what is the V_IO Pin for in a SPI header?
Vio is the supply for external interfaces of a chip.
It may be separate from the core supplymerely for electrical integrity purposes related to having enough supply pins near the load.
But often it is separate because the I/O bank will or could run at a signalling different voltage from the core.
If you intend to communicate via 3v3 signalling, something needs to supply this pin with 3v3.
And like any supply, it needs good bypassing right where it enters the chip.
In your specific case the chip consuming Vio would probably the USB bridge, and it looks like your target board is perhaps taking a supply from that board on one pin and sending it back on another, to basically say &quot;this is the signal voltage I want&quot;
But then you don't seem to have shown the power supply section of the target board, so it's possible there is a source there you are overlooking.