What is on-chip variation (OCV), and how is it different from PVT corner variation?
From PDVerse STA Mentor Guide · pdVerse Mentor Guide
Technical Explanation
OCV is the phenomenon that identical devices in different locations on the same die behave slightly differently, due to random dopant fluctuation, lithography variation, systematic gradients, and local temperature or voltage differences — distinct from the global variation captured by PVT corners.
Timing-path Reasoning
PVT corners capture global/die-to-die and lot-to-lot variation plus overall operating conditions — essentially, how one whole chip differs from another chip or how a chip's global process/voltage/temperature setpoint shifts delays uniformly. OCV instead captures local, within-die variation — how two supposedly identical gates on the very same die can still differ from each other. A common confusion is assuming that picking a slow PVT corner already accounts for OCV; it does not, because OCV is a separate, local-scale effect layered on top of whatever global corner is chosen.
Expected Result
A signoff methodology applies both a PVT corner selection (global variation) and an OCV derate or statistical model (local variation) as independent, stacked sources of pessimism/risk coverage. clock source shared trunk launch FF (derated SLOW) capture FF (derated FAST) The shared trunk cannot be both slow and fast at once — CRPR credits the difference back.
Failure Symptoms
A design analyzed only at slow/fast PVT corners without any OCV derate can miss real within-die mismatch risk, particularly on paths sensitive to local skew between clock and data, or between two supposedly matched cells.
Root Cause
Conflating global process/voltage/temperature variation (die-to-die, lot-to-lot) with local within-die variation (random dopant fluctuation, lithography, gradients, local T/V differences), which are physically distinct variation sources.
Mentor Note — Debugging Procedure
Check that the timing setup applies a derate factor (via set_timing_derate or an AOCV/POCV model) in addition to, not instead of, the chosen PVT corner. PVT corners and OCV answer two different questions — how does the whole chip vary, versus how do two gates on the same chip vary — and signoff needs both, since a slow PVT corner never substitutes for OCV.
Continue learning free
Get a practical low-power chapter
Receive the existing “Low Power and Multivoltage Fundamentals” PDF chapter and its download link by email.
Continue practising