Process corners
Why a chip is checked under several conditions, and what "slow" and "fast" mean.
On this page
The three things that varyThree common cornersWhy both ends matterA real exampleIn this labNo two chips are identical, and no chip works in only one condition. A design must work across all of them. Checking every possible combination is impossible, so engineers check the extremes, called corners.
The three things that vary#
| Variable | Slower when | Faster when |
|---|---|---|
| Process: how the transistors happened to come out | They came out slow | They came out fast |
| Voltage: the supply | It is low | It is high |
| Temperature | It is hot, in a process like this one | It is cold |
Together these are called PVT.
Three common corners#
| Corner | Combination | Used to check |
|---|---|---|
| Slow | Slow transistors, low voltage, hot | Setup: is the slowest the chip can be still fast enough? |
| Typical | Nominal everything | What to expect on an average chip |
| Fast | Fast transistors, high voltage, cold | Hold: is the fastest the chip can be still safe? |
Why both ends matter#
A setup problem is a path that is too slow, so it shows at the slow corner. A hold problem is a path that is too fast, so it shows at the fast corner. A design that passes at typical and fails at slow will fail in a hot room.
A real example#
The MAC-8 starter's specification records what happened when it was built with a 10 ns clock: it met timing at the typical corner and missed by about 2 ns at the slow corner. The starter therefore uses 15 ns, which passes at every corner.
In this lab#
The build checks timing at several corners and the chip report names each one with its margin. A design has met timing only when every corner passes. The build card's Timing met means exactly that.
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