I'm pretty new to the MCU world and whilst I'm coping OK with the software side of things my very basic understanding of electronics is making my vehicle CANBus based project slow going. Still thanks to the great resources here I'm amazed at what I have been able to achieve.

I've managed to design and have successfully tested a pcb that hosts the mbed 1768, some can controllers and a power supply amongst other things that my project requires. I'm now working on version 2 that adds some extra components and will be tested by a few more people. During the design process I noticed a potential problem and I'm wondering if there is a way to protect against it.

Finally to my question:

My project exposes the 3.3V Vout to a connector so that it can be used as a reference for some analogue sensors. On the same connector and on a neighbouring pin I have the 12V in to power the project. My concern is that if somebody (like me) accidently connects the 12V to the Vout pin then I'm assuming the mbed would be toast.

Is there a simple circuit that I could incorporate that would protect them mbed in this scenario?

Google would seem to suggest that it's more than just 1 diode that I need but I'm struggling to make sense of what google has suggested.

Protecting just the mbed from accidentally connecting 12V to the 3.3V output is just a single diode in series with the Vout pin. The down side is that your 3.3V output will then be reduced by a diode drop (about 0.2V for a schottky diode).

This wouldn't protect anything outside of the mbed that is using that 3.3V as a reference, that would still get hit by 12V. Protecting everything else would get more complicated, you would need to either clamp that power rail so it can't go over 3.3V or isolate it with some FETs until the connector was correctly attached and 12V was seen on the correct pin.