The pros and cons of running the power rails next to each other and below the block

[sridhar540]

I want the advantages and the disadvantages of running the power rails vss vcc next to each other.

And


I want the advantages and disadvantages of running the power rails above and below the block.

 

[leo_o2]

Power rails

The advantage is it will form a parasite cap. The cap will de-coupling noise and help for ESD protection.

 

[layoutmaster]

Power rails

It's a good thing to have rails since it makes much easier your connection but it's not recommended nor usual to have both placed next to each other...

Usual thing, for instance in digital blocks, is to place vcc on top and vss at bottom and then, if you make two rows of cells, the idea is to move upside-down the second row and make vcc and vss coincidents...

 

[survivor]

Power rails

For core limited case, if VDD and VSS rails are running above and below PAD, you may insert ESD devices beside PAD and in between VDD and VSS rails. That will make the I/O cell shorter but wider. You may save some area on your I/O ring. But the Latch-up will suffer if you don't add extra guard rings.
For pad limited design, if VDD and VSS rails are running next to each other, The ESD devices should be put on the same side, and the I/O cell can be made higher and narrower, that means that you can have more pads, but your I/O ring will take more area. Besides, there is no 90 degree turn on major big metal ring. That's good for both ESD and latch-up.

 

[supreet_95]

Power rails

running vgnd lines over sensitive routings is good (though not always as it may load them), but running power lines over them will add noise from power lines and hence should be avoided.

 

[forkschgrad]

Re: Power rails

they are normally routed far from each other.