BeginnerQuestion 30 of 187

What must be checked for a primary clock?

From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide

By Tabish Iqbal, who has 9+ years of VLSI Physical Design experience

Short Answer

A primary clock originates at a real design source, typically an input port — it's not derived from any other clock in the design. Verification means checking its source object and name are correct, its period and waveform are as intended, and that it's visible in the right modes.

What must be checked for a primary clock?, illustrating the physical design concept. — Technical Reference DiagramWhat must be checked for a primary clock?

Technical Reference DiagramWhat must be checked for a primary clock?

Technical Explanation

Just as important: confirm its reach — does it actually arrive at every sequential clock pin it's supposed to, or did some register get missed and end up unclocked in the constraint model.

Visual Verification

Circuit schematic showing create_clock defined at an input port propagating through clock buffers to downstream register CLK pins. — Visual VerificationPrimary Clock Definition & Sequential Register Fanout

What To Check

Warning sign: A port named clk exists but no clock object is created, or the waveform is wrong.
Inspect: Start with one named object in the report and trace it to the netlist, library, or SDC statement that created it.
Correct: Correct create_clock in the owning SDC and confirm the reported clock and representative clock-pin timing.

Command Checks & Actions

report_clocks: shows clock definitions and relationships
check_timing -include {no_clock}: finds missing or inconsistent timing setup

Run the commands in order. Each line answers a separate part of the check.

Healthy, Suspicious & Hard-stop Results

Expected: Verify its source object, name, period, waveform, mode visibility, and reach to intended sequential clock pins.
Stop before floorplanning when required logic or timing coverage is missing or unexplained.

Common Mistake

The Trap: Do not assume the check passed just because ICC2 continued. Fix or narrowly justify the named objects, then rerun the same command.

What The Interviewer Is Testing

Be ready to explain why this matters before floorplanning.

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