Abstract:
A DRAM device includes an ECC generator/checker that generates ECC syndromes corresponding to items of data stored in the DRAM device. The DRAM device also includes an ECC controller that causes the ECC syndromes to be stored in the DRAM device. The ECC controller also causes a flag bit having a first value to be stored in the DRAM device when a corresponding ECC syndrome is stored. The ECC controller changes the flag bit to a second value whenever the corresponding data bits are modified, this indicating that the stored syndrome no longer corresponds to the stored data. In such case, the ECC controller causes a new ECC syndrome to be generated and stored, and the corresponding flag bit is reset to the first value. The flag bits may be checked in this manner during a reduced power refresh to ensure that the stored syndromes correspond to the stored data.

Description:
TECHNICAL FIELD 
     This invention relates to memory devices, and, more particularly, to a method and system for efficiently checking and correcting data read from memory devices to allow the memory devices to consume relatively little power during refresh. 
     BACKGROUND OF THE INVENTION 
     As the use of electronic devices, such as personal computers, continues to increase, it is becoming ever more important to make such devices portable. The usefulness of portable electronic devices, such as notebook computers, is limited by the limited length of time batteries are capable of powering the device before needing to be recharged. This problem has been addressed by attempts to increase battery life and attempts to reduce the rate at which such electronic devices consume power. 
     Various techniques have been used to reduce power consumption in electronic devices, the nature of which often depends upon the type of power consuming electronic circuits that are in the device. For example, electronic devices such as notebook computers, typically include memory devices, such as dynamic random access memory (“DRAM”) devices, that consume a substantial amount of power. As the data storage capacity and operating speeds of memory devices continue to increase, the power consumed by such devices has continued to increase in a corresponding manner. Therefore, many attempts to reduce the power consumed by an electronic device have focused on reducing the power consumption of memory devices. 
     In general, the power consumed by a memory device increases with both the capacity and the operating speed of the memory device. The power consumed by memory devices is also affected by their operating mode. For example, a DRAM device generally consumes a relatively large amount of power when the memory cells of the DRAM device are being refreshed. As is well-known in the art, DRAM memory cells, each of which essentially consists of a capacitor, must be periodically refreshed to retain data stored in the DRAM device. Refresh is typically performed by essentially reading data bits from the memory cells in each row of a memory cell array and then writing those same data bits back to the same cells in the row. A relatively large amount of power is consumed when refreshing a DRAM because rows of memory cells in a memory cell array are being actuated in the rapid sequence. Each time a row of memory cells is actuated, a pair of digit lines for each memory cell are switched to complementary voltages and then equilibrated. As a result, DRAM refreshes tend to be particularly power-hungry operations. Further, since refreshing memory cells must be accomplished even when the DRAM is not being used and is thus inactive, the amount of power consumed by refresh is a critical determinant of the amount of power consumed by the DRAM over an extended period. Thus many attempts to reduce power consumption in DRAM devices have focused on reducing the rate at which power is consumed during refresh. 
     Refresh power can, of course, be reduced by reducing the rate at which the memory cells in a DRAM are being refreshed. However, reducing the refresh rate increases the risk that data stored in the DRAM memory cells will be lost. More specifically, since, as mentioned above, DRAM memory cells are essentially capacitors, charge inherently leaks from the memory cell capacitors, which can change the value of a data bit stored in the memory cell over time. However, current leaks from capacitors at varying rates. Some capacitors are essentially short-circuited and are thus incapable of storing charge indicative of a data bit. These defective memory cells can be detected during production testing, and can then be repaired by substituting non-defective memory cells using conventional redundancy circuitry. On the other hand, current leaks from most DRAM memory cells at much slower rates that span a wide range. A DRAM refresh rate is chosen to ensure that all but a few memory cells can store data bits without data loss. This refresh rate is typically once every 64 ms. The memory cells that cannot reliably retain data bits at this refresh rate are detected during production testing and replaced by redundant memory cells. 
     One technique that has been used to prevent data errors during refresh as well as at other times is to generate an error correcting code “ECC,” which is known as a “syndrome,” from each item of stored data, and then store the syndrome along with the data. When the data are read from the memory device, the syndrome is also read, and it is then used to determine if any bits of the data are in error. As long as not too many data bits are in error, the syndrome may also be used to correct the read data. 
