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. An apparatus for controlling a refresh frequency of dynamic random access memory (DRAM) cells, comprising:
a first time period changer for receiving an input oscillation signal having a period, and for changing the period of the input oscillation signal in response to a first factor to provide a modified oscillation signal; and
a second time period changer for receiving the modified input oscillation signal, and for changing the period of the modified input oscillation signal in response to a second factor to provide an output oscillation signal for controlling the refresh frequency.
2. The apparatus of claim 1 , further including a time period producer for providing the input oscillation signal in response to a control signal.
3. The apparatus of claim 2 , wherein the control signal is a self-refresh mode signal.
4. The apparatus of claim 1 , wherein the first time period changer includes a first frequency producer for producing a first set of m frequency signals in response to an oscillation frequency of the input oscillation signal, each of the m frequency signals having a frequency different than the oscillation frequency, m being an integer greater than one; and
a first selector for selecting one signal from the first set of m frequency signals as the modified input oscillation signal.
5. The apparatus of claim 4 , wherein the second time period changer includes a second frequency producer for producing a second set of n frequency signals in response to the modified input oscillation signal, each of the n frequency signals having a different frequency than the oscillation frequency, n being an integer greater than one; and
a second selector for selecting one signal from the second set of n frequency signals as the output oscillation signal.
6. The apparatus of claim 5 , wherein the first frequency producer includes a first frequency divider for dividing a frequency of the input oscillation signal in accordance with a first parameter to produce the first set of m frequency signals, and the second frequency producer includes a second frequency divider for dividing a frequency of the modified input oscillation signal in accordance with a second parameter to produce the second set of n frequency signals.
7. The apparatus of claim 6 , wherein the first factor includes one of a process variation factor and a temperature change factor relating to the DRAM, and the second factor includes the other of the process variation factor and the temperature change factor.
8. The apparatus of claim 7 , further including a factor provider for providing the process variation factor relating to the DRAM and the temperature change factor relating to the DRAM.
9. The apparatus of claim 8 , wherein the factor provider includes a first factor provider for designating the first parameter, so that the first frequency producer divides the frequency of the input oscillation signal in accordance with the first parameter; and
a second factor provider for designating the second parameter, so that the second frequency producer divides the frequency of the modified input oscillation signal in accordance with the second parameter.
10. The apparatus of claim 9 , wherein the first factor provider includes a process variation provider for providing a process variation code for designating the first parameter, the process variation code being derived from the process variations comprising DRAM characteristics, and the second factor provider includes a temperature change provider for providing a temperature change code for designating the second parameter, the temperature change code being derived from temperature changes comprising temperature sensed from the DRAM.
11. A method for controlling a refresh frequency of dynamic random access memory (DRAM) cells, comprising:
receiving an input oscillation signal having a period;
changing the period of the input oscillation signal in response to a first factor to provide a modified input oscillation signal; and
changing the period of the modified input oscillation signal in response to a second factor to provide an output oscillation signal for controlling the refresh frequency.
12. The method of claim 11 , wherein receiving the input oscillation signal includes providing the input oscillation signal in response to a control signal.
13. The method of claim 11 , wherein the step of changing the period of the input oscillation signal includes changing a repetition period of the input oscillation signal in accordance with the one of the first factor and the second factor to provide the modified input oscillation signal.
14. The method of claim 13 , wherein the step of changing the period of the modified input oscillation signal includes changing the repetition period of the modified input oscillation signal in accordance with the other one of the first factor and the second factor to provide the output oscillation signal.
15. The method of claim 14 , wherein changing the repetition period of the input oscillation signal includes dividing an oscillation frequency of the input oscillation signal in accordance with a first parameter to produce a first set of m frequency signals having m divided frequencies.
16. The method of claim 15 , wherein changing the repetition period of the input oscillation signal further includes selecting one frequency signal of the first set of m frequency signals as the modified input oscillation signal.
17. The method of claim 14 , wherein changing the repetition period of the input oscillation signal includes multiplying an oscillation frequency of the input oscillation signal in accordance with a first parameter to produce a first set of m frequency signals having m divided frequencies.