How to fix Power Estimation Confidence: Low: user provided insufficient toggle rate data

Question

In my design, I have a 6b x 12b multiplier that is inferred from code that is essentially if(rising_edge(clock)) then
   if (clock_enable = '1') then
      result &lt;= num_6b * num_12b;
   end if;
end if;

The design compiles fine, simulates fine, and runs fine, but the compiler throws the warning:
Warning (222013): Relative toggle rates could not be calculated because no clock domain could be identified for some nodes

And my  power analysis report gives the low confidence metric. I also have a specified POWER_DEFAULT_TOGGLE_RATE set for the design, so I'm not sure why the tool is unable to determine the clock domain or use the default toggle rate. Is this a bug in the tool? Is there some other work around for me to tell it explicitly the toggle rate for this particular multiply operation? My clock is 30MHz and my clock enable is 5KHz.

Thanks!

sstrell · Answer

Did you properly set up a .sdc file with clock constraints defining this information?  And did you enable the vectorless estimation option in the Power Analyzer?
Using the default toggle rate will always give you the low confidence metric.  You need to supply more detailed info, such as a .vcd file from a simulation or manually create Power Toggle Rate or Power Toggle Rate Percentage assignments.

susanb · Answer

The design is fully constrained and the clock is defined per the sdc. The design also has thousands of other nets that the tool associates with a clock domain, and it is using my I/O and default toggle rates (percentages I input into the tool) without any problem on those thousands of other nets.

I am trying to find out why in particular it cannot associate a clock domain automatically with those 18 nets after it infers the multiplier from my VHDL. This is a deep dive question as to why those 18 nets are behaving differently from the thousands of other nets in the design where the power analysis tool finds the clock domain without any issue. I know that it is the 18 nets of the multiplier because in the power analysis report, they are listed under "indeterminate toggle rates."

Even if my default toggle rates cause the tool to give a low confidence rating versus a vcd, I should still be able to get an answer to this question from someone in the factory who understands why the multiplier IP in the Max 10 is causing the power analysis tool to treat the registered multiply as if it is unregistered?

susanb · Answer

Also attaching the confidence metrics screen. The 39 signals are the 36 from the multiplier (18 bits mult0:17, 18 bits DATAOUT0:17) and 3 other signals where I understand and agree with the zero toggle conclusion from the tool.

farabi · Answer

Hi Susan,

Can you attach your EPE ? we can take a look. 
Another thing is did you load .csv into EPE or just manually update the parameters into EPE?

regards,
Farabi

susanb · Answer

Farabi, I am attaching the csv from Early Power Estimator. I don't see a place in the tool for me to load a .csv as input to EPE. My choices are to input a VCD or to specify the default toggle rate, which is what I did. I do notice in the EPE it does recognize the 18b multiply and that the output is registered. It shows no register on the input (the data is registered elsewhere prior to the multiply) or pipeline register, so maybe that is what is causing the zero toggle rate to be assumed? I can experiment by turning on extra registers in the multiplier to see if that fixes the power estimate, but since the design easily meets timing, I don't want to add two pipeline delays permanently to the design.

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How to fix Power Estimation Confidence: Low: user provided insufficient toggle rate data

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