Self-timed match line cascading in a partitioned content addressable memory array
View Patent ↗A CAM array includes a plurality of regular rows and a reference row. Each regular row is partitioned into a plurality of row segments, with each row segment including a number of CAM cells coupled to a corresponding match line segment. The reference row generates self-timed control signals for corresponding segments of the regular rows. Control circuits selectively enable a respective row segment in response to a logical combination of match results in a previous row segment and an associated one of the self-timed control signals.
1. A content addressable memory (CAM) device including a CAM array comprising:
a reference row including a plurality of reference CAM cells coupled to a reference match line, the reference match line configured to generate a self-timed control signal; and
a plurality of regular rows, each comprising:
a first row segment including a number of first CAM cells coupled to a first match line segment;
a second row segment including a number of second CAM cells coupled to a second match line segment; and
a control circuit configured to selectively disable the second row segment in response to match results in the first row segment and the self-timed control signal,
wherein the control circuit comprises:
a pull-down transistor coupled between the second CAM cells and ground potential, and having a gate; and
a combinational logic gate having a first input coupled to the first match line segment, having a second input to receive the self-timed control signal, and having an output coupled to the gate of the pull-down transistor.
2. The CAM device of claim 1 , wherein the control circuit is configured to disable the second row segment upon assertion of the self-timed control signal if there is a mismatch condition in the first row segment.
3. The CAM device of claim 2 , wherein the control circuit is configured to disable the second row segment by preventing the second match line segment from being discharged toward ground potential, irrespective of match results in the second row segment.
4. The CAM device of claim 1 , wherein the control circuit is configured to enable the second row segment upon assertion of the self-timed control signal if there is a match condition in the first row segment.
5. The CAM device of claim 4 , wherein the control circuit is configured to enable the second row segment by allowing the second match line segment to be selectively discharged in response to match results in the second row segment.
6. The CAM device of claim 1 , wherein the reference row is divided into at least first and second segments, wherein the first segment of the reference row includes a first subset of the reference CAM cells coupled to a first reference match line segment, and the second segment of the reference row includes a second subset of the reference CAM cells coupled to a second reference match line segment.
7. The CAM device of claim 6 , wherein match results are generated on the first reference match line segment of the reference row and the first match line segment of the regular rows concurrently.
8. The CAM device of claim 1 , wherein each regular row further comprises:
a combinational logic gate having a first input coupled to the first match line segment, having a second input coupled to the second match line segment, and an output to generate a match condition for the first and second row segments.
9. A content addressable memory (CAM) device including a CAM array comprising:
a reference row including a plurality of reference CAM cells coupled to a reference match line, the reference match line configured to generate a self-timed control signal; and
a plurality of regular rows, each comprising:
a first row segment including a number of first CAM cells coupled to a first match line segment;
a second row segment including a number of second CAM cells coupled to a second match line segment; and
a control circuit configured to selectively disable the second row segment in response to match results in the first row segment and the self-timed control signal,
wherein the reference row is divided into at least first and second segments, wherein the first segment of the reference row includes a first subset of the reference CAM cells coupled to a first reference match line segment, and the second segment of the reference row includes a second subset of the reference CAM cells coupled to a second reference match line segment, and wherein match results are generated in the first reference match line segment of the reference row just after match results are generated on the first match line segment of the regular rows.
10. A content addressable memory (CAM) device including a CAM array comprising:
at least one regular row divided into a number N of regular row segments, wherein each regular row segment includes a plurality of CAM cells coupled to a corresponding match line segment;
a reference row divided into N reference row segments, wherein each reference row segment includes a plurality of reference CAM cells coupled to a corresponding reference match line segment; and
a control circuit, coupled to the regular row and to the reference row, configured to selectively enable a next regular row segment in response to a logical combination of match results in a previous regular row segment and match results in a corresponding previous reference row segment,
wherein the control circuit comprises:
a pull-down transistor coupled between the CAM cells of a respective regular row segment and ground potential, and having a gate; and
a combinational logic gate having a first input coupled to the match line segment of a previous regular row segment, having a second input coupled to the reference match line segment of a previous reference row segment, and having an output coupled to the gate of the pull-down transistor.
11. The CAM device of claim 10 , wherein a respective reference match line segment generates an associated self-timed control signal that comprises the match results in the respective reference row segment.