     The use of ECC techniques can allow DRAM devices to be refreshed at a slower refresh rate since resulting data bit errors can be corrected as long as the refresh rate is not so low that more errors are generated than can be corrected by ECC techniques. The use of a slower refresh rate can provide the significant advantage of reducing the power consumed by DRAM devices. Prior to entering a reduced power refresh mode, each item of data is read. A syndrome corresponding to the read data is then generated and stored in the DRAM device. When exiting the reduced power refresh mode, the each item of data and each corresponding syndrome are read from the DRAM device. The read syndrome is then used to determine if the item of read data is in error. If the item of read data is found to be in error, the read syndrome is used to correct the read item of data, and the incorrect item of data is then overwritten with the corrected item of data. 
     One disadvantage of using the above-described ECC techniques in memory systems is the time and power required to generate and store ECC syndromes when entering the reduced power refresh mode. Each time the reduced power refresh mode is entered, all of the data stored in the DRAM device must be read, and a syndrome must be generated for each item or group of items of read data. The generated syndromes must then be stored. It can require a substantial period of time to accomplish these operations for the large amount of data stored in conventional high-capacity DRAM devices. During this time that the stored data are being checked, the DRAM device generally cannot be accessed for a read or a write operation. As a result, the operation of memory access devices, such as processors, is stalled until the data checking operations have been completed. Furthermore, a substantial amount of power can be consumed during the time the stored data are being checked and possibly corrected. These operations must be performed even though very little if any of the data stored in the DRAM device may have changed since the data was previously read and corresponding syndromes stored. 
     A similar problem exists where ECC techniques are being used to correct data storage errors in normal operation, i.e., not for a reduced power refresh mode. Each time a read request is coupled to a DRAM or other memory device, the syndrome corresponding to the read data must also be read, and the read data must then be checked using the read syndrome. These operations must be performed each time a read request is received even though the read data may not have changed since the read data was either written or previously read. The time required to perform these operations increases the latency of the memory device since the read data are not accessible to a memory requester until after these operations have been completed. 
     There is therefore a need for a memory system and method that uses ECC techniques to insure data integrity and allow operations in a reduced power refresh mode, but does so in a manner that does not unduly increase the read latency or power consumption of the memory device. 
     SUMMARY OF THE INVENTION 
     An error checking and correction (“ECC”) method and system includes an ECC syndrome and a respective flag bit stored for each of a plurality of groups of data bits stored in an array of memory cells. The flag bit has a first value when the ECC syndrome is stored, and a second value if any of the data bits in the respective group are modified such as by writing data to the memory cells storing the data bits. The ECC method and system may be used in a reduced power refresh mode by checking the flag bit corresponding to each group of data bits and then generating and storing a new syndrome if the flag bit has the second value indicative of at least some of the data bits in a group were modified since the previous refresh. The ECC method and system may also be used during refresh or in normal operation to determine if an ECC syndrome can be used to check and correct corresponding data. When used in this manner, the ECC syndrome is used to check the correctness of the data bits, and, if an error is found, to generate corrected data bits. The corrected data bits can then be stored in the memory device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a computer system according to one of the invention. 
         FIG. 2  is a flow chart showing the operation of the memory device of  FIG. 1  in a low power refresh mode. 
         FIG. 3  is a flow chart showing the operation of the memory device of  FIG. 1  in checking the integrity of data stored in the memory device. 
         FIG. 4  is a block diagram of a memory device according to one embodiment of the invention that may be used in the computer system of  FIG. 1 . 
         FIG. 5  is an address map showing the organization of data stored in the memory device of  FIG. 1  or  FIG. 4  according to one embodiment of the invention. 
