Content addressable memory early-predict late-correct single ended sensing

Patent metadata

Field	Value
Publication number	US-9384835-B2
Application number	US-201213482166-A
Country	US
Kind code	B2
Filing date	May 29, 2012
Priority date	May 29, 2012
Publication date	Jul 5, 2016
Grant date	Jul 5, 2016

Abstract

Circuits and methods for performing search operations in a content addressable memory (CAM) array are provided. A system for searching a CAM includes a circuit that selectively activates a main-search of a two stage CAM search while a pre-search of the two stage CAM search is still active.

First claim

What is claimed:

1. A system for searching a content addressable memory (CAM), comprising:
a circuit that selectively activates a main-search by searching a first set of cells of a two stage CAM search while a pre-search searches a second set of cells different from the first set of cells of the two stage CAM search and is still active,
wherein the circuit is configured to overlap completion of the pre-search and starting of the main-search, and
wherein a number of the first set of cells is greater than a number of the second set of cells.

2. The system of claim 1 , wherein the circuit selectively activates the main-search based on preliminary results of the pre-search indicating a match.

3. The system of claim 2 , wherein the circuit terminates the main-search when final results of the pre-search contradict the preliminary results.

4. The system of claim 1 , wherein each stored word of the CAM includes an instance of the circuit, and wherein each instance of the circuit comprises:
a start pre-search node connected to a pull-up stack comprising a first transistor and a second transistor connected between a voltage source and a pre-search match line (PML);
a start main-search node connected to a first input of a NAND gate, wherein the PML is connected to a second input of the NAND gate;
a third transistor controlled by an output of the NAND gate, wherein the third transistor selectively turns off the second transistor; and
a fourth transistor controlled by the output of the NAND gate, wherein the fourth transistor is connected between the voltage source and a main-search match line (MML).

5. The system of claim 1 , wherein the circuit comprises a pre-search match line (PML) connected to: a first plurality of pull-down stacks; pull-up devices; and an input of a NAND gate.

6. The system of claim 5 , wherein an output of the NAND gate controls a switch that turns off the pull-up devices.

7. The system of claim 6 , wherein a start main-search node is connected to another input of the NAND gate such that the NAND gate causes the switch to turn off the pull-up devices after the main-search has been started.

8. The system of claim 5 , wherein:
the circuit comprises a main-search match line (MML) connected to a second plurality of pull-down stacks; and
an output of the NAND gate is connected to a switch that controls pre-charging the MML.

9. The system of claim 8 , wherein the circuit further comprises:
a keeper stack connected between the output of the NAND gate and the MML;
a Schmitt trigger with an input connected to the MML and an output indirectly connected to a sense node; and
a tunable pin on the Schmitt trigger structured and arranged to adjust a hysteresis value of the Schmitt trigger.

10. The system of claim 1 , wherein the CAM comprises a ternary CAM (TCAM).

11. A circuit, comprising:
a pre-search match line (PML) connected to a first plurality of cells of a content addressable memory (CAM);
a main-search match line (MML) connected to a second plurality of cells of the CAM; and
a combination of devices structured and arranged to:
start a pre-search operation associated with the PML; and
start a main-search operation associated with the MML prior to completion of the pre-search operation,
wherein the circuit is configured to overlap completion of the pre-search operation and starting of the main-search operation, and

wherein the combination of devices comprises:
a start pre-search node connected to a pull-up stack comprising a first p-type field effect transistor (pFET) and a second pFET connected between a voltage source and the PML;
a start main-search node connected to a first input of a NAND gate, wherein the PML is connected to a second input of the NAND gate;
a third pFET controlled by an output of the NAND gate, wherein the third pFET turns off the second pFET; and
a fourth pFET controlled by the output of the NAND gate, wherein the fourth pFET is connected between the voltage source and the MML.

12. The circuit of claims 11 , wherein the combination of devices comprises:
an n-type field effect transistor (nFET) connected between the fourth pFET and the MML;
a sense node between the nFET and the fourth pFET;
an inverter and a latch connected to the sense node; and
a Schmitt trigger having an input connected to the MML and an output connected to the nFET.

13. The circuit of claim 12 , wherein the combination of devices comprises:
a tunable pin on the Schmitt trigger structured and arranged to adjust a hysteresis value of the Schmitt trigger; and
a keeper stack connected between the output of the NAND gate and the MML.

14. A circuit, comprising:
an inverting Schmitt trigger having a low threshold, a high threshold, and a hysteresis value; and
a silicon aware tunable pin which is external to the Schmitt trigger and is connected to an input of the Schmitt trigger, wherein the Schmitt trigger and the tunable pin are structured and arranged such that adjusting a voltage on the silicon aware tunable pin selectively changes the hysteresis value of the Schmitt trigger to correspond to the adjusted voltage, wherein:
an input of the Schmitt trigger is connected to a main-search match line (MML) connected to a plurality of cells of a content addressable memory (CAM); and
an output of the Schmitt trigger is connected to a gate of a transistor that is connected between the MML and a sense node of a comparison circuit of the CAM.

15. The circuit of claim 14 , wherein:
decreasing the voltage on the tunable pin increases the hysteresis value of the Schmitt trigger; and
increasing the voltage on the tunable pin decreases the hysteresis value of the Schmitt trigger.

16. The circuit of claim 14 , further comprising:
a second transistor connected between the sense node and a voltage source; and
a transistor stack connected between a gate of the second transistor and the MML,
wherein the second transistor is a pFET device and a gate of the second transistor is connected to a NAND gate, and
wherein the transistor stack comprises a pFET-nFET stack.

17. The circuit of claim 14 , further comprising:
an inverter connected between the sense node and a latch,
wherein the plurality of cells of the CAM are a plurality of pull-down stacks, and
the transistor is a nFET device.

18. A method of performing a memory operation in a computer memory, comprising:
starting a first stage of a two-stage memory operation in the computer memory by searching a first set of cells;
selectively starting a second stage of the two-stage memory operation by searching a second set of cells different from the first set of cells while the first stage is still executing and based on preliminary results of the first stage;
detecting a final result of the first stage after the starting the second stage; and
performing one of interrupting and completing the second stage based on the final result of the first stage,
wherein the selectively starting the second stage comprises overlapping completion of the first stage with starting of the second stage, and
wherein a number of the second set of cells is greater than a number of the first set of cells.

19. The method of claim 18 , wherein the memory operation is a search operation and the selectively starting the second stage is based on the preliminary results of the first stage indicating a match, and further comprising:
interrupting the second stage when the final result of the first stage indicates a miss; and
completing the second stage when the final resul

Assignees

• Arsovski Igor
• Dobson Daniel A
• Hebig Travis R
• Wistort Reid A
• Globalfoundries Inc

Inventors

• Arsovski Igor
• Dobson Daniel A
• Hebig Travis R
• Wistort Reid A

Classifications

• G11C15/046

  using non-volatile storage elements · CPC title

• G11C15/04 Primary

  using semiconductor elements · CPC title

• G11C15/043

  using capacitive charge storage elements · CPC title