Why does chips have multiple gnd's and vins?

Thread Starter

AgentSmithers

Joined Jan 14, 2011
77
Hi Everyone! I am taking a quick look at this chip and I notice its like this with many others. Anyone have any detailed input on why chips in general have multiple gnd outputs and why they just didn't tie it to a single pin?

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Papabravo

Joined Feb 24, 2006
13,741
Convenience. Using a single pin would complicate the layout and fabrication of the chip. This way the current flowing in and out is distributed over a number of pins so the temperature rise can on each pin can be limited. This also prevents power and ground runs from running diagonally across the chip, or worse, around the edge.
 

Thread Starter

AgentSmithers

Joined Jan 14, 2011
77
Convenience. Using a single pin would complicate the layout and fabrication of the chip. This way the current flowing in and out is distributed over a number of pins so the temperature rise can on each pin can be limited. This also prevents power and ground runs from running diagonally across the chip, or worse, around the edge.
Thank you, Running Diagonally would make sense to me as EMI can pose an issue but why the edge of the chip? What is affected in that sense?
 

Papabravo

Joined Feb 24, 2006
13,741
Thank you, Running Diagonally would make sense to me as EMI can pose an issue but why the edge of the chip? What is affected in that sense?
Inductance of a long trace. This is a function of increasing clock speeds. This could also make it difficult to connect other pins to the interior. It also depends on how many metalization layers there are. Maybe somebody who is expert in chip fabrication could opine. I've only ever done a chip design -- never a layout or fabrication.

You might just as well ask: "why don't we drive all clocks from the same source". Why do we use clock trees?
 

sagor

Joined Mar 10, 2019
207
In some cases, it is to also handle the current loads in/out of the chip. A single lead may be limited as to current capacity, whereas 2 or 3 leads at the same level can handle 2 to 3 times more current (sinking or sourcing)
Finally, some chips are newer generations of older chips, and they keep the same pin definitions to keep some pin comnpatibility, but as the newer chip gets bigger, they add more pins for power and ground.
 

ronsimpson

Joined Oct 7, 2019
684
It is common for the oscillator on a micro-computer to have its own ground pins. It is critical the capacitors on the oscillator find a very short path back to a ground pin.
Some micros have a ADC or DAC which might have its own power and ground pins. You don't want currents from digital I/O pins passing through the analog circuits. Analog & digital grounds.
As stated above pins are only good for X amount of current. (bonding wires are very small)
 
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