Content addressable memory with cascaded array
View Patent ↗A content addressable memory (CAM) device having a cascaded CAM array. The cascaded CAM array includes a first array of CAM cells and a second array of CAM cells. A first plurality of compare signal lines is coupled to the first array CAM cells and a second plurality of compare signal lines coupled to the second array of CAM cells. A plurality of storage elements have inputs coupled to the first plurality of compare signal lines and outputs coupled to the second plurality of compare signal lines.
1. A content addressable memory (CAM) device comprising:
a first storage circuit to store a comparand value in response to assertion of a first control signal;
a first array of CAM cells coupled to receive the comparand value from the first storage circuit;
a second storage circuit to store the comparand value in response to assertion of a second control signal, the second control signal being asserted a first time interval after the first control signal; and
a second array of CAM cells coupled to receive the comparand value from the second storage circuit.
2. The CAM device of claim 1 , wherein the second storage circuit is coupled to receive the comparand value from the first storage circuit.
3. The CAM device of claim 1 , wherein the first array of CAM cells comprises a plurality of compare lines coupled to receive the comparand value from the first storage circuit, and wherein the first time interval is at least as long as a time required for the first comparand value to settle on the compare lines.
4. The CAM device of claim 1 , further comprising
a first plurality of match lines coupled to respective rows of CAM cells within the first array of CAM cells;
a second plurality of match lines coupled to respective rows of CAM cells within the second array of CAM cells; and
a priority encoder having first inputs coupled to the first plurality of match lines and second inputs coupled to the second plurality of match lines.
5. The CAM device of claim 1 , further comprising:
a first plurality of match lines coupled to the first array of CAM cells to signal results of a comparison of the comparand value and contents of the first array of CAM cells;
a second plurality of match lines coupled to the second array of CAM cells to signal results of a comparison of the comparand value and contents of the second array of CAM cells; and
a storage circuit coupled to the first plurality of match lines to store the results of the comparison of the comparand value and the contents of the first array of CAM cells at least until the results of the comparison of the comparand value and the contents of the second array of CAM cells become valid.
6. A method of operation within a content addressable memory device, the method comprising:
storing a comparand value in a first storage circuit in response to assertion of a first control signal;
outputting the comparand value from the first storage circuit to a first array of CAM cells;
storing the comparand value in a second storage circuit in response to assertion of a second control signal, the second control signal being asserted a first time interval after the first control signal; and
outputting the comparand value from the second storage circuit to a second array of CAM cells.
7. The method of claim 6 wherein storing the comparand value in the second storage circuit comprises outputting the comparand value from the first storage circuit to an input of the second storage circuit.
8. The method of claim 6 wherein the outputting the outputting the comparand value from the first storage circuit to a first array of CAM cells comprises outputting the comparand value onto a plurality of compare lines coupled to the first array of CAM cells, and wherein the first time interval is at least as long as a time required for the first comparand value to settle on the plurality of compare lines.
9. The method of claim 6 further comprising:
generating first comparison results within the first array of CAM cells in response to the comparand value;
outputting the first comparison results from the first array of CAM cells to a first plurality of inputs of an address encoding circuit;
generating second comparison results within the second array of CAM cells in response to the comparand value; and
outputting the second comparison results from the second array of CAM cells to a second plurality of inputs of an address encoding circuit.