Adaptive estimation of memory cell read thresholds
A method for operating a memory ( 28 ) that includes a plurality of analog memory cells ( 32 ) includes storing data in the memory by writing first storage values to the cells. Second storage values are read from the cells, and a Cumulative Distribution Function (CDF) of the second storage values is estimated. The estimated CDF is processed so as to compute one or more thresholds. A memory access operation is performed on the cells using the one or more thresholds.
1. A method, comprising:
receiving data to be stored in a memory, wherein the memory includes a plurality of data storage cells;
writing first storage values in the plurality of data storage cells;
reading second storage values from the plurality of data storage cells;
determining a number of data memory cells of the plurality of data memory cells associated with a plurality of programming levels dependent upon the read second storage values;
estimating a probability density function (PDF) for each programming level of the plurality of programming levels;
scaling each PDF dependent upon the determined number of data storage cells;
integrating the plurality of PDFs to produce a cumulative distribution function (CDF);
computing one or more thresholds dependent upon the CDF;
reading third storage values from the plurality of data storage cells;
comparing the third storage values to the computed one or more thresholds; and
generating data bits dependent upon the comparison.
2. The method of claim 1 , wherein each data storage cell of the plurality of data storage cells comprises an analog memory cell.
3. The method of claim 1 , wherein each data storage cell of the plurality of data storage cells comprises a multi-bit analog memory cell.
4. The method of claim 1 , wherein reading second storage values comprises converting an electrical charge into digital samples.
5. The method of claim 1 , wherein writing first storage values comprises encoding the received data using an error correction code (ECC).
6. An apparatus, comprising:
a memory including a plurality of data storage cells; and
a controller coupled to the memory, wherein the controller is configured to:
receive data to be stored in the memory;
write first storage values in the plurality of data storage cells;
read second storage values from the plurality of data storage cells;
determine a number of data storage cells of the plurality of data storage cells associated with a plurality of programming levels dependent upon the read second storage values;
estimate a probability density function (PDF) for each programming level of the plurality of programming levels;
scale each PDF dependent upon the determined number of data storage cells;
integrate the plurality of PDFs to produce a cumulative distribution function (CDF);
compute one or more thresholds dependent upon the CDF;
read third storage values from the plurality of data storage cells;
compare the third storage values to the computed one or more thresholds; and
generate data bits dependent upon the comparison.
7. The apparatus of claim 6 , wherein each data cell of the plurality of data storage cells comprises an analog memory cell.
8. The apparatus of claim 6 , wherein read second storage values comprises converting an electrical charge into digital samples.
9. The apparatus of claim 6 , wherein write first storage values in the plurality of data storage cells comprises encoding the data using an error correction code (ECC).
10. The apparatus of claim 9 , wherein the controller is further configured to decode the data bits using the ECC.
11. A non-transitory computer accessible storage medium having program instructions stored therein that, in response to execution by a computer system, causes the computer system to perform operations including:
receiving data to be stored in a memory, wherein the memory includes a plurality of data storage cells;
writing first storage values in the plurality of data storage cells;
reading second storage values from the plurality of data storage cells;
determining a number of data storage cells of the plurality of memory cells associated with a plurality of programming levels dependent upon the read second storage values;
estimating a probability density function (PDF) for each programming level of the plurality of programming levels;
scaling each PDF dependent upon the determined number of data storage cells;
integrating the plurality of PDFs to produce a cumulative distribution function (CDF);
computing one or more thresholds dependent upon the CDF;
reading third storage values from the plurality of data storage cells;
comparing the third storage values to the computed one or more thresholds; and
generating data bits dependent upon the comparison.
12. The non-transitory computer accessible storage medium of claim 11 , wherein each data storage cell of the plurality of data storage cells comprises an multi-bit analog memory cell.
13. The non-transitory computer accessible storage medium of claim 11 , wherein the writing the first storage values comprises encoding the data with an error correction code (ECC).
14. The non-transitory computer accessible storage medium of claim 13 , wherein perform operations further includes decoding the data bits using the ECC.
15. The non-transitory computer accessible storage medium of claim 13 , wherein the ECC comprises a low density parity check (LDPC).