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Buffer Choice while doing clock tree Clock Tree Synthesis

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dbr@vo

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Hi,

I was reading somewhere while doing CTS, we instruct the tool to use only specefic clock tree buffers for bulding the Clock tree (usually medium drive strength clock buffers) for getting the good results.

But why do we do so, how it will help in getting bettere results ?
 

Manochitra

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Hi,

I was reading somewhere while doing CTS, we instruct the tool to use only specefic clock tree buffers for bulding the Clock tree (usually medium drive strength clock buffers) for getting the good results.

But why do we do so, how it will help in getting bettere results ?

hi

We can add any kind of buffers in CTS neither small nor large drive strength buffers..But in case of small drive strength buffers the delay will be large..We do CTS in order to match the data and clock path only so If delay is high it seems difficult..

In case of large drive strength,It may occupy large area,may cause congestion...

So it is better to add medium drive strength buffers to overcome this difficulties...

I think this may help u
 
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subhash_chevella

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Hi,

I think, we don't use medium drive strength clock buffers in all the cases while doing CTS.
We use alternative drive strength clock buffers to get better results compared to others.
I also experienced this in my previous projects.

Bue, I need to find out the reason, why alternative drive strength buffers will give good results?
If any one knows, can share your thoughts/views here. :)

regards,
Subhash C
 

dbr@vo

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hi

We can add any kind of buffers in CTS neither small nor large drive strength buffers..But in case of small drive strength buffers the delay will be large..We do CTS in order to match the data and clock path only so If delay is high it seems difficult..

In case of large drive strength,It may occupy large area,may cause congestion...

So it is better to add medium drive strength buffers to overcome this difficulties...

I think this may help u dont forget to click helped..

I agree in case of small buffers delay will be large, but while doing CTS if we give small drive strength buffers also in the buffer list, than tool is intelligent enough to select the type of buffer it wants ( like if output cap is large it will not use lower drive strength buffer and if output cap is small enough than it can use smaller drive strength). Same way if the tool is using very high drive strength buffer means its output cap. is large, lets say in place of one high drive strength buffer tool is using two medium drive strength buffers to drive that output cap. , means area occupied will be almost same but pin count will be greater in this case which will cause more congestion.

This is what I think, correct me if I am wrong.
 

verilog_always

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While using Small drive strengths - While optimizing tool may add more number of low drive strength cells in order to meet tran and other spec related .

Large Drive strengths - Power consumption will more, as for I know.
 

nav_vlsi

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There is also a flip side in providing only small drive strength buffers in CTS.
These small drive strength buffers are more sensitive to OCV compared to medium or large drive strength buffers. esp in lower nodes, when your chip's operating freq is in Ghz range , care has to taken on OCV front also.

try to give multiple large drive strength buffers & few smaller drive strength ones.
Also check for "balanced" buffers in your std cell library, that should help u.
 

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