What clock information must be available before physical design?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
The handoff must identify real clock sources, generated-clock relationships, waveforms, and the timing assumptions for each operating mode. Clock-tree synthesis later implements distribution; it does not invent the functional clock specification.
Technical Explanation
A primary clock is defined at a source such as a clock input. A generated clock derives from another clock through a divider, multiplier, or other defined relationship. Its master, source, and edge relationship matter because timing analysis must compare the correct launch and capture edges. A divided clock is not automatically asynchronous to its parent. Uncertainty models margins such as jitter and methodology-dependent allowances. Latency models clock travel time where appropriate. Before clock-tree synthesis, parts of the clock network are often idealized; after clock implementation, propagated network delays become available. Update the assumptions deliberately so estimated skew margins and actual clock behavior are not double counted. Clock gating and test multiplexing require correct mode controls and recognized timing behavior. Missing generated clocks can leave downstream registers without the intended checks. Incorrect clock-group or false-path constraints can instead hide valid transfers. Inspect the clock report and representative timed paths, rather than relying on a clock-name list alone.
Common Mistake
Declaring every different clock name asynchronous, or assuming CTS repairs missing clock constraints.
Follow-up Question & Model Response
What would you check if registers behind a divider are unconstrained? Check the divider output, generated-clock definition, source/master selection, and propagation through the active mode.
Practical Example
A divide-by-two output derived from a 10 ns source has a 20 ns period, but its phase and edge relationship still matter. Defining it as an unrelated primary clock can discard useful relationship information.
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