Dynamic random access memory device and method for self-refreshing memory cells with temperature compensated self-refresh
View Patent ↗A dynamic random access memory (DRAM) device has an array of DRAM cells of rows by columns. Each DRAM cell of the array is coupled with a wordline of a corresponding row and a bitline of a corresponding column. An entry into and an exit from the self-refresh mode are detected by a mode detector and a self-refresh mode signal is provided. An oscillation circuit generates in response to the self-refresh mode signal generates a basic time period. A first frequency divider/time period multiplier changes the basic time period in accordance with a process variation factor relating to the DRAM device. A second frequency divider/time period multiplier further changes the changed time period in accordance with a temperature change factor relating to the DRAM device. In the self-refresh mode, data stored in the DRAM cells is refreshed. In accordance with the two factors, the DRAM devices perform and achieve reliable self-refresh for variable DRAM cell retention time.
1. A method for adjusting a refresh frequency of cells of a dynamic random access memory (DRAM), comprising:
determining a change in temperature of the DRAM by sensing the change in temperature of the DRAM with a temperature sensor;
determining a process variation characteristic of the DRAM by monitoring a capacitor reference voltage corresponding to capacitance of a DRAM cell of the DRAM; and,
changing the refresh frequency in response to the change in temperature and the process variation characteristic.
2. The method of claim 1 , wherein determining the change in temperature includes providing a temperature change code derived from sensing the change in temperature of the DRAM.
3. The method of claim 2 , wherein changing includes changing the refresh frequency in response to the temperature change code.
4. The method of claim 1 , wherein determining the process variation characteristic further includes adjusting a bias voltage in response to the capacitor reference voltage, for changing the refresh frequency.
5. The method of claim 1 , wherein changing includes generating an oscillation signal, followed by changing a period of the oscillation signal in response to at least one of the change in temperature and the process variation characteristic to provide a final oscillation signal used to set the refresh frequency.
6. The method of claim 5 , wherein changing the period of the oscillation signal includes adjusting a period of the oscillation signal in response to one of the change in temperature and the process variation characteristic to provide a modified oscillation signal, the modified oscillation signal being the final oscillation signal in an absence of a change in temperature and a fixed process variation characteristic.
7. The method of claim 6 , wherein adjusting the period of the oscillation signal includes selecting one frequency signal of a set of M frequency signals having M divided frequencies as the modified oscillation signal.
8. The method of claim 7 , wherein adjusting the period of the oscillation signal includes generating the set of M frequency signals having M divided frequencies by dividing an oscillation frequency of the oscillation signal with a parameter.
9. The method of claim 7 , wherein adjusting the period of the oscillation signal includes generating the set of M frequency signals having M divided frequencies by multiplying an oscillation frequency of the oscillation signal with a parameter.
10. The method of claim 6 , wherein changing the period of the oscillation signal further includes adjusting a period of the modified oscillation signal in response to the other of the change in temperature and the process variation characteristic to provide a final oscillation signal.
11. The method of claim 10 , adjusting a period of the modified oscillation signal includes selecting one frequency signal of a set of M frequency signals having M divided frequencies as the final oscillation signal.
12. The method of claim 11 , wherein adjusting the period of the modified oscillation signal includes generating the set of M frequency signals having M divided frequencies by dividing an oscillation frequency of the modified oscillation signal with a parameter.
13. The method of claim 11 , wherein adjusting the period of the modified oscillation signal includes generating the set of M frequency signals having M divided frequencies by multiplying an oscillation frequency of the modified oscillation signal with a parameter.
14. A method for adjusting a refresh frequency of cells of a dynamic random access memory (DRAM), comprising:
determining a change in temperature of the DRAM;
determining a process variation characteristic of the DRAM; and,
changing the refresh frequency in response to the change in temperature and the process variation characteristic by generating an oscillation signal, followed by changing a period of the oscillation signal in response to at least one of the change in temperature and the process variation characteristic to provide a final oscillation signal used to set the refresh frequency.
15. The method of claim 14 , wherein determining the process variation characteristic includes determining if the process variation characteristic is fixed, and changing includes adjusting the refresh frequency in response to the change in temperature of the DRAM when the process variation characteristic is fixed for the DRAM.
16. The method of claim 15 , wherein changing includes adjusting a characteristic of an oscillation signal in response to the process variation characteristic before further adjusting the characteristic of the oscillation signal to provide a final oscillation signal in response to the change in temperature of the DRAM when the process variation characteristic varies for the DRAM, the final oscillation signal being used to set the refresh frequency.
17. The method of claim 15 , wherein changing includes first adjusting a characteristic of an oscillation signal in response to the change in temperature of the DRAM before further adjusting the characteristic of the oscillation signal to provide a final oscillation signal in response to the process variation characteristic when the process variation characteristic varies for the DRAM, the final oscillation signal being used to set the refresh frequency.
18. The method of claim 14 , wherein changing the period of the oscillation signal includes adjusting a period of the oscillation signal in response to one of the change in temperature and the process variation characteristic to provide a modified oscillation signal, the modified oscillation signal being the final oscillation signal in an absence of a change in temperature and a fixed process variation characteristic.
19. The method of claim 18 , wherein adjusting the period of the oscillation signal includes selecting one frequency signal of a set of M frequency signals having M divided frequencies as the modified oscillation signal.
20. The method of claim 19 , wherein adjusting the period of the oscillation signal includes generating the set of M frequency signals having M divided frequencies by dividing an oscillation frequency of the oscillation signal with a parameter.
21. The method of claim 19 , wherein adjusting the period of the oscillation signal includes generating the set of M frequency signals having M divided frequencies by multiplying an oscillation frequency of the oscillation signal with a parameter.
22. The method of claim 18 , wherein changing the period of the oscillation signal further includes adjusting a period of the modified oscillation signal in response to the other of the change in temperature and the process variation characteristic to provide a final oscillation signal.
23. The method of claim 22 , adjusting a period of the modified oscillation signal includes selecting one frequency signal of a set of M frequency signals having M divided frequencies as the final oscillation signal.
24. The method of claim 23 , wherein adjusting the period of the modified oscillation signal includes generating the set of M frequency signals having M divided frequencies by dividing an oscillation frequency of the modified oscillation signal with a parameter.
25. The method of claim 23 , wherein adjusting the period of the modified oscillation signal includes generating the set of M frequency signals having M divided frequencies by multiplying an oscillation frequency of the modified oscillation signal with a parameter.