         FIG. 6  is an address map showing the organization of data stored in the memory device of  FIG. 1  or  FIG. 4  according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A computer system  100  including a memory device employing ECC techniques according to one embodiment of the invention is shown in  FIG. 1 . The computer system  100  includes a central processor unit (“CPU”)  104  coupled to a system controller  106  through a processor bus  108 . The system controller  106  is coupled to input/output (“I/O”) devices (not shown) through a peripheral bus  110  and to an I/O controller  114  through an expansion bus  116 . The I/O controller  114  is also connected to various peripheral devices (not shown) through an I/O bus  118 . The system controller  106  includes a memory controller  120  that is coupled to a dynamic random access memory (“DRAM”)  122  through an address bus  126 , a control bus  128 , and a data bus  130 . The DRAM  122  includes a DRAM array  140  that stores data. The locations in the DRAM  122  to which data are written and data are read are designated by addresses coupled to the DRAM  122  on the address bus  126 . The operation of the DRAM  122  is controlled by control signals coupled to the DRAM  122  on the control bus  128 . These control signals can cause the DRAM  122  to operate in various refresh modes, such as a “self-refresh” mode in which periodic refresh cycles are periodically initiated without the need to apply control signals to the DRAM  122 . The DRAM  122  also includes ECC logic  144  that is operable to generate syndromes corresponding to data stored in the DRAM array  140 , and to check and, if necessary, correct data. The operation of the ECC logic  144  is controlled by an ECC controller  146 . The syndromes generated by the ECC logic  144  are stored in a syndrome memory  148 . 
     According to one embodiment of the invention, the DRAM  122  enters a reduced power mode refresh mode, such as a self-refresh mode, at step  150  using the process shown in  FIG. 2 . In step  152 , the ECC controller  146  initializes an address to a first address in the DRAM array  140 . This address is preferably the address for the first row of memory cells in the DRAM array  140  since the refresh of the DRAM array  140  is performed on a row-by-row basis. In step  154 , the ECC controller  146  causes data stored in the DRAM array  140  at a current address (which is initially the first address) and a corresponding syndrome and flag bit to be transferred from the DRAM array  140  and the syndrome memory  148 , respectively, to the ECC logic  144 . In transferring the data from the DRAM array  140 , the memory cells storing the data are inherently refreshed. 
     The ECC logic  144  initially does not use the syndrome to check the read data. Instead, the ECC logic  144  checks to see if a flag bit appended to the syndrome has been set at step  160 . The flag bit is an extra bit appended to the syndrome that indicates whether the stored data corresponding to the syndrome has been modified since the ECC syndrome was generated. The first time the DRAM  122  enters the reduced power refresh mode, a syndrome will not have been generated for the data, and the flag bit will not have been set. Therefore, the process branches to step  164  where the ECC logic  144  generates a syndrome corresponding to the data. The ECC controller  146  then causes the syndrome and set flag bit to be written to the syndrome memory  148  at step  168 . A check is then made at step  170  to determine if the data transferred to the ECC logic  144  was stored in the DRAM array  140  at the last address in the DRAM array  140 . If it is, the process ends at step  174  until the low power refresh is again initiated at step  150 . If the data is not from the last address in the DRAM array  140 , the address is incremented at step  176 , and the process returns to step  154 . 
     When the DRAM  122  subsequently enters the reduced power refresh mode, the process shown in  FIG. 2  again starts at  150 . The ECC controller  146  again initializes an address to a first address in the DRAM array  140  and then causes data stored in the DRAM array  140  and a corresponding syndrome and flag bit to be transferred to the ECC logic  144 . In transferring the data from the DRAM array  140 , the memory cells storing the data are again refreshed. The ECC logic  144  again checks to see if a flag bit appended to the syndrome has been set at step  160 . If the DRAM array  140  was previously refreshed in the reduced power mode and if no data have been written to the memory cells corresponding to the current address since the last refresh, the flag bit will still be set. The process shown in  FIG. 2  then branches directly to step  170 , thus bypassing step  164  where a syndrome is generated and step  168  where the syndrome and set flag bit are written to the syndrome memory  148 . The process then loops through steps  154 - 176  until the entire DRAM array  140  has been refreshed. The use of ECC techniques in the reduced power refresh mode allows refresh to occur at a rate that is sufficiently low that data retention errors can be expected since a limited number of data retention errors can be corrected As is well known in the art, ECC techniques allow a limited number of bits to be corrected. Therefore, the refresh rate in the reduced power refresh mode should not be so low that more errors are generated in a group of data than can be corrected by the ECC techniques. This reduced refresh rate can significantly reduce the power consumed by the DRAM  122 . 
