Global and Regional Resets - 2020.2 English

Vivado Design Suite User Guide: Power Analysis and Optimization (UG907)

Document ID
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2020.2 English

The Activity rate on Global Resets could change the power estimation dramatically. It conveys the state of each logic block in the design and the probability of logic output changes. If it is not set with the right switching information, you can get unrealistic power estimates. For example, ideally Reset is expected to be asserted (active) at the beginning of the run for a few cycles and remains inactive the rest of the time. This could be denoted in terms of switching activity as:

set_switching_activity -static_probability 0.01 -signal_rate 2 [get_ports glb_reset]

Report Power identifies primary ports which are found to be global resets and applies the above switching activity. It uses a very conservative and safe way to identify the global resets - the ports which are directly connected to Reset pins of leaf primitives. However this does not help much on complex designs where the Reset logic is generated internally through special logic circuits (reset generator, debouncer, reset stretching, etc). When there is logic involved to generate Reset, Report Power is not aware of design intent and does not apply any default switching information on it.

Figure 1. Reset Logic

In this situation, the Reset activity information is derived from the generated logic using a probabilistic computation and propagation algorithm. Probabilistic computation is done at the leaf primitive level of logic. At times, the probabilistic algorithm lags handling of specific logic blocks, such as deep nested feedback logic. This results in unexpected switching activity on Reset nets.

Recommended: Make sure to supply the correct switching information on global/regional Reset nets.

The designer is expected to be aware of such global reset nets in the design. Set activity rates directly on these nets in the Power tab of the Net Properties window.

Figure 2. Setting Net Activity Rate

Equivalent Tcl command:

set_switching_activity -static_probability 0.01 -signal_rate 2 [get_nets u1/clkRst_gen/user_reset]

The Power Report also helps identify the Reset nets in the design, so you can verify the switching information on these nets and take corrective action. You can run a first trial run of Report Power using the default settings to analyze the activity on Reset nets.

Figure 3. Reset Net in the Power Report

Note that the Power Report also shows the number of logic cells that are affected by this Reset net: Fanout. If the initial switching activity estimation does not seem correct, you can select the net in the Power Report (as shown above) and edit the Power properties in the Net Properties window.

Note: Report Power displays both Preset/Set and Reset nets combined in the design. The above guidelines for Reset nets also apply to Preset/Set nets.