[SOLVED] Help needed in Primt time!!!

[sakthikumaran87]

Help needed in Primetime!!!

i have a doubt for a long time. DC/RTL compiler instantiate a gate with a certain drive strength by keeping DRC constraints in mind. But in PT we are changing the drive strength of the gates to achieve timing. But wat if the change in the drive strength violates DRC/ cause funtionality mismatch to occur due to reduced drive strength. How to check this as PT itself is a sign-off tool?? Any idea??

Thanks in Advance,
SK

 

[birdy123]

PT also check the DRC constraint .. you can change the drivestrength .. but during reporting PT will give you violation corresponding to the DRC constraints.

 

[sakthikumaran87]

So you mean that swapping of cells in PT wont change the functionality or violate the rules of the design right?

 

[birdy123]

No No -- I am not saying this.. I am saying that in case after swapping the cells, if design has any violation, PT will report that. And then you have to fix it.

E.g - by changing the HVT to LVT, if there is any violation, the PT will report that violaition. Then you have to see if you can fix it it someother way, if not then you have to re-swap LVT to HVT for that particular cell.

I hope .. I got your point correctly. and replied you as per that ?

 

[sakthikumaran87]

ok how abt changing drive strenght? For ex> Buff4 to Buff1 like that??

 

[matter]

What makes you think that it is even possible to change functionality by changing drive strength ????

 

[sakthikumaran87]

Hi Matter,

My concern is that if we change the drive strength as we like and what if it actually couldnot drive the gates as it was required. For ex: if we connected a buffer of strength 4 to 5 gates and now we are swapping buffer with strength 2. My doubt is whether it can still support a fan-out of 5 as that of buffer4 and if not how to know it.

- - - Updated - - -

Thats why i asked wat will happen if it cannot drive all gates as required and whether there will any lapse with the functionality of the design.

 

[matter]

Glad that I asked you the question.

Your concern is valid, but still it is not considered as a functional violation. Functional violations are caught by design verification (formality, other DV tools) etc. These DV tools don't care about drive strength.

For the situation mentioned in your previous post, it will still be treated as a DRC violation i.e. if you replace a drive5 by drive2 and hypothetically assuming that its too small to drive the load, it will cause a max_tran violation at the loads. This will get reported by PT.

Regards

 

[sakthikumaran87]

Thanks a lot matter. i have cleared my doubt now.