Ok, Iāll try that, but one thing I donāt understand. Is the zener diode in SERIES with the input voltage, i.e., is it like this:
Input+ > zener anode āā-> zener cathod > MCU pin 8
??
Is the zener for reverse polarity protection? If so, shouldnāt it be in PARALLEL with the input, rather than in SERIES?
EDIT: This is like going down a rat-hole :(.
I shorted across the zener, but I did that my bench supply voltage dropped to 0V.
So, I did some more checking, and it looks like MCU pin 4 (GND) is shorted to MCU pin 8 (Vcc)!!
EDIT 2: I am about ready to give up on this board. Things are really messed up.
I compared this board to another board (that kind of works, powers up, at least), and:
- I removed the diode completely from this board, and EVEN THEN, the MCU pins 4 and 8 are shorted to ground!!
- Poking around, I found that on this board, the ābottomā end of resistors R1 and R2 are both tied to MCU pin 8, whereas on the other, semi-working board, only R2 is tied to MCU pin 8.
I guess that if there was a complete circuit diagram of the board, maybe thereād be a chance to figure out where the short might be, but w/o a diagram, itās really tough, because the vias run under some chips, etc. and I canāt figure out whatās going where :(!!
EDIT 3: Found this from an old post from relic: