Methods of operating PRAMS using initial programmed resistances and PRAMS using the same

1. A method of operating a Variable Resistance Memory comprising:

reading a reference cell to determine an initial programmed resistance of the reference cell; and

determining whether the initial programmed resistance has been reduced to below a predetermined reference threshold resistance; and

rewriting the reference cell to the initial programmed resistance when it is determined that the initial programmed resistance programmed to the reference cell has been reduced to less than the predetermined reference threshold resistance.

2. The method according to claim 1 wherein the predetermined reference threshold resistance is greater than a predetermined data threshold resistance used to distinguish between different data states stored in a data cell.

3. The method according to claim 2 wherein first resistances greater than the predetermined data threshold resistance represent a reset state of the data cell and second resistances less than the predetermined data threshold resistance represent a set state of the data cell.

4. The method according to claim 1 wherein reading the reference cell comprises repeatedly reading the reference cell without an intervening rewrite operation of the reference cell until determining that the initial programmed resistance of the reference cell has been reduced to below the predetermined reference threshold resistance; and then

rewriting the reference cell to have the initial programmed resistance.

5. A method according to claim 2 wherein the predetermined reference threshold resistance is less than a statistical distribution of resistances provided by programming a plurality of data cells in a reset state.

6. A method according to claim 1 wherein the initial programmed resistance of the reference cell is included in a statistical distribution of resistances provided by programming a plurality of data cells in a reset state.

7. A method according to claim 2 wherein the initial programmed resistance of the reference cell is less than a statistical distribution of resistances provided by programming a plurality of data cells in a reset state.

8. A method according to claim 1 wherein reading the reference cell comprises periodically reading the reference cell according to a reference cell time interval.

9. The method according to claim 8 wherein the reference cell time interval is reduced responsive to an increase in a temperature associated with the memory and is increased responsive to a decrease in the temperature associated with the memory.

10. The method according to claim 8 wherein the reference cell is included in a plurality of reference cells each associated with a particular region of the memory selected based on a likelihood to exhibit a high operating temperature.

11. The method according to claim 10 wherein the region of the memory is proximate to a current driver circuit.

12. The method according to claim 10 wherein the region of the memory is proximate to a central portion of a block of cells in the memory.

13. A Variable Resistance Memory comprising:

a control circuit configured to read a reference cell to determine that an initial programmed resistance of the reference cell has been reduced to below a predetermined reference threshold resistance, wherein the initial programmed resistance of the reference cell is included in a statistical distribution of resistances provided by programming a plurality of data cells in a reset state.

14. The Variable Resistance Memory according to claim 13 wherein the predetermined reference threshold resistance is greater than a predetermined data threshold resistance used to distinguish between different data states stored in a data cell in the memory.

15. The Variable Resistance Memory according to claim 14 wherein first resistances greater than the predetermined data threshold resistance represent a reset state of the data cell and second resistances less than the predetermined data threshold resistance represent a set state of the data cell.

16. The Variable Resistance Memory according to claim 13 wherein reading the reference cell comprises repeatedly reading the reference cell without an intervening rewrite operation of the reference cell until determining that the initial programmed resistance of the reference cell has been reduced to below the predetermined reference threshold resistance and then

rewriting the reference cell to have the initial programmed resistance.

17. The Variable Resistance Memory according to claim 14 wherein the predetermined reference threshold resistance is less than the statistical distribution of resistances.

18. The Variable Resistance Memory according to claim 14 wherein the initial programmed resistance of the reference cell is less than the statistical distribution of resistances.

19. A Variable Resistance Memory comprising:

a control circuit configured to read a reference cell to determine that an initial programmed resistance of the reference cell has been reduced to below a predetermined reference threshold resistance and configured to rewrite the reference cell to the initial programmed resistance when it is determined that the initial programmed resistance programmed to the reference cell has been reduced to less than the predetermined reference threshold resistance.