Granted most MCU's are configured such that to 'activate' a device on a logic high (take the simple case of an LED, or a relay or transistor for something higher power) the MCU is sourcing VCC and the other end of the component is pinned to ground completing my circuit.
My question is what would happen instead if the circuit was say, configured another way ? For example, a complete circuit from an external VCC, running through an output GPIO pin, back out of that pin and then to the load and from there to ground of the external VCC. If a logic low output were asserted on the pin would it act as a 'switch' breaking the circuit ? Or would ext VCC simply overload and override the low logic level keeping the short.
Asked, I guess in another way, can GPIO on most MCU be used as a 'switch' in this inverted way ? Presume external VCC is of the same domain as VCC of the MCU.