Method for improving accuracy of a time estimate from a memory device

• US 8,688,924 B2
• Filed: 06/08/2007
• Issued: 04/01/2014
• Est. Priority Date: 06/08/2007
• Status: Active Grant

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: Sandisk Technologies Incorporated (Western Digital Corporation)
• Inventors: Holtzman, Michael, Sela, Rotem, Barzilai, Ron, Jogand-Coulomb, Fabrice E.
• Primary Examiner(s): Shah, Sanjiv
• Assistant Examiner(s): DILLON, SAMUEL A
• Application Number:
  US11/811,284
• Publication Number:
  US 20080306710A1
• Time in Patent Office: 2,489 Days
• Field of Search: 702/176, 702/178, 702/89, 711/154, 711/163, 700/306
• US Class Current: 711/154
• CPC Class Codes: G06F 21/10 Protecting distributed programs or content, e.g. vending or licensing of copyrighted material protection in video systems or pay television H04N7/16 ; Digital rights management [DRM]
• CPC Class Codes: G06F 21/725 operating on a secure reference time value
• CPC Class Codes: H04L 2463/121 Timestamp cryptographic mechanisms or cryptographic arrangements involving time stamps H04L9/3297
• CPC Class Codes: H04L 63/123 received data contents, e.g. message integrity

Abstract

A method for improving accuracy of a time estimate from a memory device is disclosed. In one embodiment, a memory device receives a time stamp and measures active time with respect to the received time stamp. The memory device determines accuracy of previously-measured active time and generates a time estimate using the measured active time, the accuracy of previously-measured active time, and the received time stamp. In another embodiment, measured active time is adjusted, with or without generating a time estimate. Other embodiments are disclosed, and each of the embodiments can be used alone or together in combination.

Claims

1. A method for improving accuracy of a time estimate in a memory device, the method comprising:

performing the following in a memory device;

determining accuracy of previously-measured active time by comparing the previously-measured active time with an actual time from a first time stamp obtained by the memory device from a trusted time server through a host, which is untrusted;

measuring active time of the memory device relative to a second time stamp obtained by the memory device from the trusted time server through the host, wherein the measuring is performed internally in the memory device when the memory device is active, the memory device being active when it is connected to and receives power from the host;

adjusting the measured active time based on the determined accuracy;

generating a time estimate by applying the adjusted measured active time to the second time stamp; and

using the generated time estimate for a time-based operation.

2. The method of claim 1 further comprising:

measuring a number of power cycles with respect to the second time stamp; and

if the number of power cycles exceeds a threshold, requesting a new time stamp instead of generating the time estimate.

3. The method of claim 1 further comprising:

measuring a number of power cycles with respect to the second time stamp; and

if the number of power cycles equals zero, generating a time estimate by adding the measured active time to the second time stamp instead of using the adjusted measured active time.

4. The method of claim 1 further comprising:

if the measured active time exceeds a threshold, requesting a new time stamp instead of generating the time estimate.

5. The method of claim 1, wherein determining accuracy of active time previously measured by the memory device comprises determining a plurality of stretch factors, and wherein the method further comprises:

measuring a standard deviation of the plurality of stretch factors; and

if the standard deviation of the plurality of stretch factors exceeds a threshold, performing one of the following;

requesting a new time stamp instead of generating the time estimate; and

generating a time estimate by adding the measured active time to the second time stamp instead of using the adjusted measured active time.

6. The method of claim 1, wherein determining accuracy of active time previously measured by the memory device comprises determining a plurality of stretch factors and calculating a running average of the plurality of stretch factors.

7. The method of claim 1, wherein determining accuracy of active time previously measured by the memory device comprises comparing active time previously measured by the memory device with actual time between two discrete time stamps.

8. The method of claim 1, wherein determining accuracy of active time previously measured by the memory device comprises comparing down time with actual time between two discrete time stamps.

9. The method of claim 1, wherein determining accuracy of active time previously measured by the memory device comprises comparing active time previously measured by the memory device with down time.

