an old CTS question - sizes of clkcells for clock tree

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devop

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an old CTS question

Hi guys:
why we dont use the smallest and largest clkcell when building clock tree?
we usually us 3x 4x....16x clockbuf ,what about you?
thanks a lot
 

Re: an old CTS question

devop said:
Hi guys:
why we dont use the smallest and largest clkcell when building clock tree?
we usually us 3x 4x....16x clockbuf ,what about you?
thanks a lot
Click to expand...
can anyone kindly share the experience??
do you use 1x or 64x clockbuf in you design?
 

upup
one more question,would I use regular buffer instead of CLKBUF if the skew is not a big issue??
 

My Intuitive guess is

smallest clkbufs - are more sensitive to Process variations , hence clock skew(OCV)
issues.

Largest clkbufs - have more Current , Hence Electromigration(EM) /Reliability
related issues , (besides too much of Power Consumption).
 

koggestone said:
smallest clkbufs - are more sensitive to Process variations , hence clock skew(OCV)
issues.


).
Click to expand...

I am confused here,
case 1, 10 smallest buf1X connet in series to drive a load
case 2, 1 biggest buf64X drive the same load

I think case 1 will be better in OCV? what's your opinion?
 

devop said:
I am confused here,
case 1, 10 smallest buf1X connet in series to drive a load
case 2, 1 biggest buf64X drive the same load

I think case 1 will be better in OCV? what's your opinion?
Click to expand...

case 2 has Less OCV .
Refer to famous Pelgrom's 1989 paper on Mismatch.
**broken link removed**
Bottom Line is - Variations are inversely proportional to Transistor Area (W.L).
Bigger Buffers (i.e Big W ) will decrease Variations.

Hence u see all those Microprocessor Global clk design guys and Anolog
designers will love to use bigger Transistors to decrease mismatch at the
expense of more power dissipation.
 


thanks koggestone

I consider the question in another way,
case 1, 10 cells , they scatter in a big area, the ocv for the whole path will be better,


just for a certain cell ,you are right ,Bigger Buffers (i.e Big W ) will decrease Variations.

what's your opinion?
 

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