Electronic – Where do the return currents go for a MCU with multiple Vdd and grounds

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I am reading this article on mixed signal design: https://www.maximintegrated.com/en/app-notes/index.mvp/id/5450

When signals have really high speed, the return current will tend to follow right under the signal traces. Let's say a MCU has a SPI module. And power inside MCU is supplying the SPI module. The SPI module is then sourcing currents to an IC through signal trace A.

The return currents from the IC will try to follow right under trace A and go into the SPI module and then go into the power that supplying the SPI module.

The process is shown in the following diagram:
Sourcing currents and return currents

My question is: if a MCU has multiple Vdds, which power pin does the return current head to? In the following diagram, both pin 19 and pin 48 are Vdd for MCU. Which power pin supplies current for the SPI module?

I ask this question because I think it is highly related with PCB Layouts: where the return currents really go.

enter image description here

Best Answer

Doesn't matter.

Every Vdd will have a decoupling cap.

So as your return current follows your signal, it will internally go to whichever power pin has the least impedance, and then go to ground through again, the path of least impedance, which is most likely through the decoupling caps.

So knowing which power pin, doesn't matter. Have decoupling caps, and a ground plane, and the currents will do the rest.