ADAPTIVE PROGRAMMING OF ANALOG MEMORY

• US 20080130341A1
• Filed: 12/03/2007
• Published: 06/05/2008
• Est. Priority Date: 12/03/2006
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Anobit Technologies, Ltd. (Apple Inc.)
• Inventors: Cohen, Zeev, Shalvi, Ofir, Sokolov, Dotan
• Granted Patent: US 7,706,182 B2
• Time in Patent Office: Days
• US Class Current: 365/45
• CPC Class Codes: G11C 11/56 using storage elements with more than two stable states represented by steps, e.g. of voltage, current, phase, frequency
• CPC Class Codes: G11C 11/5621 using charge storage in a floating gate
• CPC Class Codes: G11C 27/005 with non-volatile charge storage, e.g. on floating gate or MNOS

Abstract

A method for storing data in a memory that includes a plurality of analog memory cells includes mapping the data to programming values, which are selected from a set of nominal programming values. The set of nominal programming values includes at least a first nominal programming value and a second nominal programming value, which is higher than the first nominal programming value.

A part of the data is stored in the analog memory cells by programming at least a first group of the cells using the first nominal programming value. A statistical characteristic of the first group of the cells is measured after programming the first group of the cells using the first nominal programming value. The second nominal programming value is modified responsively to the statistical characteristic, and at least a second group of the cells is programmed using the modified second nominal programming value.

Claims

1. A method for storing data in a memory that includes a plurality of analog memory cells, comprising:

mapping the data to programming values selected from a set of nominal programming values comprising at least a first nominal programming value and a second nominal programming value, which is higher than the first nominal programming value;

storing a part of the data in the analog memory cells by programming at least a first group of the cells using the first nominal programming value;

measuring a statistical characteristic of the first group of the cells after programming the first group of the cells using the first nominal programming value;

modifying the second nominal programming value responsively to the statistical characteristic; and

programming at least a second group of the cells using the modified second nominal programming value.

2. The method according to claim 1, wherein measuring the statistical characteristic comprises reading analog values from the first group of the cells, and processing the read analog values to produce the statistical characteristic.

3. The method according to claim 2, wherein the statistical characteristic comprises at least one characteristic type selected from a group of types consisting of a mean of a distribution of the read analog values, a variance of the distribution of the read analog values, a standard deviation of the distribution of the read analog values, a maximum of the read analog values and a k^th-largest value among the read analog values.

4. The method according to claim 2, wherein modifying the second nominal programming value comprises measuring a distance between at least one of the read analog values and the second nominal programming value, and calculating the modified second nominal programming value responsively to the distance.

5. The method according to claim 2, wherein modifying the second nominal programming value comprises increasing the second nominal programming value when the statistical characteristic indicates that the read analog values are widely-distributed around the first nominal programming value.

6. The method according to claim 2, wherein modifying the second nominal programming value comprises decreasing the second nominal programming value when the statistical characteristic indicates that the read analog values are closely concentrated around the first nominal programming value.

7. The method according to claim 1, and comprising, after programming the second group of the cells, reading the second group of the cells so as to reconstruct the data.

8. The method according to claim 7, wherein modifying the second nominal programming value comprises storing information regarding the modified second nominal programming value, and wherein reading the second group of the cells comprises retrieving the stored information regarding the modified second nominal programming value and reading the second group of the cells responsively to the retrieved information.

9. The method according to claim 1, wherein modifying the second nominal programming value comprises reading respective analog values from the cells in the second group and processing the read analog values to estimate the modified second nominal programming value.

10. The method according to claim 1, wherein measuring the statistical characteristic and modifying the second nominal programming value comprise classifying the analog memory cells to multiple classes, and measuring the statistical characteristic and modifying the second nominal programming value separately within each of the classes.

11. The method according to claim 10, wherein the classes comprise at least one class type selected from a group of types consisting of a row of the memory cells, a column of the memory cells, an odd-order column, an even-order column, a memory page, an erasure block, a memory plane, cells located near a boundary of a memory block and cells located in an interior of the memory block.

12. The method according to claim 1, and comprising designating an area of the memory as a calibration area, selecting the first group of the memory cells from the memory cells located in the calibration area, and selecting the second group of the memory cells from the memory cells located in another area of the memory different from the calibration area.

13. A data storage apparatus, comprising:

an interface, which is coupled to communicate with a memory that includes a plurality of analog memory cells; and

a processor, which is coupled to map data to programming values selected from a set of nominal programming values comprising at least a first nominal programming value and a second nominal programming value that is higher than the first nominal programming value, to store a part of the data in the analog memory cells by programming at least a first group of the cells using the first nominal programming value, to measure a statistical characteristic of the first group of the cells after programming the first group of the cells using the first nominal programming value, to modify the second nominal programming value responsively to the statistical characteristic, and to program at least a second group of the cells using the modified second nominal programming value.

14. The apparatus according to claim 13, wherein the processor is coupled to read analog values from the first group of the cells, and to process the read analog values to produce the statistical characteristic.

15. The apparatus according to claim 14, wherein the statistical characteristic comprises at least one characteristic type selected from a group of types consisting of a mean of a distribution of the read analog values, a variance of the distribution of the read analog values, a standard deviation of the distribution of the read analog values, a maximum of the read analog values and a k^th-largest value among the read analog values.

16. The apparatus according to claim 14, wherein the processor is coupled to measure a distance between at least one of the read analog values and the second nominal programming value, and to calculate the modified second nominal programming value responsively to the distance.

17. The apparatus according to claim 14, wherein the processor is coupled to increase the second nominal programming value when the statistical characteristic indicates that the read analog values are widely-distributed around the first nominal programming value.

18. The apparatus according to claim 14, wherein the processor is coupled to decrease the second nominal programming value when the statistical characteristic indicates that the read analog values are closely concentrated around the first nominal programming value.

19. The apparatus according to claim 13, wherein the processor is coupled to read the second group of the cells after programming the second group of the cells, so as to reconstruct the data.

20. The apparatus according to claim 19, wherein the processor is coupled to store information regarding the modified second nominal programming value, and to read the second group of the cells by retrieving the stored information regarding the modified second nominal programming value and reading the second group of the cells responsively to the retrieved information.

21. The apparatus according to claim 13, wherein the processor is coupled to read respective analog values from the cells in the second group and to process the read analog values to estimate the modified second nominal programming value.

22. The apparatus according to claim 13, wherein the processor is coupled to classify the analog memory cells to multiple classes, and to measure the statistical characteristic and modify the second nominal programming value separately within each of the classes.

23. The apparatus according to claim 22, wherein the classes comprise at least one class type selected from a group of types consisting of a row of the memory cells, a column of the memory cells, an odd-order column, an even-order column, a memory page, an erasure block, a memory plane, cells located near a boundary of a memory block and cells located in an interior of the memory block.

24. The apparatus according to claim 13, wherein the processor is coupled to designate an area of the memory as a calibration area, to select the first group of the memory cells from the memory cells located in the calibration area, and to select the second group of the memory cells from the memory cells located in another area of the memory different from the calibration area.

25. A data storage apparatus, comprising:

a memory, which comprises a plurality of analog memory cells; and

a processor, which is connected to the memory and is coupled to map data to programming values selected from a set of nominal programming values comprising at least a first nominal programming value and a second nominal programming value that is higher than the first nominal programming value, to store a part of the data in the analog memory cells by programming at least a first group of the cells using the first nominal programming value, to measure a statistical characteristic of the first group of the cells after programming the first group of the cells using the first nominal programming value, to modify the second nominal programming value responsively to the statistical characteristic, and to program at least a second group of the cells using the modified second nominal programming value.