Power Constraints Advisor reports the tool-computed switching activity on all control signals in the design. Control signals include resets and enables such as Reset, Set, Clear, and Preset. Providing reasonable switching activity ensures the most accurate power estimation.
In the Vivado® IDE, select to run the Power Constraints Advisor.
Review the report table and modify inaccurate switching activity on critical control signals such as inactive enables and reset signals that are asserted for excessive periods of time. The following constraints are available in the Power Constraints Advisor report:
- The nets are control sets, block RAM enables or Reg Enables.
- This field shows how accurate the switching activity is for a particular
net. Following are the thresholds used by the power tools when computing the
confidence level for nets:
- Set / Reset / Preset / Clear
Table 1. Power tool Thresholds Confidence Static Probability Low > 8% Medium Between 5% -8% High < 5%
- Block RAM Enables
Table 2. Block RAM Enable Confidence Static Probability Low < 1%
Low confidence means that the block RAM is not active in the design and should be revisited to check the possibility of removing it.
- Reg Enables
Table 3. Reg Enable Confidence Static Probability Low < 3% Medium > 3%
Low Confidence informs you that the Register in the design is not active and should be revisited. Medium Confidence informs you that the registers are enabled with reasonable amount of time either defined by you or propagated by tool.
- This field shows the fanout for each control signal, which is the number of driven leaf-level primitives. Signals with higher fanout are the most important for review and correction because they are capable of disabling downstream switching of large portions of the design. This may result in severe under-reporting of power. Low-fanout signals with inaccurate switching will have less impact and are therefore not important.
- Fanout Types
- This field specifies if the nets are control sets (set, reset, clear, preset) or bram enable. If there are multiple entries for any control net, it means that those particular nets have multiple fanouts and they are driving different pins in fanout cells.
- This field identifies the polarity for the control set. You should pay attention to the polarity while setting the static probability of a net.
- Static Probability
- This is editable filed and you need to enter the correct activity based on the fanout type and polarity of the net.
- Toggle Rate
- Toggle rate for the net. This is also editable and you need to enter this field based on the static probability.
The following process is recommended for using the Power Constraints Advisor:
- Click the Confidence column to sort it so that LOW signals are in top.
- Hold down the Ctrl key and click the Fanout column twice to sort it by descending values.
- Review and define new Static Probability and Toggle Rate for all the control nets which are LOW in confidence with fanout greater than 200.
- Click OK to apply the constraints to the design and rerun the Report Power command.
The following are some of the examples which help you to set accurate switching activity for the control sets and block RAM enables:
Active high reset with Static Probability 0.9
This indicates that the reset is high (active) 90% of the time. This means that the load cells are reset for 90% of the time, which is excessive. Change the switching activity to indicate that the reset is inactive, a more realistic condition, by setting the Static Probability to 0 and Toggle Rate to 0.
Block RAM Enable with Static Probability 0 and Toggle Rate of 0
report_power -advisory -file power_report.pwr
Advisory table is added at the end of the this report file.