Nonvolatile memory with variable read threshold
First Claim
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1. A flash memory system comprising:
- a flash memory array that includes a plurality of memory cells programmed to a plurality of programmed states;
a reading circuit connected to the memory array, the reading circuit comparing a memory cell threshold voltage to a first plurality of predetermined voltages to distinguish the plurality of programmed states in a first mode and comparing the memory cell threshold voltage to a second plurality of predetermined voltages to distinguish the plurality of programmed states in a second mode, the highest one of the second plurality of predetermined voltages being higher than the highest one of the first plurality of predetermined voltages; and
a programming circuit that programs cells to a first plurality of target voltages individually corresponding to programmed states in the first mode and programming cells to a second plurality of target voltages individually corresponding to programmed states in the second mode.
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Abstract
Data is read from a nonvolatile memory array using one or more read voltages that are adjusted during memory life. Programming target voltages and read voltages may be adjusted together over memory life to map memory states to an increasingly wide threshold window. Individual memory states are mapped to sub-ranges that are made wider, reducing errors.
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Citations
13 Claims
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1. A flash memory system comprising:
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a flash memory array that includes a plurality of memory cells programmed to a plurality of programmed states; a reading circuit connected to the memory array, the reading circuit comparing a memory cell threshold voltage to a first plurality of predetermined voltages to distinguish the plurality of programmed states in a first mode and comparing the memory cell threshold voltage to a second plurality of predetermined voltages to distinguish the plurality of programmed states in a second mode, the highest one of the second plurality of predetermined voltages being higher than the highest one of the first plurality of predetermined voltages; and a programming circuit that programs cells to a first plurality of target voltages individually corresponding to programmed states in the first mode and programming cells to a second plurality of target voltages individually corresponding to programmed states in the second mode. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)
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Specification