synthesize register and bypass circuit with design compiler

design compiler alu

Hi, friends,

I have a simple HDL model which I want to synthesize with Design compiler. It looks like this:

The output of an ALU is connected to a flip-flop. At the same time, the output of ALU is bypassed by another signal. Both the registered output and bypassed output are connected to a 2-to-1 multiplexer. You don't need to care about what circuit is before the ALU input or after the multiplexer output.

The current problem is that, when I want to synthesize the whole circuit, the design compiler will take the bypass circuit and keep on accumulating the critical path with the circuit after the multiplexer. But what I really need is to make the design compiler stop accumulating the critical path at the register and don't touch the bypass. Is there any way or commands to realize that?

Thanks very much!

regards

bypass circuit

You can do set_false_path or set_multicycle_path through the mux input for the bypass signal.

or

You can set_case_analysis on the mux select signal to only alow the registered path to go through the mux. Make sure the mux does not get optimized away.

alu design compiler

Thanks a lot! I tried the command set_case_analysis. I think it works because I can still find the multiplexer. It is not optimized away.