Static information storage and retrieval – Floating gate – Particular biasing
Reexamination Certificate
2001-01-18
2002-10-08
Elms, Richard (Department: 2824)
Static information storage and retrieval
Floating gate
Particular biasing
C365S185190, C365S185290
Reexamination Certificate
active
06462992
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to semiconductor electrically erasable programmable read only memories (EEprom), and specifically to a system of integrated circuit Flash EEprom chips.
Computer systems typically use magnetic disk drives for mass storage of data. However, disk drives are disadvantageous in that they are bulky and in their requirement for high precision moving mechanical parts. Consequently they are not rugged and are prone to reliability problems, as well as consuming significant amounts of power. Solid state memory devices such as DRAM's and SRAM's do not suffer from these disadvantages. However, they are much more expensive, and require constant power to maintain their memory (volatile). Consequently, they are typically used as temporary storage.
EEprom's and Flash EEprom's are also solid state memory devices. Moreover, they are nonvolatile, and retain their memory even after power is shut down. However, conventional Flash EEprom's have a limited lifetime in terms of the number of write (or program)/erase cycles they can endure. Typically the devices are rendered unreliable after 10
2
to 10
3
write/erase cycles. Traditionally, they are typically used in applications where semi-permanent storage of data or program is required but with a limited need for reprogramming.
Accordingly, it is an object of the present invention to provide a Flash EEprom memory system with enhanced performance and which remains reliable after enduring a large number of write/erase cycles.
It is another object of the present invention to provide an improved Flash EEprom system which can serve as non-volatile memory in a computer system.
It is another object of the present invention to provide an improved Flash EEprom system that can replace magnetic disk storage devices in computer systems.
It is another object of the present invention to provide a Flash EEprom system with improved erase operation.
It is another object of the present invention to provide a Flash EEprom system with improved error correction.
It is yet another object of the present invention to provide a Flash EEprom with improved write operation that minimizes stress to the Flash EEprom device.
It is still another object of the present invention to provide a Flash EEprom system with enhanced write operation.
SUMMARY OF THE INVENTION
These and additional objects are accomplished by improvements in the architecture of a system of EEprom chips, and the circuits and techniques therein.
According to one aspect of the present invention, an array of Flash EEprom cells on a chip is organized into sectors such that all cells within each sector are erasable at once. A Flash EEprom memory system comprises one or more Flash EEprom chips under the control of a controller. The invention allows any combination of sectors among the chips to be selected and then erased simultaneously. This is faster and more efficient than prior art schemes where all the sectors must be erased every time or only one sector at a time can be erased. The invention further allows any combination of sectors selected for erase to be deselected and prevented from further erasing during the erase operation. This feature is important for stopping those sectors that are first to be erased correctly to the “erased” state from over erasing, thereby preventing unnecessary stress to the Flash EEprom device. The invention also allows a global de-select of all sectors in the system so that no sectors are selected for erase. This global reset can quickly put the system back to its initial state ready for selecting the next combination of sectors for erase. Another feature of the invention is that the selection is independent of the chip select signal which enables a particular chip for read or write operation. Therefore it is possible to perform an erase operation on some of the Flash EEprom chips while read and write operations may be performed on other chips not involved in the erase operation.
According to another aspect of the invention, improved error correction circuits and techniques are used to correct for errors arising from defective Flash EEprom memory cells. One feature of the invention allows defect mapping at cell level in which a defective cell is replaced by a substitute cell from the same sector. The defect pointer which connects the address of the defective cell to that of the substitute cell is stored in a defect map. Every time the defective cell is accessed, its bad data is replaced by the good data from the substitute cell.
Another feature of the invention allows defect mapping at the sector level. When the number of defective cells in a sector exceeds a predetermined number, the sector containing the defective cells is replaced by a substitute sector.
An important feature of the invention allows defective cells or defective sectors to be remapped as soon as they are detected thereby enabling error correction codes to adequately rectify the relatively few errors that may crop up in the system.
According to yet another aspect of the present invention, a write cache is used to minimize the number of writes to the Flash EEprom memory. In this way the Flash EEprom memory will be subject to fewer stress inducing write/erase cycles, thereby retarding its aging. The most active data files are written to the cache memory instead of the Flash EEprom memory. Only when the activity levels have reduced to a predetermined level are the data files written from the cache memory to the Flash EEprom memory. Another advantage of the invention is the increase in write throughput by virtue of the faster cache memory.
According to yet another aspect of the present invention, one or more printed circuit cards are provided which contain controller and EEprom circuit chips for use in a computer system memory for long term, non-volatile storage, in place of a hard disk system, and which incorporate various of the other aspects of this invention alone and in combination.
Additional objects, features, and advantages of the present invention will be understood from the following description of its preferred embodiments, which description should be taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 3895360 (1975-07-01), Cricchi et al.
patent: 3898632 (1975-08-01), Spencer, Jr.
patent: 3906455 (1975-09-01), Houston et al.
patent: 3914750 (1975-10-01), Hadden, Jr.
patent: 4044339 (1977-08-01), Berg
patent: 4058799 (1977-11-01), George et al.
patent: 4064405 (1977-12-01), Cricchi et al.
patent: 4090258 (1978-05-01), Cricchi
patent: 4115850 (1978-09-01), Houston et al.
patent: 4130890 (1978-12-01), Adam
patent: 4141081 (1979-02-01), Horne et al.
patent: 4193128 (1980-03-01), Brewer
patent: 4241424 (1980-12-01), Dickson et al.
patent: 4412309 (1983-10-01), Kuo
patent: 4428047 (1984-01-01), Hayn et al.
patent: 4433387 (1984-02-01), Dyer et al.
patent: 4460982 (1984-07-01), Gee et al.
patent: 4468729 (1984-08-01), Schwartz
patent: 4521853 (1985-06-01), Guttag
patent: 4527257 (1985-07-01), Cricchi
patent: 4685084 (1987-08-01), Canepa
patent: 4758986 (1988-07-01), Kuo
patent: 4758988 (1988-07-01), Kuo
patent: 4763305 (1988-08-01), Kuo
patent: 4780855 (1988-10-01), Iida et al.
patent: 4796235 (1989-01-01), Sparks et al.
patent: 4805151 (1989-02-01), Terada et al.
patent: 4811293 (1989-03-01), Knothe et al.
patent: 4811294 (1989-03-01), Kobayashi et al.
patent: 4839705 (1989-06-01), Tigelaar et al.
patent: 4841482 (1989-06-01), Kreifels et al.
patent: 4860228 (1989-08-01), Carroll
patent: 4860261 (1989-08-01), Kreifels et al.
patent: 4860262 (1989-08-01), Chiu
patent: 4882642 (1989-11-01), Tayler et al.
patent: 4887234 (1989-12-01), Iijima
patent: 4920478 (1990-04-01), Furuya et al.
patent: 4931997 (1990-06-01), Mitsubishi et al.
patent: 4933906 (1990-06-01), Terada et al.
patent: 4939690 (1990-07-01), Momodomi et al.
patent: 4949309 (1990-08-01), Rao
patent: 4953129 (1990-08-01), Kobayashi et al.
patent: 5033023 (1991-07-01), Hsia et al.
patent: 5043940 (1991-08-01), Harari
patent: 5053990 (1991-10-01), Krei
Harari Eliyahou
Mehrotra Sanjay
Norman Robert D.
Nguyen Hien
SanDisk Corporation
Skjerven Morrill LLP
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