Content addressable memory array having virtual ground nodes
View Patent ↗Power consumption of CAM devices is reduced during compare operations between a search key and data stored in the device's array by reducing the amount of electric charge by which the match line is discharged during mismatch conditions. More specifically, for some embodiments, each row of the CAM array includes circuitry that discharges the match line to a virtual ground node rather than to ground potential during mismatch conditions. Because the electrical potential on the virtual ground node is greater than ground potential, power consumption associated with charging the match line back to a logic high state during the next compare operation is reduced.
1. A content addressable memory (CAM) device comprising an array having a plurality of rows, wherein a respective row comprises:
a plurality of CAM cells, each connected between a match line and a node;
a match line discharge transistor connected between the node and ground potential, and having a gate;
a first gating transistor connected between the match line and the gate of the match line discharge transistor, and having a gate to receive a control signal; and
a second gating transistor connected between the gate of the match line discharge
transistor and ground potential, and having a gate to receive the control signal.
2. The CAM device of claim 1 , wherein the match line discharge transistor is configured. to turn off and prevent the match line from discharging below a threshold voltage of the match line discharge transistor during a mismatch condition between a search key and data stored in the respective row.
3. The CAM device of claim 1 , wherein the control signal is to be asserted to maintain the first gating transistor in a non-conductive state and to maintain the second gating transistor in a conductive state during a pre-charge phase of a compare operation, and the control signal is to he de-asserted to maintain the first gating transistor in a conductive state and to maintain the second gating transistor in a 11 (non-conductive state during an evaluation phase of the compare operation.
4. The CAM device of claim 1 , wherein the first gating transistor is a PMOS transistor, and the second gating transistor is an NMOS transistor.
5. The CAM device of claim 1 , wherein the first gating transistor is an NMOS transistor, and the second gating transistor is a PMOS transistor.
6. The CAM device of claim 1 , wherein each CAM cell comprises:
first and second data cells to store first and second data bits, respectively; and
a compare circuit coupled to the data cells and to the match line, the compare circuit comprising:
first and second pull-down transistors connected in series between the match line and the node, wherein the first pull-down transistor has a. gate to receive a comparand bit, and the second pull-down transistor has a gate to receive the first data bit; and
third and fourth pull-down transistors connected in series between the match line and the node, wherein the third pull-down transistor has a gate to receive a complemented comparand bit, and the fourth pull-down transistor has a gate to receive the second data bit.
7. A content addressable memory (CAM) device comprising an array having a plurality of rows, wherein a respective row comprises:
a plurality of CAM cells, each connected between a match line and a node;
a match line discharge transistor connected between the node and ground potential, and having a gate; and
a gating transistor connected between the match line and the gate of the match line discharge transistor, and having a gate to receive a control signal.
8. The CAM device of claim 7 , wherein the match line discharge transistor is configured to discharge the match line only until a match line voltage reaches a threshold voltage of the match line discharge transistor when a compare operation between a search key and data stored in the respective row results in a mismatch condition.
9. The CAM device of claim 7 , wherein the control signal is to be de-asserted to maintain the gating transistor in a non-conductive state during a pre-charge phase of a compare operation, and the control signal is to be asserted to maintain the gating transistor in a conductive state during an evaluation phase of the compare operation.
10. A content addressable memory (CAM) device comprising an array having a plurality of rows, wherein a respective row comprises:
a number of CAM cells, each comprising:
first and second data cells to store first and second data bits, respectively; and
a compare circuit coupled to the data cells and to a match line, the compare circuit comprising:
first and second pull-down transistors connected in series between the match line and a node, wherein the first pull-down transistor has a gate to receive a comparand bit, and the second pull-down transistor has a gate to receive the first data bit; and
third and fourth pull-down transistors connected in series between the match line and the node, wherein the third pull-down transistor has a gate to receive a complemented comparand bit, and the fourth pull-down transistor has a gate to receive the second data bit; and
a match line discharge transistor connected between the node and ground potential, and having a gate;
a first gating transistor connected between the match line and the gate of the match line discharge transistor, and having a gate to receive a control signal; and
a second gating transistor connected between the gate of the match line discharge transistor and ground potential, and having a gate to receive the control signal.
11. The CAM device of claim 10 , wherein the match line discharge transistor is configured to turn off and prevent the match line from discharging below a threshold voltage of the match line discharge transistor during a mismatch condition between the comparand bit and data stored in the CAM cell.
12. The CAM device of claim 7 , wherein each CAM cell comprises:
first and second data cells to store first and second data bits, respectively;
a compare circuit coupled to the data cells and to the match line, the compare circuit comprising:
first and second pull-down transistors connected in series between the match line and the node, wherein the first pull-down transistor has a gate to receive a comparand bit, and the second pull-down transistor has a gate to receive the first data bit; and
third and fourth pull-down transistors connected in series between the match line and the node, wherein the third pull-down transistor has a gate to receive a complemented comparand bit, and the fourth pull-down transistor has a gate to receive the second data bit.
13. The CAM device of claim 10 , wherein the Control signal is to be asserted to maintain the first gating transistor in a non-conductive state and to maintain the second gating transistor in a conductive state during a pre-charge phase of a compare operation, and the control signal is to be de-asserted to maintain the first gating transistor in a conductive state and to maintain the second gating transistor in a non-conductive state during an evaluation phase of the compare operation.
14. The CAM device of claim 10 , wherein the first gating transistor is a PMOS transistor, and the second gating transistor is an NMOS transistor.
15. The CAM device of claim 10 , wherein the first gating transistor is an NMOS transistor, and the second gating transistor is a PMOS transistor.
16. A content addressable memory (CAM) device comprising an array having a plurality of rows, wherein a respective row comprises:
a plurality of CAM cells, each connected between a match line and a node; and
a match line discharge transistor connected between the node and ground potential, and having a gate directly connected to the match line.
17. The CAM device of claim 16 , wherein each CAM cell comprises:
first and second data cells to store first and second data bits, respectively; and
a compare circuit coupled to the data cells and to the match line, the compare circuit comprising:
first and second pull-down transistors connected in series between the match line and the node, wherein the first pull-down transistor has a gate to receive a comparand bit, and the second pull-down transistor has a gate to receive the first data bit; and
third and fourth pull-down transistors connected in series between the match line and the node, wherein the third pull-down transistor has a gate to receive a complemented comparand bit, and the fourth pull-down transistor has a gate to receive the second data bit.
18. The CAM device of claim 12 , wherein the control signal is to be de-asserted to maintain the gating transistor in a non-conductive state during a pre-charge phase of a compare operation, and the control signal is to be asserted to maintain the gating transistor in a conductive state during an evaluation phase of the compare operation.