and the parity bit is defined by a size that equally divides one word line. For example, in FIG. 6, information storage bits of 120 cells and parity bits of 8 cells are one set, and each word line WL (i) has four sets. With such a configuration, there is an effect that data can be efficiently read out to the CPU during error correction processing.

FIG. 7 is a diagram for explaining an embodiment of the control circuit (127) for giving write permission to the rewrite prohibited area in FIG. 1; (a) is a specific circuit example of the control circuit (127); (b) is a diagram showing the operation flow.
When writing into the memory mat pu, when a write command is given to an arbitrary address of the memory mat pd opposed to the sense amplifier array SA with the sense amplifier array SA interposed therebetween, the other is fixed for a certain period defined by the delay circuits D1 and D2. Write to the memory mat pu is permitted. Note that a write instruction for the first memory mat pd is not accepted.

  In FIG. 7A, the control circuit (127) includes a flip-flop circuit FF, two transistors TR1 and TR2, two delay circuits D1 and D2, two AND circuits G1 and G2, a NOT circuit NOT, a multiplexer MPLX, and the like. It is configured. Normally, one node ST1 of the flip-flop circuit FF is at a high level, the high-level signal is inverted by the NOT circuit NOT, and the AND circuit G2 is closed. Therefore, the WA from the AND circuit G2 and the Mpu from the multiplexer MPLX are both at the low level, and the memory mat pu is in the rewrite prohibited state.

  When a write instruction (write enable signal WE is at a high level) is given to memory mat pd, addresses A0 to AN input and held in the address buffer are decoded by an address predecoder, and mat pd selection signal line and mat pu are output. The selection signal line and the selection signal line in the mat are set. The output signal of the mat pd selection signal line is sent to the delay circuit D1, and is input to the AND circuit G1 after a delay time defined by the delay circuit D1. When the write enable signal WE is at a high level, the transistor TR1 is turned on by an output from the AND circuit G1, and one node ST1 of the flip-flop circuit FF is set to a low level. The low-level signal is inverted by the NOT circuit NOT, the AND circuit G2 is opened, the mat pu selection signal from the address predecoder is output as WA, and the mat pu is set in a rewrite permission state.

The multiplexer MPLX selects one of the two inputs of the mat pu selection signal line and the WA, and outputs it as Mpu.
When the write enable signal WE is at a low level, that is, in a read operation, the mat pu selection signal line is output as Mpu, and one of the word lines in the mat pu is activated via the X decoder X-DEC and the X driver X-DRV. Signal.
When the write enable signal WE is at a high level, that is, in a write operation, the mat pu selection signal line is output as Mpu only when the mat is in the rewrite permitted area. If the mat is a rewrite prohibited area, WA is output as Mpu. Whether the area is a rewrite permitted area or a rewrite prohibited area is determined by information stored in the storage unit of the rewrite prohibited mat.

  When one node ST1 of the flip-flop circuit FF goes low, the other node ST2 of the flip-flop FF goes high, and thereafter, after a predetermined delay time defined by the delay circuit D2 has elapsed, the transistor TR2 is turned on. And set ST2 to low level. Thereby, ST1 returns to the high level again.

FIG. 7B is a time chart of each signal at the time of the write operation when the mat pu is in the rewrite prohibited area.
In FIG. 7A, one node ST1 of the flip-flop FF is at a high level because the transistor Tr1 is normally off, and the output WA of the AND circuit G2 is always at a low level because the signal passes through the knot circuit NOT. In the rewrite prohibited area, the write enable signal WE is always at the low level. In the rewrite prohibited area, when the write enable signal WE is at a high level, the output Mpu of the multiplexer MPLX matches WA (in this case, WA is at a low level), so that the mat pu is not selected at the time of a write instruction.

