Common clock path pessimism removal (CPPR) – Part 3


It’s been 5 days since my last post (and that was intentional). I wanted to go slow on this topic, as this is an important and critical one for reducing a lot of pessimism during a chip tape-out.

We saw negative slack being created due to OCV derates. But, below is the catch

And let me give you a hint. Can you run at 2 different speed at the same time instant ? Think ….. Think …..

If you are a real person with 2 eyes, 2 ears, 1 nose, 2 hands, and 2 legs, you can’t :). It doesn’t mean that animals can … They can’t either ….. No living being on this earth can run at 2 different speeds at same time instant ‘t’

Circuits (non-living being) also behave in the same way … fortunately. Look at the below image, and with the above calculations, we are trying to say, that cell ‘b1’, which is in common path of launch and capture clock, has 2 delays at same instant of time ‘t’, i.e. 43ps and 34ps

It can either have 34ps or 43ps, but not both. So for our calculations, either we take 43ps for both OR 34ps for both, in the common clock path.

Now since, the algorithm has already done the calculations, smart engineers came up with a simple solution, without changing the algorithm.

What they did, they allowed the slack calculation to happen in a traditional way, as we have shown in above image and last post, AND, introduced a new term “Clock Path Pessimism Removal“, which says, remove the additional pessimism from final Slack calculation.

Let’s see how

Above launch clock common path has a delay of 128ps and capture clock common path has a delay of 102.4ps. So the additional pessimism is 25.6ps (again….used a hand calculator … I think i am pissing you off now, by saying that every time :))

The additional “25.6ps”, we can either 1) add it in “Data required time” OR 2) subtract from “Data arrival time” Why? Because, we want the “delays” in common path to be same

Let’s do 1) for this example and see what we get

There you go ….. without doing even a single ECO (Engineering Change Order : For now, google this one, I will plan a separate post on this one) and using pure concepts, we were able to meet this path and attain the required frequency.

You see that, That’s the power of having strong basics … You save a lot of money, time, effort. That’s what I call “SMART WORK” 🙂  “The pleasure of life is to work hard towards your goals but a little bit smart and achieving them”

I would be now blessed by all STA people :).



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