Syndrome weight based evaluation of memory cells performance using multiple sense operations

• US 10,388,394 B2
• Filed: 07/25/2017
• Issued: 08/20/2019
• Est. Priority Date: 07/25/2017
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Tate, Yonathan, Ish-Shalom, Tomer
• Primary Examiner(s): McMahon, Daniel F.
• Application Number:
  US15/658,430
• Publication Number:
  US 20190035485A1
• Time in Patent Office: 756 Days
• Field of Search: None
• CPC Class Codes: G06F 11/1012 using codes or arrangements adapted for a specific type of error G06F11/1048 takes precedence
• CPC Class Codes: G06F 11/1076 Parity data used in redundant arrays of independent storages, e.g. in RAID systems
• CPC Class Codes: G11C 29/42 using error correcting codes [ECC] or parity check
• CPC Class Codes: G11C 29/44 Indication or identification of errors, e.g. for repair

Abstract

A memory system includes an interface and storage circuitry. The interface is configured to communicate with a plurality of memory cells that store data by setting the memory cells to analog voltages representative of respective storage values. The storage circuitry is configured to read from a group of the memory cells a code word encoded using an Error Correction Code (ECC), by sensing the memory cells using at least first and second read thresholds for producing respective first and second readouts, to calculate, based on at least one of the first and second readouts, (i) a syndrome weight that is indicative of an actual number of errors contained in the code word, and (ii) a mid-zone count of the memory cells for which the first readout differs from the second readout, and, to evaluate a performance measure for the memory cells, based on the calculated syndrome weight and mid-zone count.

Claims

1. A memory system, comprising:

an interface, configured to communicate with a plurality of memory cells that store data by setting the memory cells to analog voltages representative of respective storage values; and

storage circuitry including a processor, configured to;

read from a group of the memory cells a code word that was encoded using an Error Correction Code (ECC), by sensing the memory cells in the group using at least first and second read thresholds for producing respective first and second readouts; and

based on at least one of the first readout and the second readout, the processor is configured to determine for the code word (i) a syndrome weight that is indicative of an actual number of errors contained in the code word, and (ii) a mid-zone count of the memory cells for which the first readout differs from the second readout,

wherein the processor is further configured to evaluate a performance measure for the memory cells in the group, based on the determined syndrome weight and on the mid-zone count, by estimating a channel matrix that is indicative of a readout performance of the memory cells in the group.

2. The memory system according to claim 1, wherein the storage circuitry processor is configured to determine the syndrome weight based only on one of the first readout and the second readout.

3. The memory system according to claim 1, wherein the storage circuitry processor is configured to assign respective soft metric values to elements of the first readout, based on the first readout and on the second readout, wherein each soft metric value comprises a sign and a magnitude, and to determine the syndrome weight based on signs of the soft metrics that were assigned to the elements of the first readout.

4. The memory system according to claim 1, wherein the storage circuitry processor is configured to produce a syndrome vector by multiplying the first readout by a parity-check matrix representing the ECC, and to calculate the syndrome weight by counting a number of nonzero elements in the syndrome vector.

5. The memory system according to claim 1, wherein the syndrome weight is a function of a linear combination of the mid-zone count and a conditional probability of the channel matrix, and wherein the storage circuitry processor is configured to estimate the channel matrix by estimating the conditional probability, based on the function and on the calculated syndrome weight and on the mid-zone count.

6. The memory system according to claim 1, wherein the storage circuitry is configured to select between hard and soft ECC decoding, based on the evaluated performance measure.

7. The memory system according to claim 1, wherein the storage circuitry is configured to determine a number of read thresholds to use in subsequent read operations based on the evaluated performance measure.

8. The memory system according to claim 1, wherein the memory cells in the group are configured to store multiple bits per cell, in a set of multiple programming levels predefined within a range of threshold voltages, wherein the storage circuitry is configured to:

sense the memory cells using multiple read thresholds, including the first and second read thresholds, and produce multiple respective readouts, including the first and second readouts, wherein the multiple read thresholds divide the range of threshold voltages into multiple zones;

identify among the multiple zones two or more middle zones corresponding to mid-ranges between respective programming levels;

calculate, by the storage circuitry processor, a syndrome weight based on one of the multiple readouts; and

evaluate, by the storage circuitry processor, the performance measure based on the syndrome weight that was calculated based on one of the multiple readouts, and on a number of memory cells for which the readouts fall in the middle zones.

9. A method, comprising:

in a memory system comprising a plurality of memory cells that store data by setting the memory cells to analog voltages representative of respective storage values, reading from a group of the memory cells a code word that was encoded using an Error Correction Code (ECC), by sensing the memory cells in the group using at least first and second read thresholds for producing respective first and second readouts;

based on at least one of the first readout and the second readout, calculating, by a processor of the memory system, for the code word (i) a syndrome weight that estimates an actual number of errors contained in the code word, and (ii) a mid-zone count of the memory cells for which the first readout differs from the second readout; and

evaluating, by the processor, a performance measure for the memory cells in the group, based on the calculated syndrome weight and on the mid-zone count, by estimating a channel matrix that is indicative of a readout performance of the memory cells in the group.

10. The method according to claim 9, wherein calculating the syndrome weight comprises calculating the syndrome weight based only on one of the first readout and the second readout.

11. The method according to claim 9, wherein calculating the syndrome weight comprises assigning respective soft metric values to elements of the first readout, based on the first readout and on the second readout, wherein each soft metric value comprises a sign and a magnitude, and calculating the syndrome weight based on signs of the soft metrics that were assigned to the elements of the first readout.

12. The method according to claim 9, wherein calculating the syndrome weight comprises producing a syndrome vector by multiplying the first readout by a parity-check matrix representing the ECC, and counting a number of nonzero elements in the syndrome vector.

13. The method according to claim 9, wherein the syndrome weight is a function of a linear combination of the mid-zone count and a conditional probability of the channel matrix, and wherein estimating the channel matrix comprises estimating the conditional probability, based on the function and on the calculated syndrome weight and on the mid-zone count.

14. The method according to claim 9, and comprising selecting between hard and soft ECC decoding, based on the evaluated performance measure.

15. The method according to claim 9, and comprising determining a number of read thresholds to use in subsequent read operations based on the evaluated performance measure.

16. The method according to claim 9, wherein the memory cells in the group store multiple bits per cell, in a set of multiple programming levels predefined within a range of threshold voltages, wherein sensing the memory cells comprises sensing the memory cells using multiple read thresholds, including the first and second read thresholds, to produce multiple respective readouts, including the first and second readouts, wherein the multiple read thresholds divide the range of threshold voltages into multiple zones, the method further comprises identifying among the multiple zones two or more middle zones corresponding to mid-ranges between respective programming levels, wherein calculating the syndrome weight comprises calculating the syndrome weight based on one of the multiple readouts, and wherein evaluating the performance measure comprises evaluating the performance measure based on the syndrome weight that was calculated based on one of the multiple readouts, and on a number of memory cells for which the readouts fall in the middle zones.