Read-threshold calibration in a solid state storage system

1. A read-threshold calibration method in a solid state storage system, comprising:

measuring, with a controller, a threshold voltage distribution of solid state storage elements;

locating, with the controller, a first minimum point of the threshold voltage distribution;

estimating, with the controller, a first optimal read threshold based on the located first minimum point;

decoding, with the controller, data using the estimated first optimal read threshold;

if decoding fails:

removing, with the controller, an excessive number of minimum points from the threshold voltage distribution by filtering the threshold voltage distribution of the solid state storage elements;

locating, with the controller, a subsequent minimum point of the filtered threshold voltage distribution;

estimating, with the controller, a subsequent optimal read threshold based on the located subsequent minimum point; and

decoding, with the controller, the data that previously failed to decode using the estimated subsequent optimal read threshold.

2. The read-threshold calibration method in a solid state storage system according to claim 1 , wherein the excessive number of minimum points are removed from the threshold voltage distribution by filtering with a filter length in a range of two to eight.

3. The read-threshold calibration method in a solid state storage system according to claim 2 , further comprising changing the filter length if decoding fails.

4. The read-threshold calibration method in a solid state storage system according to claim 1 , wherein the measuring the threshold voltage distribution of solid state storage elements is performed by reading the solid state storage elements using a variety of read threshold voltages.

5. The read-threshold calibration method in a solid state storage system according to claim 1 , further comprising terminating the read-threshold calibration method when the decoding is successful.

6. The read-threshold calibration method in a solid state storage system according to claim 1 , wherein the determining a threshold voltage includes selecting the threshold voltage to reduce the number of read errors.

7. A system, comprising:

solid state storage elements; and

a storage controller suitable for:

measuring a threshold voltage distribution of the solid state storage elements;

locating a first minimum point of the threshold voltage distribution;

estimating a first optimal read threshold based on the located first minimum point;

decoding data using the estimated first optimal read threshold;

wherein, if decoding fails, the storage controller is further suitable for:

removing an excessive number of minimum points from the threshold voltage distribution by filtering the threshold voltage distribution of the solid state storage elements;

locating a subsequent minimum point of the filtered threshold voltage distribution;

estimating a subsequent optimal read threshold based on the located subsequent minimum point; and

decoding data using the estimated subsequent optimal read threshold.

8. The system according to claim 7 , wherein the excessive number of minimum points are removed from the threshold voltage distribution by filtering with a filter length in a range of two to eight.

9. The system according to claim 8 , wherein the storage controller is further suitable for changing the filter length if decoding fails.

10. The system according to claim 7 , wherein the read-threshold calibrator is suitable for measuring the threshold voltage distribution of solid state storage elements by reading the solid state storage elements using a number of read threshold voltages.

11. The system according to claim 7 , wherein the storage controller is further suitable for terminating operation if the decoding is successful.

12. The system according to claim 7 , wherein the read-threshold calibrator is suitable for determining the threshold voltage by selecting the threshold voltage to reduce the number of read errors.

13. A computer implemented process for calibrating a read threshold, the computer implemented process being implemented in a non-transitory computer readable storage medium and comprising computer instructions for:

measuring a threshold voltage distribution of solid state storage elements;

locating a first minimum point of the threshold voltage distribution;

estimating a first optimal read threshold based on the located first minimum point;

decoding data using the estimated first optimal read threshold;

if decoding fails:

removing an excessive number of minimum points from the threshold voltage distribution by filtering the threshold voltage distribution of the solid state storage elements;

locating a subsequent minimum point of the filtered threshold voltage distribution;

estimating a subsequent optimal read threshold based on the located subsequent minimum point; and

decoding the data that previously failed to decode using the estimated subsequent optimal read threshold.

14. The computer implemented process according to claim 13 , wherein the excessive number of minimum points are removed from the threshold voltage distribution by filtering with a filter length in a range of two to eight.

15. The computer implemented process according to claim 13 , wherein the instructions further comprises terminating operation if the decoding is successful.