Software refreshed memory device and method
View Patent ↗A software refreshed memory device comprises a plurality of memory cells that must be periodically refreshed to avoid losing data. Preferably, the memory cells can avoid losing data even though the time interval between successive memory refresh operations is relatively long, as compared to the time interval between successive memory refresh operations in a conventional volatile memory device, such as a DRAM. A processor can perform periodic memory refresh operations by executing a set of memory refresh instructions implemented in software, rather than in hardware. Accordingly, the memory device can advantageously be simplified, because the need for memory refresh circuitry and for a unique refresh control signal are advantageously eliminated. Moreover, the processor executing the memory refresh instructions can typically perform more sophisticated algorithms, as compared to memory refresh circuitry implemented in hardware, for determining when to perform a memory refresh operation. For example, the processor can determine whether each individual memory cell needs to be refreshed, thereby advantageously avoiding performing unnecessary refresh operations on memory cells that do not need to be refreshed.
1. A method of refreshing a memory cell, the method comprising:
determining whether a memory refresh operation is complete;
if the memory refresh operation is not yet complete, determining whether system resources are available to refresh a memory cell;
if the memory refresh operation is not yet complete and system resources are available to refresh the memory cell, refreshing the memory cell; and
if the memory operation is not yet complete and system resources are not available to refresh the memory cell, forcing at least some of the system resources to become available to refresh the memory cell.
2. The method of claim 1 , further comprising starting a timer.
3. The method of claim 1 , wherein refreshing the memory cell comprises refreshing a memory cell at an address corresponding to a current value of a counter, and further comprising incrementing the counter after refreshing the memory cell.
4. The method of claim 3 , further comprising resetting the counter if the memory refresh operation is complete.
5. The method of claim 1 , wherein determining whether a memory refresh operation is complete comprises determining whether each memory cell in a memory array has been refreshed.
6. The method of claim 1 , wherein determining whether a memory refresh operation is complete comprises determining whether each memory cell in a particular block of memory has been refreshed.
7. The method of claim 2 , wherein forcing at least some of the system resources to become available to refresh the memory cell comprises forcing at least some of the system resources to become available to refresh the memory cell responsive to determining that the timer has exceeded a maximum wait time.
8. The method of claim 7 , wherein the maximum wait time comprises the maximum time that the memory cell can retain an assigned state.
9. The method of claim 7 , wherein the maximum wait time corresponds to the maximum time that a processor can wait before it forces at least some of the system resources to become available to refresh remaining memory cells.
10. The method of claim 9 , wherein the remaining memory cells comprise the remaining memory cells in a memory array being refreshed.
11. The method of claim 9 , wherein the remaining memory cells comprise the remaining memory cells in a block of memory being refreshed.
12. The method of claim 7 , further comprises monitoring whether system resources have become available to refresh the memory cell if the timer has not exceeded the maximum wait time.
13. The method of claim 1 , further comprising repeating the method periodically.
14. The method of claim 1 , wherein the memory cell comprises a variable resistance memory cell.
15. A memory device comprising:
a plurality of variable resistance memory cells; and
a processor coupled to the plurality of variable resistance memory cells, said processor configured to determine whether a memory refresh operation is complete; if the memory refresh operation is not yet complete, determine whether system resources are available to refresh a memory cell; if the memory refresh operation is not yet complete and system resources are available to refresh the memory cell, refresh the memory cell; and if the memory operation is not yet complete and system resources are not available to refresh the memory cell, force at least some of the system resources to become available to refresh the memory cell.
16. The memory device of claim 15 , wherein the variable resistance memory cells comprise programmable conductor random access memory cells.
17. The memory device of claim 15 , wherein the processor monitors whether system resources have become available to refresh the memory cell if the timer has not exceeded the maximum wait time.
18. The memory device of claim 15 , wherein when the processor forces at least some of the system resources to become available to refresh the memory cell, the processor forces at least some of the system resources to become available to refresh the memory cell responsive to determining that the timer has exceeded a maximum wait time.
19. The memory device of claim 18 , wherein the maximum wait time comprises the maximum time that each of the plurality of variable resistance memory cells can retain an assigned state.
20. The method of claim 18 , wherein the maximum wait time corresponds to the maximum time that a processor can wait before it forces at least some of the system resources to become available to refresh remaining memory cells.