The Clock Interaction report summarizes how two clocks are timed together at a high-level:
- Do the two clocks have a common primary clock? When you properly define clocks, clocks originating from the same source in the design share the same primary clock.
- Do the two clocks have a common period? This shows in the setup or hold path requirement column (unexpandable), when the timing engine cannot determine the most pessimistic setup or hold relationship.
- Are the paths between the two clocks partially or completely covered by clock groups or timing exception constraints?
- Is the setup path requirement between the two clocks tight? When two clocks are synchronous, this can happen if their period is not specified as an exact multiple. This can occur due to rounding off. Over multiple clock cycles, the edges can drift apart, causing the worst case timing requirement to be tight.