flow: don't use cell sg13g2_sdfbbp_1

[htfab]

Yosys can only infer flip-flops where the next_state property in the liberty file is a single pin or its negation. For sg13g2_sdfbbp_1 it is (SCE*SCD)+(SCE'*D) so it prints a warning and ignores the cell. By adding the cell to the DONT_USE_CELLS list we can get rid of the warning.

[maliberty]

@povik a scan flop seems like an ordinary thing - why can't yosys handle this cell?

[povik]

@maliberty With YosysHQ/yosys#4652 we are extending the next_state handling to deal with arbitrary expressions, by which we should get rid of the warning in question.

Is there anything you would expect a synthesis tool to do with a scan flop other than ignore it?

[maliberty]

I would hope it would quietly ignore it. Putting it in the DONT_USE lists suggests that it should never be used (should DFT ignore it too?).

Is there an ETA to land that PR?

[povik]

Is there an ETA to land that PR?

My estimate is the start of December release.

[maliberty]

@htfab are you ok to wait ~1 month to have this resolved by yosys? If not we can merge but will need to track reverting this then.

[htfab]

I'm ok with waiting. We are using it in Tiny Tapeout, but we already have a script that patches it on our side, so it just means we keep that script for somewhat longer.

[maliberty]

This will resolve automatically when the right yosys update happens.