Low power match-line sensing circuit
View Patent ↗A low power matchline sensing scheme where power is distributed according to the number of mismatching bits occurring on a matchline is disclosed. In particular, match decisions involving a larger number of mismatched bits consume less power compared to match decisions having a lesser number of mismatched bits. The low power matchline sensing scheme is based upon a precharge-to-miss sensing architecture, and includes a current control circuit coupled to each matchline of the content addressable memory array for monitoring the voltage level of the matchline during a search operation. The current control circuit provides a voltage control signal to the current source of the matchline to adjust the amount of current applied to the matchline in response to the voltage of the matchline. In otherwords, matchlines that are slow to reach the match threshold voltage due to the presence of one or more mismatching bits will receive less current than matchlines having no mismatching bits. Significant power reduction without compromising search speed is realized since matchlines carrying a match result are provided with the maximum amount of current.
1. A dynamic current controller for controlling a current provided by a current source coupled to a signal line, the dynamic current controller comprising:
a dynamic bias voltage generator including a low threshold transistor for sensing a voltage level of the signal line and for adjusting a control voltage in response to the sensed voltage level, the current applied to the signal line being increased or decreased in response to the control voltage.
2. The dynamic current controller of claim 1 , wherein the dynamic bias voltage generator and the current source are enabled for a predetermined period of time in response to an active logic level of an enable signal.
3. The dynamic current controller of claim 2 , wherein the enable signal is generated from a control circuit coupled to a reference dynamic bias voltage generator sensing a reference signal line, the enable signal being driven to an inactive logic level after the predetermined period of time.
4. The dynamic current controller of claim 3 , wherein the predetermined period of time corresponds to a time for the reference signal line to reach a sense voltage threshold from a precharge voltage level in response to an applied current from a reference current source controlled by the reference dynamic bias voltage generator during sensing of the reference signal line.
5. The dynamic current controller of claim 4 , wherein the control circuit includes:
a sense amplifier circuit for sensing the sense voltage threshold and providing a sense output when the sense voltage threshold is sensed, and
a logic circuit for driving the enable signal to the inactive logic level in response to the sense output.
6. The dynamic current controller of claim 1 , wherein the dynamic bias voltage generator includes a level shifting circuit for sensing the voltage level of the signal line and for providing a level shifted voltage level to the dynamic bias voltage generator.
7. The dynamic current controller of claim 2 , wherein the current source includes:
a current transistor for providing the current in response to the control voltage; and
a switch transistor for receiving the enable signal and connected in series with the current transistor for selectively coupling the transistor to the signal line when the enable signal is at the active logic level.
8. The dynamic current controller of claim 2 , wherein the dynamic bias voltage generator includes:
a transistor for receiving a bias voltage;
a diode-connected transistor connected in series with the transistor, the control voltage being provided from the diode-connected transistor gate; and
a sense transistor having a gate coupled to the signal line, and connected in series with the diode-connected transistor.
9. The dynamic current controller of claim 8 , wherein the dynamic bias voltage generator includes a precharge circuit for precharging the voltage control signal to a predetermined voltage level.