BeginnerQuestion 50 of 50Source: Synopsys PrimeTime User Guide: Power Analysis; Synopsys IC Compiler II Implementation User Guide: Annotating the Switching Activity

What is switching activity (SAIF/VCD), and why does power analysis need it?

From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide

By Tabish Iqbal · 9+ Years VLSI Physical Design Experience

Short Answer

Switching activity is how often each net toggles and how long it sits at 1. Dynamic power depends directly on it, so the power number is only as good as the activity you feed in: SAIF gives toggle counts, VCD gives full waveforms, and without either the tool falls back to default activity.

A clock and a data net trace over 10 cycles with 4 toggles counted on the data net, feeding a box labelled read_saif (PT) with toggle count and static probability, then a dynamic power number. — Technical Reference DiagramWhat is switching activity (SAIF/VCD), and why does power analysis need it?

Technical Explanation

Dynamic power scales with toggle rate: a net that never switches burns no switching power, however big its load.
SAIF stores toggle counts and time at 0 and 1 per net over a window. It is compact and good for average power.
VCD stores every value change with its time, so it can show peaks and cycle-by-cycle power, but files get large.
Static probability, the share of time a net sits at 1, also feeds leakage, because a cell's leakage depends on its input state.
In PrimeTime power analysis you load them with read_saif (PT) or read_vcd (PT); the full flow is covered elsewhere.
With no activity, tools apply a default toggle rate at inputs and propagate it, which is a guess, not your workload.
Activity from the wrong test, or with names that do not match the netlist, gives confident but wrong power numbers.

Common Mistake

The Trap: Signing off power with default or vectorless activity alone.

The number can be off by a large factor, and the power grid or battery budget is then sized for a workload that never happens.
Zero-delay RTL activity has a smaller gap of its own: it misses glitches, so gate-level activity with delays usually reads higher.

Follow-up Question & Model Response

"Which workload should the activity come from?"

Candidate Model Response: It should come from the mode you are signing off. For average power and battery life, use a realistic long run of the main use case. For peak power and IR drop, use the busiest short window. Record both, and check the annotation rate so you know how many nets actually got activity.

Practical Example

Design Scenario: (illustrative) Net U_DSP/acc[3] toggles 4 times in 10 cycles of a 500 MHz clock, a toggle rate of 0.4 per cycle. SAIF from a 1 ms filter test gives PD_DSP 42 mW. A default activity guess gave 18 mW, less than half, so the power grid would have been undersized. The team also checked that about 97% of nets received activity from the SAIF before trusting the 42 mW. For IR-drop signoff they used a separate VCD from the busiest 2 µs of the same test.

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