STEP 9: PVT Corners and Variations
This step explains how real silicon variations affect timing and why Static Timing Analysis must be performed under multiple operating conditions. A design that works correctly under one condition may fail under another. PVT analysis ensures that the design is robust across all expected manufacturing and operating scenarios.
PVT (Process, Voltage, Temperature)
PVT represents the three primary sources of variation that influence circuit timing. These variations are unavoidable in real silicon and must be accounted for during timing analysis.
Process variation refers to differences in transistor and interconnect characteristics that occur during fabrication. These differences can cause devices to be faster or slower than nominal values.
Voltage variation refers to changes in supply voltage due to IR drop, power delivery limitations, or external conditions. Lower voltage generally slows down circuits, while higher voltage speeds them up.
Temperature variation refers to changes in operating temperature. Higher temperatures typically reduce carrier mobility, increasing delay, while lower temperatures generally result in faster operation.
Static Timing Analysis evaluates timing across combinations of process, voltage, and temperature to ensure reliable operation under all expected conditions.
SS, TT, FF Corners
Corners are predefined combinations of process, voltage, and temperature that represent extreme or nominal operating conditions.
The slow-slow corner represents a condition where transistors are slow, supply voltage is low, and temperature is high. This corner produces the largest delays and is critical for setup timing analysis.
The typical-typical corner represents nominal process, voltage, and temperature conditions. It is often used for reference or functional analysis.
The fast-fast corner represents a condition where transistors are fast, supply voltage is high, and temperature is low. This corner produces the smallest delays and is critical for hold timing analysis.
By analyzing these corners, STA ensures that both worst-case and best-case timing behaviors are covered.
OCV (On-Chip Variation)
On-chip variation accounts for differences in delay that occur within the same chip. Even on a single die, different regions can experience slightly different process conditions, voltages, and temperatures.
OCV introduces additional timing margin to account for these intra-chip variations. It ensures that timing analysis does not assume perfect uniformity across the chip.
AOCV (Advanced On-Chip Variation)
Advanced on-chip variation improves upon basic OCV by applying variation margins based on the depth of the timing path. Longer paths experience less relative variation due to averaging effects, while shorter paths experience more variation.
AOCV provides more realistic and less pessimistic timing analysis compared to simple OCV.
POCV (Parametric On-Chip Variation)
Parametric on-chip variation is a more accurate and statistical approach to modeling variation. It uses statistical data to model how delay varies with process parameters across the chip.
POCV reduces unnecessary pessimism while maintaining accuracy, making it suitable for advanced technology nodes and signoff analysis.
Why One Corner Passes and Another Fails
Different corners stress different aspects of the design. A path that meets timing at a fast corner may fail at a slow corner because delays increase. Similarly, a path that passes hold timing at a slow corner may fail at a fast corner because data propagates too quickly.
This is why STA must analyze multiple corners rather than relying on a single condition.
Setup Versus Hold Corner Dependency
Setup timing is most critical at slow corners, where delays are largest and data arrives later. Hold timing is most critical at fast corners, where delays are smallest and data arrives earlier.
Understanding this dependency helps designers know which corners to focus on when debugging setup or hold violations.
Goal of PVT Corners and Variations
The goal of this step is to develop an understanding of variation-aware timing. By analyzing PVT corners and on-chip variations, Static Timing Analysis ensures that the design functions correctly across all realistic manufacturing and operating conditions, not just under ideal or nominal scenarios.