Clock uncertainty is the total amount of possible time variation between any pair of clock edges. The uncertainty consists of the computed clock jitter (system, input, and discrete); the phase error introduced by certain hardware primitives; and any clock uncertainty specified by the user in the design constraints (
For primary clocks, the jitter is defined by
set_system_jitter. For clock generators such as MMCM and PLL, the tool computes the jitter based on user-specified jitter on its source clock and its configuration. For other generated clocks (such as flop based clock dividers), the jitter is the same as that of its source clock.
The user-specified clock uncertainty is added to the uncertainty computed by the Vivado® Design Suite timing engine. For generated clocks (such as from MMCM, PLL, and flop-based clock dividers), uncertainty specified by the user on source clock does not propagate through the clock generators.
For more information on jitter and phase error definitions, see the Vivado Design Suite User Guide: Using Constraints (UG903).
The clock uncertainty has two purposes:
- Reserve some amount of margin in the slack numbers for representing any noise on the clock that could affect the hardware functionality. Because the delay and jitter numbers are conservative, Xilinx does not recommend adding extra uncertainty to ensure proper hardware functionality.
- Over-constrain the paths related to a clock or a clock pair during one or several implementation steps. This increases the QoR margin that can be used to help the next steps to close timing on these paths. By using clock uncertainty, the clock waveforms and their relationships are not modified, so the rest of the timing constraints can still apply properly.