     It is possible for the data stored in the DRAM array  140  to be modified between refreshes by, for example, writing data to the DRAM array  140 . For this reason, each time data are written to the DRAM array  140 , the ECC logic  144  resets the flag bit appended to the syndrome corresponding to the data stored in the memory cells to which the data are written. 
     One of the advantages of using the process shown in  FIG. 2  is that, in many cases, it will be necessary to generate and store syndromes for very few memory cells in the DRAM array  140 . When the DRAM  122  is idle, as it generally will be when in a reduced power refresh mode, such as a self-refresh mode, data will not be written to the DRAM array  140 . As a result, the flag bit appended to almost all syndromes will still be set, thus making it unnecessary to generate and store syndromes for almost all of the memory cells in the DRAM array  140 . As a result, the power consumed by the DRAM  122  is reduced by the amount of power that would be consumed in performing these syndrome generating and storing operations. Without using the process shown in  FIG. 2 , it would be necessary to generate and store syndromes for all of the data stored in the DRAM array  140  each time the reduced power refresh mode was entered thereby consuming substantially more power. 
     The reduced power refresh mode of the DRAM  122  also may be conducted using alternate processes. For example, prior to entering a reduced power refresh mode, the ECC logic  144  can generate a syndrome from all of the data stored in the DRAM array  140 , and each generated syndrome and a set flag bit can then be stored in the syndrome memory  148 . As a result, the ECC logic  144  will not detect a flag bit that has not been set when performing the first refresh in the reduced refresh mode. 
     A process that is similar to the process shown in  FIG. 2  can also be used to reduce power consumption when background ECC techniques are being used to insure the integrity of data stored in the DRAM  122 . As with the process shown in  FIG. 2 , each time data are written the DRAM array  140 , the flag bit of a corresponding syndrome is reset. The process, which is shown in  FIG. 3 , is entered at  200  when the integrity of a group of data, such as data stored in an entire row, is to be checked. The ECC controller  146  initializes an address to a first address in the DRAM array  140  at step  202 . This address is preferably the address for the first row of memory cells in the DRAM array  140 . In step  204 , the ECC controller  146  causes data stored in the DRAM array  140  at the current address, a corresponding syndrome and a corresponding flag bit to be transferred from the DRAM array  140  and the syndrome memory  148 , respectively, to the ECC logic  144 . The ECC logic  144  checks to see if a flag bit appended to the syndrome is set at step  208 . If the flag bit is set, meaning that the data has not been modified since the last integrity check, the ECC logic  144  uses the syndrome to determine if any data retention errors have arisen at step  210 . If the syndrome indicates the data are in error, the syndrome is used to correct the error at step  214 . The corrected data are then written to the DRAM array  140  at step  218  before progressing to step  220 . If no data retention error was detected at step  210 , the process branches directly to step  220 . 
     If the ECC logic  144  determines at step  208  that the flag bit is not set, meaning that the data corresponding to the syndrome have been modified, the process branches to step  224  where the ECC logic  144  generates a new syndrome. This syndrome, as well as a set flag bit, are then written to the syndrome memory  148  at step  226  before branching to step  220 . 
     At step  220 , a check is made to determine if the data transferred to the ECC logic  144  for integrity checking was stored in the final address of the DRAM array. If so, the process ends at step  222  until the integrity check is again initiated at step  200 . Otherwise, the address is incremented at step  228 , and the process returns to step  204 . 
     The use of the process shown in  FIG. 3  can considerably reduce the power consumed by the DRAM  122  since it will often not be necessary to generate and store syndromes for the data stored in the DRAM array  140 . Instead, it will be necessary to generate and store a syndrome for data only if the data have been modified. If there was no way of determining if the data had changed; it would be necessary to generate and store a syndrome each time data was written to the DRAM  122 . Furthermore, if the syndrome did not match data stored in the DRAM array  140 , there would be no way to determine if a data retention error had occurred (in which case the syndrome should be used to generate and store corrected data) or if new data had been written to that location (in which case the syndrome should not be used to generate and store corrected data). 