10. The method of claim 1, wherein determining accuracy of active time previously measured by the memory device comprises determining a stretch factor per power cycle of the memory device, wherein the method further comprises measuring a number of power cycles with respect to the second time stamp, and wherein the time estimate is generated based on the stretch factor per power cycle and the number of power cycles.

11. The method of claim 1, wherein active time is measured by incrementing a value in a time counter in response to an interrupt signal, and wherein the measured active time is adjusted by multiplying the value in the time counter by an amount based on the accuracy of active time previously measured by the memory device instead of by a frequency at which the interrupt signal is generated.

12. The method of claim 1, wherein active time is measured by incrementing a value in a time counter in response to an interrupt signal and multiplying the value in the time counter by a frequency at which the interrupt signal is generated, and wherein the measured active time is adjusted by multiplying the measured active time by an amount based on the accuracy of active time previously measured by the memory device.

13. The method of claim 1, wherein at least one of the first and second time stamps is signed by the time server.

14. The method of claim 1, wherein the memory device stores digital rights management (DRM) keys and licenses to unlock protected content stored on the memory device.

15. The method of claim 1 further comprising using the time estimate in one or more of the following operations:

an authentication operation, a host revocation operation, and a digital rights management (DRM) operation.

16. A method for improving accuracy of measured active time in a memory device, the method comprising:

performing the following in a memory device;

determining accuracy of previously-measured active time by comparing the previously-measured active time with an actual time from a first time stamp obtained by the memory device from a trusted time server through a host, which is untrusted;

measuring active time of the memory device relative to a second time stamp obtained by the memory device, wherein the measuring is performed internally in the memory device when the memory device is active, the memory device being active when it is connected to and receives power from the host;

adjusting the measured active time based on the determined accuracy; and

using the adjusted measured active time for a time-based operation.

17. The method of claim 16 further comprising:

generating a time estimate using the adjusted measured active time.

18. The method of claim 16 further comprising:

measuring a number of power cycles with respect to the second time stamp; and

if the number of power cycles exceeds a threshold, requesting a new time stamp instead of adjusting the measured active time.

19. The method of claim 16 further comprising:

measuring a number of power cycles with respect to the second time stamp;

wherein the measured active time is not adjusted if the number of power cycles equals zero.

20. The method of claim 16, wherein the measured active time is not adjusted if the measured active time exceeds a threshold.

21. The method of claim 16, wherein determining accuracy of active time previously measured by the memory device comprises determining a plurality of stretch factors, and wherein the method further comprises:

measuring a standard deviation of the plurality of stretch factors; and

performing one of the following;

adjusting the measured active time only if the standard deviation of the plurality of stretch factors does not exceed a threshold; and

requesting a new time stamp instead of adjusting the measured active time if the standard deviation of the plurality of stretch factors exceeds a threshold.

22. The method of claim 16, wherein determining accuracy of active time previously measured by the memory device comprises determining a plurality of stretch factors and calculating a running average of the plurality of stretch factors.

23. The method of claim 16, wherein determining accuracy of active time previously measured by the memory device comprises comparing active time previously measured by the memory device with actual time between two discrete time stamps.

24. The method of claim 16, wherein determining accuracy of active time previously measured by the memory device comprises comparing down time with actual time between two discrete time stamps.

25. The method of claim 16, wherein determining accuracy of active time previously measured by the memory device comprises comparing active time previously measured by the memory device with down time.

26. The method of claim 16, wherein determining accuracy of active time previously measured by the memory device comprises determining a stretch factor per power cycle of the memory device, wherein the method further comprises measuring a number of power cycles with respect to a received time stamp, and wherein measured active time is adjusted using the stretch factor per power cycle and the number of power cycles.

27. The method of claim 16, wherein active time is measured by incrementing a value in a time counter in response to an interrupt signal, and wherein the measured active time is adjusted by multiplying the value in the time counter by an amount based on the accuracy of active time previously measured by the memory device instead of by a frequency at which the interrupt signal is generated.

