A one-cycle pulse is the most commonly lost signal in multi-clock designs. This topic covers why a plain synchroniser misses it, the two signoff-accepted ways to carry it, and the spacing rule both of them need. For the design-side construction see the CDC handshakes RTL lessons; here the emphasis is on what a reviewer and the checks must confirm.
Lesson | After it you can... |
|---|---|
Explain the loss mechanism for a short pulse into a slower clock. | |
State the toggle rules, including the no-early-re-toggle rule. | |
Judge when stretching is acceptable instead of a toggle. | |
Specify and check the minimum gap between pulses. |
Suggested order: read the four lessons in page order, then continue to Handshake Protocols Across Domains.
The pulse can rise and fall entirely between two destination sampling edges, so the synchroniser never sees it. The event must first be turned into a level that persists.