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Technology Stocks : WDC/Sandisk Corporation -- Ignore unavailable to you. Want to Upgrade?

To: Binx Bolling who wrote (16800)	11/22/2000 7:13:31 PM
From: Binx Bolling	Read Replies (1) \| Respond to of 60323

United States Patent 6,151,246 So, et. al. Nov. 21, 2000 -------------------------------------------------------------------------------- Multi-bit-per-cell flash EEPROM memory with refresh Abstract A multibit-per-cell non-volatile memory divides the suitable threshold voltages of memory cells into ranges corresponding to allowed states for storage of data and ranges corresponding to forbidden zones indicating a data error. A read process in accordance automatically checks whether a threshold voltage is in a forbidden zone. In an alternative embodiment, a refresh process includes reprogramming the threshold voltage into an allowed state. In the case of a flash memory, a refresh reads a sector of the memory and saves corrected data from the sector in a buffer or another sector. The corrected data from the buffer or other sector can be written back in the original sector, or the corrected data can be left in the other sector with addresses of the original sector being mapped to the other sector. Refresh process for the non-volatile memory can be perform in response to detecting a threshold voltage in a forbidden zone, as part of a power-up procedure for the memory, or periodically with a period on the order of days, weeks, or months. As a further aspect, the allowed states correspond to gray coded digital values so that allowed states that are adjacent in threshold voltage correspond to multibit values that differ in only a single bit. Error detection and correction codes can be used to identify data errors and generate corrected data for refresh operations. -------------------------------------------------------------------------------- Inventors: So; Hock C. (Redwood City, CA); Wong; Sau C. (Hillsborough, CA). Assignee: SanDisk Corporation (Sunnyvale, CA). Appl. No.: 200,220 Filed: Nov. 25, 1998 Related U.S. Application Data Continuation-in-part of Ser No. 924,909, Sept. 8, 1997. Intl. Cl. : G11C 16/06 Current U.S. Cl.: 365/185.09; 365/185.24; 371/21.4 Field of Search: 365/185.24, 185.25, 185.03, 184, 185.09; 371/21.4, 21.1 -------------------------------------------------------------------------------- References Cited \| [Referenced By] -------------------------------------------------------------------------------- U.S. Patent Documents 4,964,079 Oct., 1990 Devin 365/168 5,689,465 Nov., 1997 Sukegawa et al. 371/10 5,751,639 May, 1998 Ohsawa 365/227 5,761,125 Jun., 1998 Himeno 365/185.24 5,909,449 Jun., 1999 So et al. 371/21.4 Primary Examiner: Nelms; David Assistant Examiner: Lam; David Attorney, Agent or Firm: Majestic, Parsons, Siebert & Hsue 23 Claims, 9 Drawing Figures