Multiple match detection circuit and method
View Patent ↗A multiple matchline sense circuit for detecting a single, more than one, or no match conditions during a search-and-compare operation of a content addressable memory is disclosed. The circuit compares the rising voltage rate of a multiple matchline to the rising voltage rate of a reference multiple matchline in order to generate a multibit result representing one of the three conditions. The circuit generates a self-timed control signal to end the search-and-compare operation, and to set the circuit to a precharge state.
1. A multiple match detection circuit having a current source for changing a voltage of a sense line from a miss voltage level to a hit voltage level and a sense circuit for detecting the voltage of the sense line and for generating a multiple match signal at a variable delay time corresponding to the number of matches upon activation of the current source comprising:
a reference circuit for generating a reference match signal in a fixed delay time corresponding to a single match upon activation of the current source; and,
a detection circuit for providing an output corresponding to the difference between the variable delay time of the multiple match signal and the fixed delay time of the reference match signal.
2. The multiple match detection circuit of claim 1 , wherein the output is represented as two bits of data.
3. The multiple match detection circuit of claim 2 , wherein the output represents a single match, no match or at least two matches.
4. The multiple match detection circuit of claim 1 , wherein multiple discharge transistors are coupled in parallel between the sense line and ground, the gate of each of the multiple discharge transistors being coupled to an output from a respective matchline sense circuit.
5. The multiple match detection circuit of claim 4 , wherein the reference circuit includes a reference current source for changing a voltage of a reference sense line from the miss voltage level to the hit voltage level and a reference sense circuit for detecting the voltage of the reference sense line.
6. The multiple match detection circuit of claim 5 , wherein a configuration of the reference current source, the reference sense line and the reference sense circuit is identical to a configuration of the current source, the sense line and the sense circuit.
7. The multiple match detection circuit of claim 6 , wherein multiple reference discharge transistors are coupled in parallel between the reference sense line and ground, the gate of each of the multiple reference discharge transistors being connected to ground except one transistor having a gate connected to VDD.
8. The multiple match detection circuit of claim 1 , wherein the detection circuit includes
a logic gate for receiving the multiple match signal and the reference match signal for providing a control signal;
a first flip flop circuit for receiving the multiple match signal and the control signal, for providing a first data signal; and
a second flip flop circuit for receiving the reference match signal and the control signal, for providing a second data signal.
9. The multiple match detection circuit of claim 8 , wherein the control signal enables and disables the current source.
10. The multiple match detection circuit of claim 9 , wherein the first and second flip flop circuits latch the first and second data signals in response to a transition in the voltage level of the control signal.
11. The multiple match detection circuit of claim 8 , wherein the reference circuit includes a reference current source for changing a voltage of a reference sense line from the miss voltage level to the hit voltage level and
a reference sense circuit for detecting the voltage of the reference sense line, the control signal enabling and disabling the current source and the reference current source.
12. The multiple match detection circuit of claim 11 , wherein the sense line and the reference sense line are precharged to the miss voltage level when the current source and the reference current source are disabled.