Patent number 8612830 is assigned to
The following quote was obtained by the news editors from the background information supplied by the inventors: "A technology disclosed herein relates to a nonvolatile memory device which uses a flash memory.
"In the memory device which uses the flash memory, error correction is essential because of characteristics of the flash memory. Errors are classified into a transient error and a stationary error. The transient error is detected when certain data is read, but not detected next time the same data is read. Such an error occurs in a state where an output can be detected as both '0' and '1' because of a noise or an input near threshold voltage value.
"The stationary error is a permanent error always detected, if detected when certain data is read, when the data is read thereafter. Such a stationary error occurs due to a change in threshold voltage value of a transistor which has a floating gate structure for a memory cell of the flash memory. When a threshold voltage value changes, '1' is read even though '0' is stored, or '0' is read even though '1' is stored. A representative cause of such a stationary error is a read disturbance.
"The read disturbance changes a threshold value of the transistor. Thus, written data is recognized as a different value when it is read. There are tendencies of a changing direction of the threshold value and recognition easiness of a different value. However, in a so-called multivalue flash memory (in other words, flash memory where each memory cell stores a plurality of bits of information by setting a plurality of threshold voltage values in one memory cell) and the like, both cases where '1' is mistaken for '0' and '0' is mistaken for '1' may equally occur. This is because, in the multivalue flash memory, a hamming distance is 1 between a plurality of bits corresponding to one of a plurality of voltage ranges divided based on a plurality of threshold voltage values and a plurality of bits corresponding to its adjacent voltage range.
"JP 2004-326867 A discloses measures to deal with the read disturbance.
"Generally, a storage medium using a flash memory, such as a compact flash memory card, an SD memory card, or a solid state drive, includes a controller in a module, and the controller controls a flash memory chip. This controller generally executes error correction. A flash memory chip that has an error correction function therein is also available.
"As the flash memory controller (or flash memory chip) executes error correction, how many bits of errors have been corrected is generally unknown to the outside. In this case, an error can be recognized for the first time when error correction becomes impossible.
"In data transfer through an interface of the conventional nonvolatile memory device, upon issuance of a read command, data corresponding to the read command is returned as a response. Interfaces that can identify status information of a command execution result before data transfer and after the data transfer are available. In both cases, however, the status information is a result of executing the command in principle. In other words, as a command and a status correspond to each other one to one, for example, the number of statuses to be returned is one with respect to commands of transferring a plurality of sectors."
In addition to the background information obtained for this patent, VerticalNews journalists also obtained the inventors' summary information for this patent: "A phenomenon of increase in errors such as a read disturbance is inherent in the memory device using the flash memory. Thus, even with an error correction capability of a plurality of bits, errors increase more than the error correction capability over time, causing a problem of impossibility of storing accurate information.
"The read disturbance is affected by the number of deletion and the like. Thus, even when correctable errors occur, a possibility that the level of the errors will progress over the error correction capability is different from one deletion unit to another.
"In the case of the nonvolatile memory device that uses the flash memory, an upper limit value of the number of deletion times is set for each deletion unit of the flash memory. Accordingly, the flash memory is disabled from deletion for each deletion unit during use to reach the end of its life. Thus, a unit that manages and controls the nonvolatile memory device always has to monitor a status of the nonvolatile memory device, and to execute a process to prevent a data loss before the device reaches the end of its life to be unusable. As the process to prevent a data loss, for example, a so-called refreshing process of copying data of a deletion unit approaching the end of life to another deletion unit may be carried out.
"According to a representative invention disclosed in this application, there is provided a nonvolatile memory device comprising: a plurality of memory cells; and a memory controller coupled to the plurality of memory cells to control data writing and data reading in the plurality of memory cells, wherein: each of the plurality of memory cells is a field effect transistor which includes a floating gate; the plurality of memory cells are divided into a plurality of deletion blocks which are deletion units; the plurality of deletion blocks include a first deletion block and a second deletion block; the plurality of memory cells in each of the plurality of deletion blocks store data containing error correcting codes; the nonvolatile memory device holds management information indicating the number of deletion times executed in each of the plurality of deletion blocks; and the memory controller is configured to: read data stored in the first deletion block; detect and correct an error contained in the read data by decoding the error correcting codes; execute, when the number of bits of the detected error exceeds a threshold value, a refreshing process to store the corrected data in the second deletion block; set a smaller value as the threshold value as an error frequency detected in the first deletion block is higher; and set a smaller value as the threshold value as the number of deletion times executed in the first deletion block is larger.
"According to an embodiment of this invention, in the flash memory device, before errors generated in the flash memory increase more than the error correction capability, proper refreshing can be carried out. Thus, a highly reliable memory device that uses up a life of a flash memory can be provided."
URL and more information on this patent, see: Kitahara, Jun; Mizushima,
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