Content addressable memory having column redundancy
View Patent ↗A content addressable memory (CAM) having a main array including a plurality of columns of CAM cells, a spare column of CAM cells selectable to functionally replace a defective column of CAM cells in the main array, and circuitry to shift data corresponding to the defective column and data corresponding to all subsequent columns to corresponding adjacent non-defective columns.
1. A method for functionally replacing a defective column of content addressable memory (CAM) cells in a CAM array with a spare column of CAM cells, comprising:
identifying the defective column of CAM cells; and
disabling the defective column of CAM cells without including fuses coupled to the defective column.
2. The method of claim 1 , further comprising:
enabling the spare column of CAM cells to functionally replace the defective column of CAM cells without including fuses coupled to the spare column.
3. The method of claim 2 , wherein each of a plurality of columns of CAM cells in the CAM array is coupled to a bit line pair and to a comparand line pair, and the spare column of CAM cells is coupled to a spare bit line pair and a spare comparand line pair.
4. The method of claim 3 , wherein disabling the defective column of CAM cells comprises:
forcing the comparand line pair to the same predetermined state if the corresponding column is identified as defective.
5. The method of claim 3 , wherein disabling the defective column of CAM cells and enabling the spare column of CAM cells comprise:
storing, in a global mask circuit associated with the CAM array, one or more mask patterns for compare operations in the CAM array.
6. The method of claim 2 , wherein disabling the defective column of CAM cells and enabling the spare column of CAM cells comprises, for each column of CAM cells:
driving the comparand line pair of the column of CAM cells in response to a mask bit, a comparand bit, and a column control signal indicating whether the column of CAM cells is defective.
7. The method of claim 6 , wherein driving the comparand line pair comprises:
masking the comparand bit if the column control signal indicates the corresponding column is defective.
8. The method of claim 6 , wherein driving the comparand line pair comprises:
latching the column control signal in response to an enable signal;
storing the mask bit; and
selectively masking the comparand bit in response to the mask bit and the column control signal.
9. The method of claim 2 , wherein disabling the defective column of CAM cells and enabling the spare column of CAM cells further comprises:
driving the spare comparand line pair of the spare column of CAM cells in response to a mask bit, a comparand bit, and a spare column enable signal indicating the existence of the defective column.
10. The method of claim 9 , wherein driving the spare comparand line pair comprises:
masking the comparand bit if the spare column enable signal indicates there is not a defective column in the CAM array.
11. The method of claim 9 , wherein driving the spare comparand line pair comprises:
latching the spare column enable signal in response to a control signal;
storing the mask bit; and a
selectively masking the comparand bit in response to the mask bit and the spare column enable signal.