Why do clock buffers specifically need equal rise and fall delay, and what's the practical fallback when that's hard to guarantee?

Short Answer

Buffers used to taper clock paths need equal rise and fall delay time to maintain the original duty cycle and prevent clock signal overlap from differing propagation delays -- critical for very high-speed designs. Because perfect rise/fall balance is genuinely hard to achieve in a real buffer, a common remedy is to use inverters instead of buffers for clock tapering; incorrectly selected clock buffers with unequal rise/fall cause clock pulse width degradation as the signal propagates.

Technical Explanation

• Buffers used to taper clock paths need equal rise and fall delay time -- this maintains the original duty cycle and prevents clock signal overlap from differing propagation delays.
• This matters most for very high-speed designs, where even small rise/fall imbalance compounds across many buffering levels.
• Incorrectly selected clock buffers with unequal rise/fall delay cause clock pulse width degradation as the signal propagates through the tree.
• Since perfect rise/fall balance is genuinely hard to guarantee in a real buffer, a common practical remedy is to use inverters instead of buffers for clock tapering.
• report_clock_qor -type transition is how a rise/fall imbalance from the wrong buffer/inverter choice would actually surface as a DRC transition violation.

Follow-up Question & Model Response

"Why would using inverters (rather than buffers) actually help solve the rise/fall balance problem?"

Candidate Model Response: Using inverters means pairing them (an even count) preserves polarity while sidestepping the harder problem of guaranteeing identical rise and fall delay in a single buffer cell -- an inverter's asymmetry is easier to characterize and pair consistently than a buffer's symmetry is to guarantee.