12. The CAM device of claim 11 , wherein the control circuit is configured to disable the next regular row segment upon assertion of the associated self-timed control signal if there is a mismatch condition in the previous regular row segment.
13. The CAM device of claim 12 , wherein the control circuit is configured to disable the next regular row segment by preventing the corresponding next match line segment from being discharged toward ground potential, irrespective of match results in the next regular row segment.
14. The CAM device of claim 10 , wherein the control circuit is configured to enable the next regular row segment upon assertion of a self-timed control signal generated by the previous reference row segment if there is a match condition in the previous regular row segment.
15. The CAM device of claim 14 , wherein the control circuit is configured to enable the next regular row segment by allowing the corresponding next match line segment to be selectively discharged in response to match results in the next regular row segment.
16. The CAM device of claim 10 , wherein each row further comprises:
a combinational logic gate having a first input coupled to a last match line segment, having a second input coupled to a next-to-last match line segment, and having an output to generate a match condition for the row.
17. A content addressable memory (CAM) array including a plurality of regular rows, each regular row comprising:
a first row segment including a number of first CAM cells coupled to a first match line segment;
a second row segment including a number of second CAM cells coupled to a second match line segment; and
a control circuit configured to selectively disable the second row segment in response to match results in the first row segment and a self-timed control signal generated by a reference row of CAM cells,
wherein the control circuit comprises:
a pull-down transistor coupled between the second CAM cells and ground potential, and having a gate; and
a combinational logic gate having a first input coupled to the first match line segment, having a second input to receive the self-timed control signal, and having an output coupled to the gate of the pull-down transistor.
18. The CAM device of claim 17 , wherein the reference row comprises:
a first reference row segment including a number of first reference CAM cells coupled to a first reference match line segment; and
a second reference row segment including a number of second reference CAM cells coupled to a second reference match line segment.
19. The CAM device of claim 18 , wherein the first reference match line segment generates the self-timed control signal.
20. The CAM device of claim 17 , wherein the control circuit is configured to disable the second row segment upon assertion of the self-timed control signal if there is a mismatch condition in the first row segment.
21. The CAM device of claim 20 , wherein the control circuit is configured to disable the second row segment by preventing the second match line segment from being discharged toward ground potential, irrespective of match results in the second row segment.
22. The CAM device of claim 17 , wherein the control circuit is configured to enable the second row segment upon assertion of the self-timed control signal if there is a match condition in the first row segment.
23. The CAM device of claim 22 , wherein the control circuit is configured to enable the second row segment by allowing the second match line segment to be selectively discharged in response to match results in the second row segment.
24. The CAM device of claim 17 , wherein each row further comprises:
a combinational logic gate laving a first input coupled to the first match line segment, having a second input coupled to the second match line segment, and an output to generate a match condition for the first and second row segments.
25. A method for selectively disabling at least one of a plurality of segments of a regular row within a content addressable memory (CAM) array, wherein each regular row segment includes a plurality of CAM cells coupled to an associated match line segment, the method comprising:
generating match results in a first regular row segment;
generating a self-timed control signal in a first segment of a reference row that includes a number of reference CAM cells coupled to an associated reference match line segment, wherein the reference row is divided into at least first and second segments, wherein the first segment of the reference row includes a first subset of the reference CAM cells coupled to a first reference match line segment, and the second segment of the reference row includes a second subset of the reference CAM cells coupled to a second reference match line segment, and wherein the generating a self-timed control signal comprises generating match results on the first reference match line segment of the reference row just after match results are generated in the first regular row segment; and
selectively disabling a second regular row segment in response to a logical combination of the match results of the first regular row segment and the self-timed control signal.
26. The method of claim 25 , wherein the self-timed control signal comprises match results provided on the associated reference match line segment.
27. The method of claim 25 , wherein the second regular row segment is disabled upon assertion of the self-timed control signal if there is a mismatch condition in the first regular row segment.
28. The method of claim 27 , wherein the second regular row segment is disabled by:
preventing the match line segment of the second regular row segment from being discharged toward ground potential irrespective of match results in the second regular row segment.
29. The method of claim 25 , wherein the second regular row segment is enabled upon assertion of the self-timed control signal if there is a match condition in the first regular row segment.
30. The method of claim 29 , wherein the second regular row segment is enabled by:
allowing the match line segment of the second regular row segment to he selectively discharged in response to match results in the second regular row segment.