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AR# 60809

2014.4 Power/XPE - Why is the Combinatorial Logic Toggle Rate at 100% from .xpe import flow?


Why is the Combinatorial Logic Toggle Rate at 100% from the XPE flow?


Because XPE computes the Signal Rate from the Clock and Toggle Rates, A clockFreq and toggleRate tag for combinatorial logic must be manufactured in the .xpe file in order for the signal rates to match in report_power and XPE. 

This is specific to the import flow.

Because there is no clock associated with LUT outputs, the switching information is meaningful only in terms of signal_rate.

Report Power reports the switching information in signal_rate and no clock information or toggle rate. This is clean.

On the other hand, XPE deals with toggle rate, even for LUTs.

  • To match with XPE behavior, currently Report_Power dumps the activity rates in Toggle Rate.
  • To do so, Report_Power converts the signal_rate into toggle_rate by assuming some clock frequency and creates the .xpe file.

These frequencies appear to be random (by keeping the toggle rate to 100% and deriving the clock frequency), but the creation of the .xpe file is done in reverse, by starting with the total power, LUT count, and 100% toggle rate, and then calculating the matching clock frequency.  

This can be confusing in the case of multiple clocks, as the clock frequency will likely not match any clock in the design.

AR# 60809
Date 03/27/2015
Status Active
Type General Article
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