AR# 1354: My CBxxx counter is not fast enough for my design
My CBxxx counter is not fast enough for my design
Keywords: CB, CC, counter, speed
In most cases, using a CBxxx counter for a XC4000/XC5200 architecture is not the best choice, since these macros do not utilize dedicated fast carry logic. Furthermore, without RLOCs on the flip-flops within the CBxxx macros, there is no guarantee that the place-and-route software will place the counter flip-flops optimally.
Using the XBLOX COUNTER module is the best choice in most situations, since XBLOX will choose the best configuration for a given architecture. Another choice is to use one of the CCxxx counter macros, which do use dedicated fast carry logic and use RLOCs for effective placement of logic resources. If the counter you desire is larger than any of largest counters in the library (CC16xxx), you may need to create your own counter macro, which would be based on a CC16xxx macro, for the proper number of counter bits. CCxxx macros should NOT be cascaded= this creates a less-than-optimal counter due to the added delay from coming off and getting back on the fast carry path, which occurs at the point where the two CCxxx macros would be connected.