Gain control for read operations in flash memory
View Patent ↗A technique for performing read operations with reduced errors in a memory device such as flash memory. An automatic gain control approach is used in which cells which have experienced data retention loss are read by a fine M-level quantizer which uses M-1 read threshold voltage levels. In one approach, M-quantized threshold voltage values are multiplied by a gain to obtain gain-adjusted threshold voltage values, which are quantized by an L-level quantizer, where L<M, by comparing the gain-adjusted threshold voltage values to read threshold voltage levels of a fresh memory device. In another approach, the read threshold voltage levels of the fresh device are gain adjusted for reading non-gain-adjusted threshold voltage values from the cells which have experienced data retention loss.
1. A method of reading data from a unit of a flash memory device, the method comprising:
reading data from cells of the flash using a quantizer with M-1 read threshold voltage levels, thereby assigning to each cell in the page a representation value from a predefined set of M values;
based on said representation values compute a set of L-1 read threshold voltage levels where L is less than M; and
assigning a read threshold voltage value to each cell in the unit, said read threshold voltage value is taken from a set of L values and computed as a function of the representation value of the cell and said L-1 read threshold voltage levels.
2. The method of claim 1 , wherein said computing step comprises computing said L-1 read threshold voltage levels according to a function that depends on values read from a set of cells.
3. The method of claim 2 wherein said set of cells is a set of cells containing data of the unit.
4. The method of claim 2 wherein said set of cells is separate from the cells containing data of the unit.
5. The method of claim 2 , wherein said computing step comprises:
multiplying said M-1 read threshold voltage levels by a function of a magnitude measure of said set of cells thereby generating a set of M-1 modified read threshold voltage levels; and
choosing a set of L-1 read threshold voltage levels as a subset of said set of M-1 modified read threshold voltage levels.
6. The method of claim 5 further comprising dividing said modified read threshold voltage levels by a number R.
7. The method of claim 6 , wherein R is a function of a magnitude measure of a set of reference values associated with said set of cells.
8. The method of claim 5 , wherein said magnitude measure is a Euclidian norm.
9. The method of claim 5 , wherein said magnitude measure is an L 1 norm.
10. The method of claim 5 , wherein said magnitude measure is an L ∞ norm.
11. A method of reading data from a unit of a flash memory device, the method comprising:
reading data from cells of the flash using a quantizer with M-1 read threshold voltage levels, thereby assigning to each set of N read cells a representation vector from a predefined set of M N vectors;
based on said representation vector compute a set of L read threshold voltage levels where L is less than M N ; and
assigning a vector of read threshold voltage values to each set of N cells in the unit, each said vector of read threshold voltage values is taken from a set of L read threshold vectors and computed as a function of the representation vector of said N cells and said L read threshold voltage levels.
12. The method of claim 11 wherein said L is a power of 2.
13. The method of claim 12 wherein each said set of N cells read from the flash device is programmed to one of L states, representing log 2 (L) bits.