Ternary CAM cell for reduced matchline capacitance
View Patent ↗A ternary content addressable memory (CAM) cell is disclosed for providing reduced or minimized matchline (ML) capacitance and for increasing current between matchline and tail-line in the case of a mismatch. The speed of a CAM cell is generally inversely proportional to its ML capacitance, and proportional to the current. Conventional ternary CAM cells have many matchline transistors, each contributing to the matchline capacitance. Embodiments of the present invention have a single matchline transistor between a matchline and a ground line, or tail-line, of the CAM cell. The single matchline transistor couples the matchline to the tail-line in response to a discharge signal from a compare circuit. The compare circuit can be divided into a pull-up section for driving a gate voltage level control node and a discharge section for discharging the gate voltage level control node, the discharge signal being provided at the gate voltage level control node.
1. A ternary content addressable memory cell having bit lines, first and second search lines, a word line, a matchline, a tail-line, and first and second memory cells, the CAM cell comprising:
a compare circuit for receiving stored data from the first and second memory cells, and for receiving search data from the search lines, the compare circuit performing a comparison of the stored data and the search data in response to the search data, and for providing a discharge signal when the comparison results in a mismatch condition, the compare circuit including a pull-up section for driving a gate voltage level control node, and a discharge section for discharging the gate voltage level control node, the discharge signal being provided at the gate voltage level control node; and
a matchline transistor for coupling the matchline to the tail-line in response to the discharge signal.
2. The ternary content addressable memory cell of claim 1 wherein the matchline transistor has a gate terminal coupled to the gate voltage level control node for receiving the discharge signal.
3. The ternary content addressable memory cell of claim 2 wherein the matchline transistor is n-type and has a drain terminal coupled to the matchline, and a source terminal coupled to the tail-line.
4. The ternary content addressable memory cell of claim 3 wherein the pull-up section includes first and second n-type pull-up transistors, the first n-type pull-up transistor for coupling the first search line to the gate voltage level control node, and the second n-type pull-up transistor for coupling the second search line to the gate voltage level control node.
5. The ternary content addressable memory cell of claim 4 wherein the first n-type pull-up transistor has a gate terminal for receiving stored data from the first memory cell, and the second n-type pull-up transistor has a gate terminal for receiving stored data from the second memory cell.
6. The ternary content addressable memory cell of claim 3 wherein the discharge section includes first and second n-type discharge transistors connected to each other in series between the gate voltage lever control node and a voltage supply.
7. The ternary content addressable memory cell of claim 6 wherein the first n-type discharge transistor has a gate terminal for receiving stored data from the first memory cell, and the second n-type discharge transistor has a gate terminal for receiving stored data from the second memory cell.
8. The ternary content addressable memory cell of claim 3 wherein the pull-up section includes a pull-up search stack having two branches, each branch for coupling the gate voltage level control node to a voltage supply.
9. The ternary content addressable memory cell of claim 8 wherein each branch includes a pair of serially connected p-type transistors, and one branch receives, at gate terminals of the p-type transistors, date from the first search line and stored data from the first memory cell, and the other branch receives, at gate terminals of the p-type transistors, data from the second search line and stored data from the second memory cell.
10. The ternary content addressable memory cell of claim 8 wherein the pull-up section further includes a pull-up pre-charge transistor for coupling the voltage supply to the pull-up search stack in response to a pre-charge signal.
11. The ternary content addressable memory cell of claim 3 wherein the discharge section includes a discharge search stack having two branches, each branch for coupling the gate voltage level control node to a voltage supply.
12. The ternary content addressable memory cell of claim 11 wherein each branch includes a pair of serially connected n-type transistors, and one branch receives, at gate terminals of the n-type transistors, data from the second search line and stored data from the first memory cell, and the other branch receives, at gate terminals of the n-type transistors, date from the first search line and stored data from the second memory cell.
13. The ternary content addressable memory cell of claim 3 wherein the discharge section includes a discharge transistor for coupling the gate voltage level control node to a voltage supply in response to a pre-charge signal.
14. The ternary content addressable memory cell of claim 2 wherein the matchline transistor is p-type and has a gate terminal coupled to the gate voltage level control node for receiving the discharge signal.
15. The ternary content addressable memory cell of claim 14 wherein the pull-up section includes first and second p-type pull-up transistors connected to each other in series between the gate voltage level control node and a voltage supply.
16. The ternary content addressable memory cell of claim 15 wherein the first p-type pull-up transistor has a gate terminal for receiving stored data from the first memory cell, and the second p-type pull-up transistor has a gate terminal for receiving stored data from the second memory cell.
17. The ternary content addressable memory cell of claim 14 wherein the discharge section includes first and second p-type discharge transistors, the first p-type discharge transistor for coupling the first search line to the gate voltage level control node, and the second p-type discharge transistor for coupling the second search line to the gate voltage level control node.
18. The ternary content addressable memory cell of claim 17 wherein the first p-type discharge transistor has a gate terminal for receiving stored data from the first memory cell, and the second p-type discharge transistor has a gate terminal for receiving stored data from the second memory cell.
19. The ternary content addressable memory cell of claim 1 wherein the first and second memory cells include SRAM cells.
20. The ternary content addressable memory cell of claim 1 wherein the first and second memory cells include DRAM cells.