Impact of setup and hold requirements with increase in drive strengths of flip flops

[helpful]

Hi,

What is the impact on the setup and hold requirement of a flip-flop with the increase in its drive strength? and also reason behind the impact?

Thanks
Srikanth

 

[birdy123]

I recommend you to follow this article

**broken link removed**

and

10 ways to fix setup and hold violation

 

[helpful]

Hi Birdy,

Thanks for your response.
The material provided doesn't answer my question completely.

To be more specific about my question.

Consider a timing path with two flops F1, F2 of drive strength '1'.

F1---------Combinational Logic--------F2

If 'x' is the setup requirement for flop F2 and if I upsize the flop F2 to say drive strength '2', what will be the impact on the setup requirement, will it be 'x' or greater than 'x' or lesser than 'x' and why?

Can you please help with this question.?

Thanks
Srikanth

 

[KaptainBug]

Basically when you upsize a flop/transistor it will reduce the delay of the gate. In this case it can affect only the Tc-q delay of the flop.

Setup eq:
Tclk >= Tc-q + Tcomb + Tsu

Hold eq:
Tc-q + Tcomb >= Thold

With all other parameters remaining constant, upsizing the flop or reducing the delay of Tc-q will relax the setup time and hold time is more constrained.

Please correct me if this is wrong.

 

[birdy123]

There will be no impact on setup or Hold of F2 just by increasing the driving strength.

 

[KaptainBug]

There will be no impact on setup or Hold of F2 just by increasing the driving strength.

That's right. Increasing drive strength of F2 does not have any effect on setup and hold. But increasing drive strength of F1 will have an effect on its setup and hold. Correct me if I am wrong.

 

[lostinxlation]

It will affect setup and hold of F2 only if you change the drive strength of F1 to the extent that it has much impact on rise and fall time at F2 data input.
If the logic between F1 and F2 is deep enough and has cells with reasonably good drive strengths, it doesn't make much difference in the rise/fall at the end of the combo logic regardless of rise/fall at F2 output.