The idea of 3.3V and 5.0V is that you use that for the VCC of your logic on the shield.
The ioref is then used as the Arduino-facing side of a dual-voltage buffer/driver chip -- optocoupler, level converter, that kind of thing.
The idea of 3.3V and 5.0V is that you use that for the VCC of your logic on the shield.
The ioref is then used as the Arduino-facing side of a dual-voltage buffer/driver chip -- optocoupler, level converter, that kind of thing.