Adaptive match line charging
A content addressable memory (CAM) device having any number of rows, each of the rows including a match line connected to a plurality of CAM cells, a match line detector circuit, and a pre-charge circuit. The detector circuit detects a voltage of the match line and generates a feedback signal based on the detected match line voltage. The pre-charge circuit adaptively charges the match line in response to the feedback signal.
1. A content addressable memory (CAM) device having any number of rows, each of the rows comprising:
a plurality of CAM cells coupled to a match line;
a match line detector circuit coupled to the match line and configured to detect a voltage of the match line and to generate a pre-charge feedback signal in response to the detected match line voltage, wherein the match line detector circuit asserts the feedback signal if the detected match line voltage exceeds a threshold voltage of the match line detector circuit, and de-asserts the feedback signal if the detected match line voltage is below the threshold voltage of the match line detector circuit; and
a pre-charge circuit coupled between the match line and a supply voltage and having a first input to receive the feedback signal, wherein the pre-charge circuit is configured to adaptively charge the match line in response to the feedback signal, wherein the pre-charge circuit charges the match line if the feedback signal is de-asserted, and clamps the match line voltage if the feedback signal is asserted.
2. The CAM device of claim 1 , wherein during a pre-charge phase of a compare operation, the pre-charge circuit is configured to clamp the match line voltage if the feedback signal indicates the match line voltage exceeds a threshold voltage of the match line detector circuit.
3. The CAM device of claim 1 , wherein the pre-charge circuit further comprises a second input to receive a pre-charge signal.
4. The CAM device of claim 3 , wherein the pre-charge circuit is selectively enabled in response to the pre-charge signal.
5. The CAM device of claim 1 , wherein each CAM row further comprises a valid bit cell for storing a valid bit indicating whether the CAM row contains valid data.
6. The CAM device of claim 5 , wherein the pre-charge circuit further comprises a second input coupled to the valid bit cell.
7. The CAM device of claim 6 , wherein the pre-charge circuit is enabled for adaptively charging the match line if the valid bit is asserted to indicate that valid data is stored in the CAM row.
8. The CAM device of claim 6 , wherein the pre-charge circuit is disabled if the valid bit is de-asserted to indicate that valid data is not stored in the CAM row.
9. The CAM device of claim 5 , wherein the pre-charge circuit comprises:
a static pre-charge circuit coupled between the match line and the supply voltage, the static pre-charge circuit providing a relatively small charge current for the match line; and
a dynamic pre-charge circuit coupled between the match line and the supply voltage, the dynamic pre-charge circuit selectively providing a relatively large pre-charge current for the match line in response to the valid bit and the feedback signal.
10. The CAM device of claim 9 , wherein the dynamic pre-charge circuit is enabled to charge the match line during a pre-charge phase of a compare operation only if the row contains valid data.
11. The CAM device of claim 10 , wherein the dynamic pre-charge circuit is disabled during an evaluation phase of the compare operation.
12. The CAM device of claim 11 , wherein the static pre-charge circuit charges the match line during the evaluation phase of the compare operation.
13. The CAM device of claim 9 , wherein the pre-charge circuit further comprises:
a discharge circuit coupled between the match line and ground potential, wherein the discharge circuit is configured to discharge the match line if the row does not contain valid data.
14. A content addressable memory (CAM) device having any number of rows, each of the rows comprising:
a plurality of CAM cells coupled to a match line;
a match line detector circuit coupled to the match line and configured to detect a voltage of the match line and to generate a pre-charge feedback signal in response to the detected match line voltage;
a pre-charge circuit coupled between the match line and a supply voltage and having a first input to receive the feedback signal, wherein the pre-charge circuit is configured to adaptively charge the match line in response to the feedback signal; and
a delay circuit coupled between the match line detector circuit and the pre-charge circuit and including a first control input to receive a delay enable signal, wherein assertion of the delay enable signal causes the delay circuit to delay propagation of the feedback signal to the pre-charge circuit, thereby allowing the pre-charge circuit to selectively increase the voltage of the match line on a per-row basis.
15. The CAM device of claim 14 , wherein the delay circuit includes a second control input to receive a program signal that controls the amount of delay provided by the delay circuit.
16. A content addressable memory (CAM) device having any number of rows, each of the rows comprising:
a plurality of CAM cells coupled to a match line;
a match line detector circuit having an input coupled to the match line and having an output coupled to a sensed match line, wherein the match line detector circuit selectively drives the sensed match line in response to a detected voltage on the match line;
a pre-charge circuit coupled between the match line and a supply voltage and having a first input coupled to the sensed match line; and
a delay circuit coupled between the match line detector circuit and the pre-charge circuit and including a first control input to receive a delay enable signal, wherein assertion of the delay enable signal causes the delay circuit to delay propagation of the feedback signal to the pre-charge circuit, thereby allowing the pre-charge circuit to selectively increase the voltage of the match line on a per-row basis.
17. The CAM device of claim 16 , wherein the pre-charge circuit adaptively charges the match line in response to a feedback signal derived from the sensed match line.
18. The CAM device of claim 17 , wherein the match line detector circuit asserts the feedback signal if the detected match line voltage exceeds a threshold voltage of the match line detector circuit, and the match line detector circuit de-asserts the feedback signal if the detected match line voltage is below the threshold voltage of the match line detector circuit.
19. The CAM device of claim 18 , wherein the pre-charge circuit charges the match line only if the feedback signal is de-asserted.
20. The CAM device of claim 18 , wherein the pre-charge circuit clamps the match line voltage if the feedback signal is asserted.
21. The CAM device of claim 17 , wherein each CAM row further comprises a valid bit indicating whether the CAM row contains valid data, and the pre-charge circuit further comprises a second input to receive the valid bit.
22. The CAM device of claim 18 , wherein the pre-charge circuit is enabled for adaptively charging the match line only if the valid bit is asserted to indicate that valid data is stored in the CAM row.
23. The CAM device of claim 16 , wherein the delay circuit includes a second control input to receive a program signal that controls the amount of delay provided by the delay circuit.