     A synchronous DRAM (“SDRAM”)  300  according to one embodiment of the invention is shown in  FIG. 4 . The SDRAM  300  includes an address register  312  that receives bank addresses, row addresses and column addresses on an address bus  314 . The address bus  314  is generally coupled to a memory controller like the memory controller  120  shown in  FIG. 1 . Typically, a bank address is received by the address register  312  and is coupled to bank control logic  316  that generates bank control signals, which are described further below. The bank address is normally coupled to the SDRAM  300  along with a row address. The row address is received by the address register  312  and applied to a row address multiplexer  318 . The row address multiplexer  318  couples the row address to row address latch &amp; decoder circuit  320   a - d  for each of several banks of memory cell arrays  322   a - d , respectively. 
     One of the latch &amp; decoder circuits  320   a - d  is enabled by a control signal from the bank control logic  316  depending on which bank of memory cell arrays  322   a - d  is selected by the bank address. The selected latch &amp; decoder circuit  320  applies various signals to its respective bank  322  as a function of the row address stored in the latch &amp; decoder circuit  320 . These signals include word line voltages that activate respective rows of memory cells in the banks  322   a - d.    
     The row address multiplexer  318  also couples row addresses to the row address latch &amp; decoder circuits  320   a - d  for the purpose of refreshing the memory cells in the banks  322   a - d . The row addresses are generated for refresh purposes by a refresh counter  330 . The refresh counter  330  periodically increments to output row addresses for rows in the banks  322   a - d . During operation in the low power, reduced refresh rate mode described above, the refresh counter  330  causes the memory cells in the banks  322   a - d  to be refreshed at a rate that is sufficiently low that data errors are likely to occur. Refreshing the memory cells at this low rate causes relatively little power to be consumed during self-refresh or other reduced refresh periods. During operation in a normal refresh mode, the refresh counter  330  periodically increments at a normal refresh rate that generally does not result in data retention errors during a normal refresh mode. The refresh of the memory cells is typically performed every 64 ms. 
     After the bank and row addresses have been applied to the address register  312 , a column address is applied to the address register  312 . The address register  312  couples the column address to a column address counter/latch circuit  334 . The counter/latch circuit  334  stores the column address, and, when operating in a burst mode, generates column addresses that increment from the received column address. In either case, either the stored column address or incrementally increasing column addresses are coupled to column address &amp; decoders  338   a - d  for the respective banks  322   a - d . The column address &amp; decoders  338   a - d  apply various signals to respective sense amplifiers  340   a - d  through column interface circuitry  344 . The column interface circuitry  344  includes conventional I/O gating circuits, DQM mask logic, read data latches for storing read data from the memory cells in the banks  322   a - d  and write drivers for coupling write data to the memory cells in the banks  322   a - d.    
     The column interface circuitry  344  also includes an ECC generator/checker  346  that essentially performs the same function as the ECC logic  144  in the DRAM  122  of  FIG. 1 . The ECC generator/checker  346  may be implemented by conventional means, such as by chains of exclusive OR gates implementing a Hamming code. Syndromes corresponding to the data stored in the memory cells in the banks  322   a - d  and corresponding flag bits may be stored in one or more of the banks  322   a - d . Data from one of the banks  322   a - d  are sensed by the respective set of sense amplifiers  342   a - d . When data are transferred from the memory cells of the banks  322   a - d  during the reduced power refresh mode, the corresponding syndrome and flag bit is coupled to the ECC generator checker  346 . The ECC generator/checker  346  then checks and, if necessary, corrects the data as explained above. In the event data are being coupled from the banks  322   a - d  for a read operation, the data are coupled to a data output register  348 , which applies the read data to a data bus  350 . Data read from one of the banks  322   a - d  may be coupled to the data bus  350  through the data output register  348  without be processed by the ECC generator/checker  346 . Alternatively, the read data may be processed by the ECC generator/checker  346  to detect and correct errors in the read data. 