28. The method of claim 16, wherein active time is measured by incrementing a value in a time counter in response to an interrupt signal and multiplying the value in the time counter by a frequency at which the interrupt signal is generated, and wherein the measured active time is adjusted by multiplying the measured active time by an amount based on the accuracy of active time previously measured by the memory device.

29. The method of claim 16 further comprising receiving the first time stamp.

30. The method of claim 29, wherein at least one of the first and second time stamps is signed by the time server.

31. The method of claim 16, wherein the memory device stores digital rights management (DRM) keys and licenses to unlock protected content stored on the memory device.

32. The method of claim 16 further comprising using the adjusted measured time in one or more of the following operations:

an authentication operation, a host revocation operation, and a digital rights management (DRM) operation.

33. A memory device comprising:

a memory array; and

circuitry in communication with the memory array and operative to;

measure active time of the memory device, only when the memory device is active, wherein the memory device is active when it is connected to and receives power from a host;

determine accuracy of active time previously measured by the memory device over a time period by comparing it with actual time over that time period, wherein accuracy is determined by comparing active time previously measured by the memory device with actual time between two discrete time stamps obtained by the memory device from a trusted time server through the host, which is untrusted;

adjust the measured active time based on the determined accuracy; and

use the adjusted measured active time for a time-based operation.

34. The memory device of claim 33, wherein the circuitry is further operative to:

generate a time estimate using the adjusted measured active time.

35. The memory device of claim 33, wherein the circuitry is further operative to:

measure a number of power cycles with respect to a received time stamp; and

if the number of power cycles exceeds a threshold, request a new time stamp instead of adjusting the measured active time.

36. The memory device of claim 33, wherein the circuitry is further operative to:

measure a number of power cycles with respect to a received time stamp;

wherein the measured active time is not adjusted if the number of power cycles equals zero.

37. The memory device of claim 33, wherein the measured active time is not adjusted if the measured active time exceeds a threshold.

38. The memory device of claim 33, wherein determining accuracy of active time previously measured by the memory device comprises determining a plurality of stretch factors, and wherein the circuitry is further operative to:

measure a standard deviation of the plurality of stretch factors; and

perform one of the following;

adjust the measured active time only if the standard deviation of the plurality of stretch factors does not exceed a threshold; and

request a new time stamp instead of adjusting the measured active time if the standard deviation of the plurality of stretch factors exceeds a threshold.

39. The memory device of claim 33, wherein determining accuracy of active time previously measured by the memory device comprises determining a plurality of stretch factors and calculating a running average of the plurality of stretch factors.

40. The memory device of claim 33, wherein determining accuracy of time previously measured by the memory device comprises comparing down time with actual time between two discrete time stamps.

41. The memory device of claim 33, wherein determining accuracy of active time previously measured by the memory device comprises comparing active time previously measured by the memory device with down time.

42. The memory device of claim 33, wherein determining accuracy of active time previously measured by the memory device comprises determining a stretch factor per power cycle of the memory device, wherein the circuitry is further operative to measure a number of power cycles with respect to a received time stamp, and wherein measured active time is adjusted using the stretch factor per power cycle and the number of power cycles.

43. The memory device of claim 33, wherein active time is measured by incrementing a value in a time counter in response to an interrupt signal, and wherein the measured active time is adjusted by multiplying the value in the time counter by an amount based on the accuracy of active time previously measured by the memory device instead of by a frequency at which the interrupt signal is generated.

44. The memory device of claim 33, wherein active time is measured by incrementing a value in a time counter in response to an interrupt signal and multiplying the value in the time counter by a frequency at which the interrupt signal is generated, and wherein the measured active time is adjusted by multiplying the measured active time by an amount based on the accuracy of active time previously measured by the memory device.

45. The memory device of claim 33, wherein the circuitry is further operative to receive a time stamp.

46. The memory device of claim 45, wherein the time stamp is signed by the time server.

47. The memory device of claim 33, wherein the memory device stores digital rights management (DRM) keys and licenses to unlock protected content stored on the memory device.

48. The memory device of claim 33, wherein the circuitry is further operative to adjust measured time in one or more of the following operations:

an authentication operation, a host revocation operation, and a digital rights management (DRM) operation.