CDC: Two Flip-Flops Are Not Magic

Fri, 02 Jan 2026 05:19:00 +0000

Timing Series: Part 5 of 6

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The Bug You Can’t Reproduce

You’re debugging a data corruption bug. It happens once every few hours under heavy load. Sometimes once a day. The data path looks fine. The control logic looks fine. You add ILA triggers. You wait. You catch it.

A control signal that pulses for one cycle is sometimes missed entirely. The path crosses clock domains. You have a 2-flop synchronizer.

Constraints: The Contract You Forgot to Sign

Tue, 23 Dec 2025 15:04:00 +0000

Timing Series: Part 1 of 6

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Signoff Gates: Build Must Fail If…

Before reading further, know this: your build should fail if any of these checks fail.

Gate 1: Unconstrained endpoints must be zero

check_timing -verbose
report_exceptions -verbose

Fail if: Any constraint-related issue about clocks, unconstrained endpoints, or missing I/O delays. Also fail if any exception matches zero objects (silent no-op) or hits an unexpectedly large set.

CDC on bitwiz

CDC: Two Flip-Flops Are Not Magic

The Bug You Can’t Reproduce

Constraints: The Contract You Forgot to Sign

Signoff Gates: Build Must Fail If…