About routing constant values to VDD/VSS power rails

[OuDret]

Is there any way to route logical constant 1 & 0 to Vdd & Vss power rails instead of using tie cells? Right now I'm using a cell with short circuit between output and power pins, which is really bad in terms of area.

[maliberty]

No there is no facility for that. The supply rails are often noisy and the tie cell provides a more stable output. What is the issue with using a tie cell?

[OuDret]

I am using an emerging electronic technology and the number of transistor that we can introduce is a strong restriction, in this way and we experience a huge difference using them or not in our designs. I guess I will have to create copies of each cell for all possible shortened inputs to replace them after synthesis.

[maliberty]

You could use one tie hi & one tie low per design if that's acceptable.

[maliberty]

You could also make a dummy tie cell whose internal implementation is just a short.

[OuDret]

Using only one tie cell per design would increase routing congestion which is critical since i only have 2 routing layers. Dummy tie cell is what I'm currently using but it take 1 stdcell site + pad_site area.

[stefanottili]

Constant propagation should be able to remove all constants unless they're connected to ram/rom/pad/analog macro's, no ?
Which synthesis tool has been used to create the netlist ?
Can yosys do constant propagation ? If so, is it on by default or does it need to be enabled ?

[maliberty]

Yosys does constant propagation in the normal flow.

[OuDret]

I have ran hilomap -singleton -locell LOGIC0 Z -hicell LOGIC1 Z in yosys synthesis.

If you don't handle it in sI have ran hilomap -singleton -locell LOGIC0 Z -hicell LOGIC1 Z in yosys synthesis.

If you don't handle it in synthesis step, constants like 1'h0 are replaced for _zero keyword when calling read_verilog .

Openroad ``insert_tie_cells``` command replaces those keywords for tie_cells.

I don't know much about yosys but I assume it can be done some constant propagation. Will look for it