Precharge circuits and methods for content addressable memory (CAM) and related devices
An integrated circuit device can include a plurality of compare cell circuits that selectively provide charge transfer path between a result line and a reference node according to a comparison between a stored data value and an applied compare data value during a compare time period. A first precharge circuit can have a controllable impedance path coupled between the result line and a precharge voltage node. A control circuit can place the first precharge circuit into a low impedance state during a first portion of the compare time period, and into a high impedance state during a second portion of the compare time period.
1. An integrated circuit device, comprising:
a plurality of compare cell circuits that selectively provide charge transfer paths between a result line and a reference node according to a comparison between a stored data value and an applied compare data value during a compare time period;
a first precharge circuit having a controllable impedance path coupled between the result line and a precharge voltage node; and
a control circuit that places the first precharge circuit into a low impedance state during a first portion of the compare time period, and into a high impedance state during a second portion of the compare time period.
2. The integrated circuit device of claim 1 , wherein:
the control circuit places the first precharge circuit into the high impedance state in response to at least a periodic control signal.
3. The integrated circuit device of claim 2 , wherein:
the control circuit includes a delay circuit coupled to receive an input periodic signal, the delay circuit delaying the input periodic signal by a predetermined delay amount to generate the periodic control signal, the predetermined delay amount being selectable.
4. The integrated circuit device of claim 1 , wherein:
the plurality of compare cell circuits place their respective charge transfer paths into a high impedance state during a precharge time period that is different from the compare time period.
5. The integrated circuit device of claim 4 , wherein:
each compare cell circuit receives a first and second compare data signal, each first and second compare data signal being driven to a first logic value in the precharge time period.
6. The integrated circuit device of claim 1 , wherein:
the control circuit places the first precharge circuit into the high impedance state in response to at least the potential of the result line.
7. The integrated circuit device of claim 6 , wherein:
the control circuit places the first precharge circuit into the high impedance state in response to at least a logic combination of the potential of the result line and a control signal.
8. The integrated circuit device of claim 1 , wherein:
each compare cell circuit comprises at least a first compare transistor of a first conductivity type having a source-drain path connected between the result line and the reference node; and
the first precharge circuit comprises at least a first precharge transistor of a second conductivity type having a source-drain path coupled between the precharge voltage node and the result line, and a gate coupled to the control circuit.
9. The integrated circuit of claim 1 , further including:
a second precharge circuit having a controllable impedance path coupled between the result line and a precharge voltage node; and
the control circuit that places the second precharge circuit into a low impedance state prior to the compare time period and into a high impedance state during the compare time period.
10. The integrated circuit of claim 9 , wherein:
the compare time period is defined according to a compare control signal; and
the control circuit places the second precharge circuit into the low impedance state prior to the compare time period in response to a timing signal and places the second precharge circuit into the low impedance state after the compare time period in response to at least the compare control signal.
11. The integrated circuit of claim 10 , wherein:
the control circuit places the second precharge circuit into the low impedance state after the compare time period in response to a logical combination of the compare control signal and the potential of the result line.
12. The integrated circuit of claim 1 , further including:
an isolation circuit coupled between the plurality of compare cell circuits and the reference node; and
the control circuit places the isolation circuit into a low impedance state during the first portion of the compare time period to allow current to flow through the compare cell circuits, and into a high impedance state during a second portion of the compare time period to limit the current flowing through the compare cell circuits.
13. The integrated circuit of claim 1 , wherein:
each compare cell circuit comprises a portion of a content addressable memory (CAM) cell that includes a storage circuit for storing at least one bit value of the stored data value for comparison with at least one compare data value signal, and the result line provides a match result between the stored data value and the applied compare data value.
14. The integrated circuit device of claim 1 , wherein:
each compare cell circuit comprises a portion of a range compare memory cell that includes a storage circuit for storing at least one bit value of the stored data value for comparison with at least one bit of the corresponding applied compare data value, and the result line provides a range compare result between the stored data value and the applied compare data value.
15. The integrated circuit device of claim 1 , wherein:
each compare cell circuit comprises a portion of an AND type content addressable memory (CAM) cell, the plurality of compare cell circuits includes controllable current paths arranged in series with one another between a result node and a reference node, the result line comprising at least two of the charge transfer paths.
16. An integrated circuit device, comprising:
compare means for selectively coupling a result line to a reference voltage according to a comparison result between a stored data value and an applied compare data value during a compare time period;
first precharge means for coupling the result line to a precharge voltage during a first portion of the compare time period and isolating the result line from the precharge voltage during a second portion of the compare time period that follows the first portion; and
second precharge means for coupling the result line to the precharge voltage outside of the compare time period and isolating the result line from the precharge voltage during the compare time period.
17. The integrated circuit device of claim 16 , wherein:
the first precharge means isolates the result line from the precharge voltage in response to any selected from the group of: a periodic signal, or the potential of the result line.
18. The integrated circuit of claim 16 , wherein:
the second precharge means couples the result line to the precharge voltage prior to the compare period and after the compare period in two separate precharge operations.
19. The integrated circuit of claim 16 , wherein:
the second precharge means couples the result line to the precharge voltage prior to the compare period and selectively after the compare period according to at least a potential of the result line.
20. A method of reducing current consumption in an integrated circuit device, comprising:
selectively coupling a result line to a reference node in response to a compare data value being applied to a plurality of compare cell circuits;
precharging the result line to the precharge potential via a first precharge path while the compare data value is being applied;
after precharging the result line via the first precharge path, isolating the result line via the first path while the compare data is still being applied; and
precharging the result line to the precharge potential via a second precharge path before the compare data value is applied and after the compare data is applied, in two separate precharge operations.
21. The method of claim 20 , wherein:
isolating the result line via the first path is done in response to at least a periodic signal.
22. The method of claim 20 , wherein:
isolating the result line via the first path is done in response to at least a potential of the result line.
23. The CAM device of claim 20 , further including:
conditionally precharging the result line to the precharge potential via the second precharge path after the compare data is applied depending upon the potential of the result line.