Reading a multi-bit value from a memory cell

摘要

Adaptive write operations for non-volatile memories select programming parameters according to monitored programming performance of individual memory cells. In one embodiment of the invention, programming voltage for a memory cell increases by an amount that depends on the time required to reach a predetermined voltage and then a jump in the programming voltage is added to the programming voltage required to reach the next predetermined voltage. The adaptive programming method is applied to the gate voltage of memory cells; alternatively, it can be applied to the drain voltage of memory cells along a common word line. A circuit combines the function of a program switch and drain voltage regulator, allowing independent control of drain voltage of selected memory cells for parallel and adaptive programming. Verify and adaptive read operations use variable word line voltages to provide optimal biasing of memory and reference cells during sensing.

主权项

1. A method for reading a non-volatile memory comprising:
applying first read voltages to a selected memory cell storing a multi-bit data value and to a first reference cell having a first threshold voltage that is between a highest of a plurality of target threshold voltages that represent data values and a lowest of the target threshold voltages; comparing a first response that the selected memory cell has to the first read voltages to a response that the first reference cell has to the first read voltages; if the first response of the selected memory cell is greater than the response of the first reference cell, applying second read voltages to the selected memory cell and to a second reference cell having a second threshold voltage that is between the first threshold voltage and the lowest of the target threshold voltages and then comparing a second response of the selected memory cell to a response of the second reference cell; and if the first response of the selected memory cell is less than the response of the first reference cell, applying third read voltages to the selected memory cell and to a third reference cell having a third threshold voltage that is between the first threshold voltage and the highest of the target threshold voltages and then comparing a third response of the selected memory cell to a response of the third reference cell.