When a write command to an address in the mat pd is generated by the chip selection signal CS (the write enable signal WA is at a high level), the mat pd selection signal goes to a high level. As a result, after the delay time of the delay circuit D1, the transistor Tr1 turns on, and one node ST1 of the flip-flop FF changes to low level. In this state, since the AND circuit G2 is turned on, the output WA matches the mat pu selection signal line. At this time, when a write command to the mat pu (the write enable signal WE is at a high level) is given, the output Mpu of the multiplexer MPLX goes high in accordance with the mat pu selection signal. As a result, the word line corresponding to the in-mat select signal from the address predecoder is activated, and the write operation is performed.
Note that rewriting permission for the mat pd may be performed by giving a write command to the mat pu in contrast.

  According to the embodiment shown in FIG. 7, there is an effect that a highly reliable system can be obtained. That is, it is possible to define the rewrite-inhibited area from the software side by a program, but according to the present embodiment in which the circuit is used to define the rewrite-inhibited area, there is a possibility that the rewritable area is erroneously written in the normal operation. It is greatly reduced. Further, since the signals for two adjacent mat sets are used, there is an advantage that the control circuit (127) can be easily arranged close to each mat set.

It is a basic block diagram of the system of the present invention. 2 is a flowchart of an error correction process in the system of FIG. FIG. 5 is a diagram illustrating how an error correction processing start command is generated according to the present invention. FIG. 3 is a diagram illustrating an example of a storage area of an error correction processing program according to the present invention. FIG. 4 is a diagram illustrating a configuration example of an address storage unit in a rewrite-protected area according to the present invention. FIG. 3 is a diagram illustrating an example of a mat configuration of a ferroelectric memory according to the present invention. FIG. 3 is a control circuit for giving write permission to a rewrite prohibited area and an operation waveform diagram. It is a system configuration example including a conventional ECC circuit.

Explanation of reference numerals

80: DRAM (Dynamic Random Access Memory)
81: memory cell array 82: information storage bit area 83: parity bit area 84: peripheral circuit section 85: ECC circuit 100: system of the present invention 110: CPU (central processing unit)
120: Ferroelectric memory 121: Memory cell array 122: Rewrite prohibited area 123: Rewrite permitted area 124: Information storage bit area 125: Parity bit area 126: Peripheral circuit section 127: Control circuit for giving write permission to the rewrite prohibited area Mat (i) u: upper mat mat (i) d: lower mat
WL (i): Word line
X-DRV: X driver
X-DEC: X decoder
FF: flip-flop circuit
MPLX: Multiplexer
D1, D2: delay circuit
NOT: Knot circuit
Tr1, Tr2: Transistor
G1, G2: AND circuit
WE: Write enable signal
A0 to AN: Address signal
WA, Mpu: Signal line
CS: Chip select signal

Claims (8)

In a system having a first mode and a second mode,
A first memory block including a plurality of memory cells each having a ferroelectric capacitor and a switching transistor;
A CPU connected to the first memory block,
The first memory block has a first area for storing normal data, and a second area for storing error correction information for error correction of the normal data,
In the first mode, the CPU accesses the normal data without performing a correction process including an error check using the error correction information, and a write operation to the first memory block is prohibited,
In the second mode, a write operation to the first memory block is permitted, and the system performs an error check on data using the normal data and the error correction information, and detects error data by the error check. A correction process for correcting the error data and writing back the error data. The system according to claim 1,
The system wherein the correction processing is performed by the CPU according to a correction processing program. The system according to claim 1 or 2,
The system generates a trigger signal based on a predetermined condition and starts the correction process. The system according to any one of claims 1 to 3,
In the second mode, a memory cell into which correction data is written is the same as a memory cell from which the error data has been read. The system according to any one of claims 1 to 4,
The system, wherein the system is a mobile device. The system according to any one of claims 1 to 5,
The plurality of memory cells store data according to a polarization direction of the ferroelectric capacitor,
The system wherein the data is read from a corresponding memory cell by detecting the polarization direction. The system according to any one of claims 1 to 6,
The system of claim 1, wherein the first memory block is used to store an OS program of the system. The system according to any one of claims 1 to 7,
The system according to claim 1, wherein the error correction information of the second area is a parity bit of normal data of the first area.

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