About power consumption report

[b224hisl]

I would like to know:

1. Whether the power consumption in the report represents per-cycle power consumption.
2. How does the tool calculate the power (It seems that there is a class called PowerResult, but I don't find its definition)
   Thank you very much!!

[maliberty]

The consumption is based on average activity and is reported in Watts (J/s) not per-cycyle. The Liberty file gives models of power consumption for each cell. The activity can either be read from a .vcd file or estimated statistically if not otherwise specified.

[b224hisl]

Thank you very much, but how does the openroad generate the .vcd file? Does OpenRoad use the .vcd file or just add the power consumption of the netlist's cells from the lib file?

[maliberty]

OR just reads the vcd file. It is generated from a Verilog simulator. The vcd data, if present, is used to set the toggle rate of the signals.

[b224hisl]

So if there is no .vcd data, OR will add the power consumption of the netlist's cells from the lib file, am I correct?
And if I set the .vcd file, will the power report show per-cycle power consumption?

[maliberty]

There are two different steps going on

1. Estimating the toggling activity - done either from vcd or statistically
2. Computing the power used - always done from .lib and always in Watts (never per-cycle). This step uses the activity (eg if the input to an inverter never toggles then it only consumes leakage power).

[b224hisl]

OK thank you very much!
And I have another two questions.

1. How to point to a specific .vcd file in OR?
2. If I don't set the .vcd file, how does OR estimate the toggling activity statistically? Like Iterate through each case of each cell, take the average, and then add up all the cells in the netlist?

[maliberty]

read_power_activities -vcd

It propagates average toggling through gates statistically (eg nand2's output will toggle less often then its inputs).

[b224hisl]

ok thank you, I think the problem has been solved, I will close this discussion