Power savings in a content addressable memory device using masked pre-compare operations
A CAM device for comparing a search key with a plurality of ternary words stored in a CAM array includes one or more population counters, a pre-compare memory, and a pre-compare circuit. The present embodiments reduce the power consumption of CAM devices during compare operations between a search key and ternary words stored in a CAM array by selectively enabling the match lines in the CAM array in response to pre-compare operations between a set of population counts corresponding to the masked search key and a set of population counts corresponding to the ternary words stored in the CAM array.
1. A content addressable memory (CAM) device, comprising:
a CAM array having a plurality of rows of CAM cells, each row of the plurality of rows storing an associated word and being coupled to a corresponding match line; and
a pre-compare circuit, coupled to the CAM array, configured to compare a first data population count with a first key population count and a second data population count with a second key population count to determine whether a search key can match the associated word stored in each row of the plurality of rows.
2. The CAM device of claim 1 , wherein the pre-compare circuit is further configured to selectively enable the match lines of corresponding rows of the CAM array to be pre-charged in response to the comparisons.
3. The CAM device of claim 2 , wherein the pre-compare circuit is further configured to assert a corresponding enable signal to allow the match lines of corresponding rows of the CAM array to be pre-charged when the first key population count is greater than or equal to the first data population count and the second data population count is greater than or equal to the second key population count.
4. The CAM device of claim 3 , wherein the pre-compare circuit is further configured to de-assert the corresponding enable signal when either:
the second data population count is less than the second key population count; or
the first key population count is less than the first data population count.
5. The CAM device of claim 3 , wherein:
the pre-compare circuit comprises a comparator configured to conduct the comparisons sequentially to generate a first comparison result and a second comparison result, wherein:
the first comparison result is stored in a memory; and
the second comparison result is combined with the first comparison result to generate the corresponding enable signal.
6. The CAM device of claim 3 , further comprising match-line control circuitry configured to pre-charge the match lines of corresponding rows of the CAM array in response to the corresponding enable signal.
7. The CAM device of claim 1 , wherein the pre-compare circuit comprises:
a first input to receive the first and second data population counts from a pre-compare memory;
a second input to receive the first and second key population counts from a population counter; and
a plurality of outputs coupled to corresponding rows of the CAM array.
8. The CAM device of claim 1 , wherein:
the first and second data population counts are based on bits of the associated word stored in each row of the plurality of rows of the CAM array; and
the first and second key population counts are based on bits of a masked search key.
9. A method of comparing a search key with a plurality of words stored in corresponding rows of a content addressable memory (CAM) array, the method comprising:
generating first data population count and a second data population count for the plurality of words;
generating a first key population count and a second key population count for the search key;
pre-charging match lines corresponding to rows of the CAM array when the first key population count is greater than or equal to the first data population count and the second data population count is greater than or equal to the second key population count.
10. The method of claim 9 , wherein:
the first and second data population counts are based on bits of the word stored in each row of the CAM array;and
the first and second key population counts are based on bits of a masked search key.
11. The method of claim 9 , further comprising:
de-asserting an enable signal when the second data population count is less than the second key population count or the first key population count is less than the first data population count.
12. The method of claim 9 , wherein the pre-charging the match lines comprises:
conducting a comparison sequentially to generate a first comparison result and a second comparison result;
storing the first comparison result in a memory; and
combining the first comparison result with the second comparison result to generate a corresponding enable signal, wherein the corresponding enable signal indicates that the match lines are to be pre-charged.
13. A content addressable memory (CAM) device for comparing a search key with a plurality of ternary words, comprising:
a CAM array having a plurality of rows of CAM cells, each row of the plurality of rows storing an associated word and being coupled to a corresponding match line;
a pre-compare memory that stores a first data population count and a second data population count for a corresponding word stored in the CAM array; and
a pre-compare circuit configured to compare the first data population count with a first key population count and the second data population count with a second key population count.
14. The CAM device of claim 13 , wherein the pre-compare circuit is further configured to selectively enable the match lines of corresponding rows of the CAM array to be pre-charged in response to the comparisons.
15. The CAM device of claim 14 , wherein the pre-compare circuit is further configured to assert a corresponding enable signal to allow the match lines of corresponding rows of the CAM array to be pre-charged when the first key population count is greater than or equal to the first data population count and the second data population count is greater than or equal to the second key population count.
16. The CAM device of claim 15 , wherein the pre-compare circuit is further configured to de-assert the corresponding enable signal when either:
the second data population count is less than the second key population count; or
the first key population count is less than the first data population count.
17. The CAM device of claim 15 , wherein:
the pre-compare circuit comprises a comparator configured to conduct the comparisons sequentially to generate a first comparison result and a second comparison result, wherein:
the first comparison result is stored in a memory; and
the second comparison result is combined with the first comparison result to generate the corresponding enable signal.
18. The CAM device of claim 15 , further comprising match-line control circuitry configured to pre-charge the match lines of corresponding rows of the CAM array in response to the corresponding enable signal.
19. The CAM device of claim 13 , wherein pre-compare circuit comprises:
a first input to receive the first and second data population counts from a pre-compare memory;
a second input to receive the first and second key population counts from a population counter; and
a plurality of outputs coupled to corresponding rows of the CAM array.
20. The CAM device of claim 13 , wherein:
the first and second data population counts are based on bits of the word stored in each row of the plurality of rows of the CAM array; and
the first and second key population counts are based on bits of a masked search key.