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arrangement of the cell rows

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jitendravlsi

jitendravlsi

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What is the need for flipping the cell rows?


I think one reason is to minimize the core area.
is it correct?

any other reason?
 

flipping rows lets the rows share power rails
 

    jitendravlsi

    jitendravlsi

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then what is the advantage of sharing the power rails?

or


why it is required to share the power rails?
 

here's a few picutures that might make it clearer. the first group of three cells aren't flipped. so each cell has a power rail at the top (the plusses) and a ground rail at the bottom (the minuses) and they're abutted together so the rails of each cell are connected. there's a gap between the first and second row because obviously the power and ground rails can't sit on top of eachother, and there will be some design rule that states a minimum spaciing. in the second group the middle row is flipped (the W and M are my cheesy way of showing orientation) so that the ground rail of the top row can also be the ground rail of the second row, and the power rail of the second row can be the power rail of the third row and so on. there's no need for spacing and as a result it's more compact.

there's no requirement to do it that way, it's just more area efficient. hope that helps.


Code: 
without shared rails:
+++++++++++++++++++++++++++++++++
|       |       |       |       |
|   W   |   W   |   W   |   W   |
|       |       |       |       |
---------------------------------

+++++++++++++++++++++++++++++++++
|       |       |       |       |
|   W   |   W   |   W   |   W   |
|       |       |       |       |
---------------------------------

+++++++++++++++++++++++++++++++++
|       |       |       |       |
|   W   |   W   |   W   |   W   |
|       |       |       |       |
---------------------------------


with shared rails:

+++++++++++++++++++++++++++++++++
|       |       |       |       |
|   W   |   W   |   W   |   W   |
|       |       |       |       |
---------------------------------
|       |       |       |       |
|   M   |   M   |   M   |   M   |
|       |       |       |       |
+++++++++++++++++++++++++++++++++
|       |       |       |       |
|   W   |   W   |   W   |   W   |
|       |       |       |       |
---------------------------------
 

    jitendravlsi

    jitendravlsi

    Points: 2
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Hi ,

Main Advantage of sharing power rails is that , you are actually using lesser metal1 for power rails. Lesser metal means lesser resistance for Power Distribution network ( rather lesser IR drop in Power Distribution) which results in an better distribution of VDD & VSS to core

Regrads
Nav
 

    jitendravlsi

    jitendravlsi

    Points: 2
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Thanks

Denki & Nav

Nice explanation
 

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