IR drop analysis reports negative values

[ishwars1618]

When using OpenROAD flow on certain circuits (usually large circuits, having around 10^4 cell instances), I found that the analyze_power_grid tool reports negative voltage values at each instance, even though I had set the VDD to 0.77V.

I have attached a package with the test case below for the final_report step.

Can you please let me know how this can be resolved?

Thanks,
Ishwar S.

Please download the test case package from here: https://drive.google.com/file/d/1BpxVOgMD08aVjMaJrdFYml4kR0VSF-jj/view?usp=drive_link

[maliberty]

I don't see a negative value reported:

########## IR report #################
Corner: default
Worstcase voltage: 1.69e-01 V
Average IR drop  : 4.60e-01 V
Worstcase IR drop: 6.01e-01 V
######################################
########## IR report #################
Corner: default
Worstcase voltage: 5.93e-01 V
Average IR drop  : 4.60e-01 V
Worstcase IR drop: 5.93e-01 V
######################################

How are you observing this negative value?

[ishwars1618]

I see negative values in results/asap7/tc1_1e4/base/6_instance_voltage_pwr.txt, which is produced by scripts/info-beta.tcl in line 76. The vsrc file used in analyze_power_grid can be seen here. This contains vsrc nodes with 0.7V instead of 0.77V.
I changed both vsrcs to 0.77V and reran the 6th step, but this still results in negative voltage values, which can be seen in this file.

[maliberty]

Without the vsrc file I don't see the negative values (the test case was not setup this way as originally packaged). With it I do. The file is incorrect as it only has three values per line not four as described in https://github.com/The-OpenROAD-Project/OpenROAD/blob/master/src/psm/doc/Vsrc_description.md

I'll add some error checking to the file reader.

[maliberty]

I see you can leave off the fourth value but in any case the third value is the bump size not the voltage. For example with

10,10,0.7
40,40,0.7

I don't see any negative values