Memory refresh method and system

1. A memory refresh method, applied in a system memory having a plurality of memory ranks, comprising:

determining whether an access request corresponds to the memory rank;

calculating an idle auto-refresh number of the memory rank, if no corresponding access request corresponds to the memory rank; and

switching the memory rank from an auto-refresh mode to a self-refresh mode, if the idle auto-refresh number of the memory rank reaches a predetermined value.

2. The memory refresh method as claimed in claim 1 , wherein the memory initially is in auto-refresh mode, and performs auto-refresh every single refresh period.

3. The memory refresh method as claimed in claim 1 , further comprising: issuing a refresh command for switching the memory rank to the self-refresh mode, if the idle auto-refresh number of the idle memory reaches the predetermined value.

4. The memory refresh method as claimed in claim 3 , wherein the refresh command is an auto-refresh command or a self-refresh command.

5. The memory refresh method as claimed in claim 1 , further comprising: maintaining the memory rank with corresponding access request in the auto-refresh mode.

6. The memory refresh method as claimed in claim 1 , wherein calculation of the idle auto-refresh number comprises:

determining whether an access request is sent to the memory rank having no corresponding access request; and

updating the idle auto-refresh number of the memory rank during the auto-refresh mode, if no access request is sent to the memory rank.

7. The memory refresh method as claimed in claim 6 , further comprising: resetting the idle auto-refresh number of the memory rank, if an access request is sent to the memory rank during calculation of the predetermined value.

8. The memory refresh method as claimed in claim 1 , further comprising: detecting whether an access request is sent out for the memory rank in the self-refresh mode.

9. The memory refresh method as claimed in claim 1 , further comprising: switching the memory rank from the self-refresh mode to the auto-refresh mode after a predetermined time, if an access request is sent to the memory rank in the self-refresh mode.

10. The memory refresh method as claimed in claim 1 , further comprising: keeping the memory rank in the self-refresh mode, if no access request corresponds to the memory rank.

11. A memory refresh method, applied in a system memory having a plurality of memory ranks, comprising:

calculating an idle time of an idle memory rank; and

switching the idle memory rank to a self-refresh mode, if the idle time of the idle memory rank reaches a predetermined value.

12. The memory refresh method as claimed in claim 11 , wherein the idle memory rank is the memory rank having no corresponding access request.

13. The memory refresh method as claimed in claim 11 , wherein the idle time is calculated based on auto-refresh times of the idle memory rank.

14. The memory refresh method as claimed in claim 13 , further comprising: recalculating the idle time of the idle memory rank if an access request is sent to the memory rank during calculation of the idle time.

15. The memory refresh method as claimed in claim 11 , further comprising: detecting whether an access request is sent out for the memory rank in the self-refresh mode.

16. The memory refresh method as claimed in claim 11 , further comprising: switching the memory rank from the self-refresh mode to the auto-refresh mode after a predetermined time, if an access request is sent to the memory rank in the self-refresh mode.

17. The memory refresh method as claimed in claim 11 , further comprising: keeping the memory rank in the self-refresh mode if no access request corresponds to the memory rank.