KEY The derate increases with the cell's distance from the clock path's diverging point.

Raising Frequency on a Signed-Off Database

Raising the frequency shifts the timing windows of every timing arc in the design. This alters how the arc windows overlap, which in turn can increase or decrease the crosstalk effects across the design.

KEY Higher frequency shifts arc timing windows, changing window overlap and crosstalk effects.

Uncertainty vs Frequency for Performance

Change the frequency. Closing timing by simply increasing uncertainty does not guarantee real performance because it does not address noise-related issues. Changing the frequency actually shifts the crosstalk arrival windows and meaningfully affects timing, so closing timing at the new frequency gives a genuine guarantee of the target speed.

KEY Adjust frequency, not uncertainty - it really shifts crosstalk windows and guarantees performance.

AOCV on Cell Delays, Not Wire Delays

Although wire-width variation from OPC and etching is also non-uniform across the die, AOCV is not applied to nets for the following reasons.

• Cells contain doping, which is sensitive to the process; AOCV is essentially a function of process effects such as Vt and doping.
• Etching-related variation is already accounted for in the parasitic extraction characterization - the etch values are tabulated in the TLUPlus/nxtgrd files, where wire delay varies with thickness, net length, temperature and similar factors.
• Wire delay varies mainly with local process and temperature, not with voltage. When a timing path stretches over a long distance, the local temperature of a net in one location can differ from an identical net elsewhere, so flat derates (OCV) are used for wires to cover that variation.