CAM match line precharge

• US 6,101,115 A
• Filed: 08/07/1998
• Issued: 08/08/2000
• Est. Priority Date: 08/07/1998
• Status: Expired due to Term

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Inventors: Ross, Mark
• Primary Examiner(s): PHAN, TRONG Q
• Application Number:
  US09/130,747
• Time in Patent Office: 732 Days
• Field of Search: 365/49, 365/168, 365/203
• US Class Current: 365/49.1
• CPC Class Codes: G11C 15/04 using semiconductor elements

Abstract

The invention provides a method and system for improved CAM match line precharge, in which the amount of power consumed by match line precharge and discharge is significantly reduced. An independent match precharge voltage is introduced to the CAM, which is used to separately precharge the match line and is available for use by comparison circuits to attempt to match the input tag. The match precharge voltage is selected so as to reduce the relative power consumption for match line precharge and discharge by a factor of about five. Optionally, sense amplifiers in the comparison circuits are disposed so as to compare each discharged match line with the match precharge voltage itself.

Claims

1. An independent match-line precharge voltage within a content addressable memory, including

an input port disposed for receiving a selected match precharge voltage substantially independent of an operating voltage of said memory;

wherein said selected match precharge voltage provide for relatively lower power consumption of said memory when said selected match precharge voltage is substantially less than said operating voltage.

2. A memory as in claim 1, wherein said memory includes a mask value for at least some lines of said memory.

3. A memory as in claim 1, wherein said memory includes a ternary content addressable memory.

4. A memory as in claim 1, wherein said power consumption is between about 20% to about 50% of that for a match precharge voltage greater than about 2.00 volts.

5. A memory as in claim 1, wherein said power consumption is between about 20% to about 50% of that for a match precharge voltage substantially equal to said operating voltage.

6. A memory as in claim 1, wherein said selected match precharge voltage is about 1.8 volts.

7. A memory as in claim 1, wherein said selected match precharge voltage is about 1.0 volt.

8. A memory as in claim 1, wherein said selected match precharge voltage is less than about 2.0 volts.

9. A memory as in claim 1, including a sense amplifier;

wherein said selected precharge voltage provides a noise margin substantially above a switching point for said sense amplifier.

10. An independent match line precharge voltage within a content addressable memory, including

an input port disposed for receiving a selected match precharge voltage substantially independent of an operating voltage of said memory;

a set of sense amplifiers each coupled to at least one match line of said memory, each said sense amplifier being responsive to said selected match precharge voltage.

11. A memory as in claim 10, wherein said memory includes a mask value for at least some lines of said memory.

12. A memory as in claim 10, wherein said memory includes a ternary content addressable memory.

13. A memory as in claim 10, wherein said power consumption is between about 20% to about 50% of that for a match precharge voltage greater 2.0 volts.

14. A memory as in claim 10, wherein said power consumption is between about 20% to about 50% of that for a match precharge voltage substantially equal to said operating voltage.

15. A memory as in claim 10, wherein said selected match precharge voltage is about 1.8 volts.

16. A memory as in claim 10, wherein said selected match precharge voltage is about 1.0 volt.

17. A memory as in claim 10, wherein said selected match precharge voltage is less than about 2.0 volts.

18. A memory as in claim 10, including a sense amplifier;

wherein said selected precharge voltage provides a noise margin substantially above a switching point for said sense amplifier.

19. An independent match-line precharge voltage within a content addressable memory, including

an input port disposed for receiving a selected match precharge voltage substantially independent of an operating voltage of said memory;

a set of sense amplifiers each coupled to at least one match line of said memory, each said sense amplifier being responsive to said selected match precharge voltage;

wherein said selected match precharge voltage provides for relatively lower power consumption of said memory when said selected match precharge voltage is substantially less than said operating voltage.

20. A memory as in claim 19, wherein said memory includes a mask value for at least some lines of said memory.

21. A memory as in claim 19, wherein said memory includes a ternary content addressable memory.

22. A memory as in claim 19, wherein said power consumption is between about 20% to about 50% of that for a match precharge voltage greater than about 2.0 volts.

23. A memory as in claim 19, wherein said power consumption is between about 20% to about 50% of that for a match precharge voltage substantially equal to said operating voltage.

24. A memory as in claim 19, wherein said selected match precharge voltage is about 1.8 volts.

25. A memory as in claim 19, wherein said selected match precharge voltage is about 1.0 volt.

26. A memory as in claim 19, wherein said selected match precharge voltage is less than about 2.0 volts.

27. A memory as in claim 19, wherein said selected precharge voltage provides a noise margin substantially above a switching point for said set of sense amplifiers.

28. A method of operating a content addressable memory, said method including steps for

receiving a selected match precharge voltage substantially independent of an operating voltage of said memory;

and

consuming relatively lower power during operating of said memory responsive to said selected match precharge voltage, when said selected match precharge voltage is substantially less than said operating voltage.

29. A method as in claim 28, including steps for matching, for at least some lines of said memory, an input value for said memory against a mask value.

30. A method as in claim 28, wherein said power consumption is between about 20% to about 50% of that for a match precharge voltage greater than about 2.0 volts.

31. A method as in claim 28, wherein said power consumption is between about 20% to about 50% of that for a match precharge voltage substantially equal to said operating voltage.

32. A method of operating a content addressable memory, said method including steps for

receiving a selected match precharge voltage substantially independent of an operating voltage of said memory;

and

matching at least one match line of said memory responsive to selected match precharge voltage.

33. A method as in claim 32, including steps for matching, for at least some lines of said memory, an input value for said memory against a mask value.

34. A method as in claim 32, wherein said power consumption is between about 20% to about 50% of that for a match precharge voltage greater than about 2.0 volts.

35. A method as in claim 32, wherein said power consumption is between about 20% to about 50% of that for a match precharge voltage substantially equal to said operating voltage.

36. A method of operating a content addressable memory, said method including steps for

receiving a selected match precharge voltage substantially independent of an operating voltage of said memory;

and

matching at least one match line of said memory responsive to selected match precharge voltage;

consuming relatively lower power during operating of said memory responsive to said selected match precharge voltage, when said selected match precharge voltage is substantially less than said operating voltage.

37. A method as in claim 36, including steps for matching, for at least some lines of said memory, an input value for said memory against a mask value.

38. A method as in claim 36, wherein said power consumption is between about 20% to about 50% of that for a match precharge voltage greater 2.0 volts.

39. A method as in claim 36, wherein said power consumption is between about 20% to about 50% of that for a match precharge voltage substantially equal to said operating voltage.