I am working on a board which is user-insertable into a DIP socket. Incorrect insertion is always a concern, and in this specific case insertion "shifted-by-one-pin" is especially worrying.
What then happens is that a 3V3 digital logic output line, which normally goes from an FPGA to a TTL input in the socket, instead gets connected to 12V power supply. Once the power is on, this immediately fries the FPGA and renders the board useless.
Given the geometry of the situation, purely mechanical misalignment prevention is going to be difficult to say the least, making it even more important to have a protection circuit in place. Now, I looked into Zeners, TVS diodes, signal diodes in series with the signal line, etc, but couldn't really find a solution that would combine these three items:
- Pass ~1 MHz digital signal reliably from the FPGA to the TTL input.
- Have the reaction time short enough to prevent FPGA from getting damaged (EPM1270, so basically anything above 4V is unacceptable).
- Be able to withstand sustained over-voltage for at least several seconds.
Any suggestions where to look next?
Update: I am choosing my answer, because it works for my case, but since it has its caveats, other responses may be better depending on circumstances.