1.1.7. Timing Pessimism
Minimum and maximum delay variation can occur when two different delay values are used for the same clock path. For example, in a simple setup analysis, the maximum clock path delay to the source register is used to determine the data arrival time. The minimum clock path delay to the destination register is used to determine the data required time. However, if the clock path to the source register and to the destination register share a common clock path, both the maximum delay and the minimum delay are used to model the common clock path during timing analysis. The use of both the minimum delay and maximum delay results in an overly pessimistic analysis since two different delay values, the maximum and minimum delays, cannot be used to model the same clock path.
Segment A is the common clock path between reg1 and reg2. The minimum delay is 5.0 ns; the maximum delay is 5.5 ns. The difference between the maximum and minimum delay value equals the common clock path pessimism removal value; in this case, the common clock path pessimism is 0.5 ns. The Timing Analyzer adds the common clock path pessimism removal value to the appropriate slack equation to determine overall slack. Therefore, if the setup slack for the register-to-register path in the example equals 0.7 ns without common clock path pessimism removal, the slack is 1.2 ns with common clock path pessimism removal.
You can also use common clock path pessimism removal to determine the minimum pulse width of a register. A clock signal must meet a register’s minimum pulse width requirement to be recognized by the register. A minimum high time defines the minimum pulse width for a positive-edge triggered register. A minimum low time defines the minimum pulse width for a negative-edge triggered register.
Clock pulses that violate the minimum pulse width of a register prevent data from being latched at the data pin of the register. To calculate the slack of the minimum pulse width, the Timing Analyzer subtracts the required minimum pulse width time from the actual minimum pulse width time. The Timing Analyzer determines the actual minimum pulse width time by the clock requirement you specified for the clock that feeds the clock port of the register. The Timing Analyzer determines the required minimum pulse width time by the maximum rise, minimum rise, maximum fall, and minimum fall times.
With common clock path pessimism, the minimum pulse width slack can be increased by the smallest value of either the maximum rise time minus the minimum rise time, or the maximum fall time minus the minimum fall time. In the example, the slack value can be increased by 0.2 ns, which is the smallest value between 0.3 ns (0.8 ns – 0.5 ns) and 0.2 ns (0.9 ns – 0.7 ns).