| ??? 10/16/07 22:31 Modified: 10/16/07 22:33 Read: times |
#145827 - re: defaults? Responding to: ???'s previous message |
Andy said:
The default output means that you get "xx" for all of the 253 cases not covered by the three cases you specify. Of course that is for simulation only, as real hardware has no such state. You'd be wise to make your default assignment to 2'b00 so the synthesizer is clear about what is should do for the other cases. Maybe we are getting somewhere here. If I "got wise" and did assign 2'b00 for the default case, then it would have to consider bits 2, 3, 4, and 6, as you say, in order to work correctly. But the point is that I don't want to tell the synthesizer what to do for the 253 unspecified vectors. Rather, I want to leave it free to do anything it wants with the unassigned cases in order to minimize the logic that gets generated. Does that make sense? I still don't understand why you consider that bits 2, 3, 4 and 6 in your eight-bit vector are somehow unimportant. If they were the SAME for the three choices, then sure, the comparison could be reduced to the bit assignment, but they are not! I'm not sure how to respond to this without repeating myself. I think if you will address the "challenge" in my previous note, we may get this straightened out. Thanks, -- Russ |