     Data to be written to the memory cells in one or more of the banks  322   a - d  are coupled from the data bus  350  through a data input register  352  directly to write drivers in the column interface circuitry  344  without interfacing with the ECC generator/checker  346 . However, the flag bit corresponding to the write data is reset as explained above to indicate that any data stored in the location where the data are written has been modified. Alternatively, write data may be coupled to the ECC generator/checker  346  so it can generate a corresponding syndrome. The write data, the corresponding syndrome and a set flag bit are then coupled to write drivers in the column interface circuitry  344 , which couple the data, syndrome and flag bit to the memory cells in one of the banks  322   a - d . A pair of complementary data mask signals “DQML” and “DQMH” may be applied to the column interface circuitry  344  and the data output register  348  to selectively alter the flow of data into and out of the column interface circuitry  344 , such as by selectively masking data to be read from the banks of memory cell arrays  322   a - d.    
     The above-described operation of the SDRAM  300  is controlled by control logic  356 , which includes a command decoder  358  that receives command signals through a command bus  360 . These high level command signals, which are typically generated by a memory controller such as the memory controller  120  of  FIG. 1 , are a clock chip select signal CS#, a write enable signal WE#, a column address strobe signal CAS#, and a row address strobe signal RAS#, with the “#” designating the signal as active low. Various combinations of these signals are registered as respective commands, such as a read command or a write command. The control logic  356  also receives a clock signal CLK and a clock enable signal CKE, which allow the SDRAM  300  to operate in a synchronous manner. The control logic  356  generates a sequence of control signals responsive to the command signals to carry out the function (e.g., a read or a write) designated by each of the command signals. The control logic  356  also applies signals to the refresh counter  330  to control the operation of the refresh counter  230  during refresh of the memory cells in the banks  322 . The control signals generated by the control logic  356 , and the manner in which they accomplish their respective functions, are conventional. Therefore, in the interest of brevity, a further explanation of these control signals will be omitted. 
     The control logic  356  also includes a mode register  364  that may be programmed by signals coupled through the command bus  360  during initialization of the SDRAM  300 . The mode register  364  then generates a mode bit that is used by the control logic  356  to enable the reduced power ECC modes described above with respect to  FIGS. 2 and 3 . Finally, the control logic  356  also includes an ECC controller  370  that essentially performs the functions of the ECC controller  146  in the DRAM  122  of  FIG. 1 . The ECC controller  146  causes the control logic  356  to issue control signals to the ECC generator/checker  346  and other components to generate syndromes and flag bits for storage in the banks  322   a - d , and to check and correct data read from the banks  322   a - d  using the stored syndromes and flag bits. 
     Although the SDRAM device  300  can have a variety of configurations, in one embodiment the storage of data in the SDRAM device  300  is organized as shown in  FIG. 5 . As shown in  FIG. 5 , each row of memory cells in the DRAM array  140  contains 128 column groups, and each column group contains 128 bits of data arranged as 8 16-bit words plus an additional 9 bits that are used to store an 8-bit ECC syndrome and 1 flag bit. The 8 syndrome bits are capable of correcting a single bit error in the respective column group. If the ability to correct a larger number of bits is desired, then the number of syndrome bits can be increases accordingly. 
     One disadvantage of the arrangement for storing data as shown in  FIG. 5  is that each column group contains an odd number of bits, i.e., 128 data bits, 8 syndrome bits and 1 flag bit. However, memory devices generally use rows with an even number of columns. To alleviate this disadvantage, data can be stored in the SDRAM device  300  using organization shown in  FIG. 6 . As shown in  FIG. 6 , each row of memory cells in the DRAM array  140  contains 256 column groups, and each column group contains 64 bits of data arranged as 4 16-bit words plus an additional 8 bits that are used to store a 7-bit ECC syndrome and 1 flag bit. The 7 syndrome bits are capable of correcting a single bit error in the 64 bits in the respective column group. As a result, each column group now contains an even number of bits, i.e., 64 data bits, 7 syndrome bits and 1 flag bit. 
     Although the present invention has been described with reference to the disclosed embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, although the reduced power refresh mode has been described in the context of a self-refresh reduced power mode, it will be understood that it may also be used in other refresh modes. Other variations will also be apparent to one skilled in the art. Accordingly, the invention is not limited except as by the appended claims.