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VLSI Physical Design  ›  Ch 7. Setup, Hold & Timing

KEY No - hold violations are frequency-independent functional failures and block sign-off.

Clock-Gating Setup and Hold Checks

Clock gating blocks clock pulses using an enable signal and an AND gate. Because the signal being gated is the clock itself, the enable must not change the pulse shape or introduce glitches.

The enable must arrive sufficiently before the clock's rising edge so it does not chop that edge - this is the clock-gating setup (default max) check. Likewise the enable's turning-off edge must occur well after the clock's falling edge so it is not chopped - this is the clock-gating hold (default min) check.

Diagram 0

KEY Clock-gating setup/hold checks ensure the enable does not chop or glitch the gated clock pulse.

What Sets the Maximum Frequency

The worst (smallest) maximum-path margin sets the maximum frequency, because setup failure is frequency dependent.

Hold failure is frequency independent, so it has no bearing on the maximum-frequency calculation and is therefore not factored in.

KEY Max frequency is set by the worst setup margin; hold is frequency-independent, so it is excluded.