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Number of ground pad sand power pads

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tweethy

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Are the no:eek:f power pads and ground pads in a design equal?If not,why?.
 

soloktanjung

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I think yes because as we add an io/core power pad, we have to also add an io/core ground pad.
 

shobhit

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Hi,

It is not necessary that the power pads & the ground pads are same on a chip.....

After the power is laid down after floorplan, we run IR drop analysis on the power as well as ground and a check is made on
1) How much a VDD is possible to fall at the power pins of a hard macro/ memory
2) How much a bump is possible upto the VSS pins of memory/macro
3) there is limit on both bump in VSS and fall in VDD. The output of IR drop tool is a IR drop map and based on which side the drop/bump is more, we add VDD/VSS pads
to compensate the drop.

Thus the no. of pads can definitely vary. It all depend on the way you build the power/ground mesh...

regards,
Shobhit
 

itsmeteja

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In general for the digital chips, we use the same ground across the IO and core.

You need to instantiate 1vdd_io,1vss pad for every 5 signals having 5:1:1 as the same ground is shared for the supplying the core vss as well.

Also, based on the power consumption and EM limit of the pad we will come up with number of pads for core vdd supply. Rule of thumb for vss is (no of core power pads)/3

So, there may be chances you may have the same count of core vdd pads and #core_vdd/3+vss_io.

If you have vss_io = 2/3 vdd_core in this case you will have the same number of vdd and vss pads.

- - - Updated - - -

In general for the digital chips, we use the same ground across the IO and core.

You need to instantiate 1vdd_io,1vss pad for every 5 signals having 5:1:1 as the same ground is shared for the supplying the core vss as well.

Also, based on the power consumption and EM limit of the pad we will come up with number of pads for core vdd supply. Rule of thumb for vss is (no of core power pads)/3

So, there may be chances you may have the same count of core vdd pads and #core_vdd/3+vss_io.

If you have vss_io = 2/3 vdd_core in this case you will have the same number of vdd and vss pads.
 

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