I am attempting to debug a W65C02 and I am using an Arduino Mega to look at the inputs and outputs of the device, which is behaving weird.

I have Pin 22 to 52 connected as INPUT on the MEGA and they are all connected to A0-A15 and D0-D7 on the 6502. However when I unplug my Arduino and keep the 6502 powered on it seems to stay on in that the power light is on. Is something fried or is this normal behavior?

Nothing else is plugged in to the Arduino except GND and the digital inputs.


2 Answers 2


Arduino get the power from the port of 6502 via the protecting diod in the Arduino port. This the normal behavior. // Mikael

  • 5
    It may be "normal" behaviour, but it is far from desired behaviour. Damage to the Arduino can be caused by it as the internal protection diodes get overpowered and blow. Best case: the diodes fail open circuit and you don't notice any difference. Worst case: diode fails short and the pin is permanently HIGH.
    – Majenko
    Apr 13, 2020 at 9:45
  • The best case is bad too, because it means that pin no longer has a protection diode, and can be burned out. This answer should be edited to say "DON'T DO THAT" in very strong terms.
    – Duncan C
    Apr 13, 2020 at 20:48

tl;dr: Don't do that.

As a general rule with CMOS digital circuits you don't want logic pin input voltages to be significantly higher than the VCC power input. (The same is true for TTL for that matter.) Doing that risks damage to the circuit.

That means that if a chip/board is not powered, you should not drive the logic lines at all.

Carlmikael says the behavior you are seeing is normal. I would call it expected, but I would not call it normal. It's "a normal error response" to a circuit being driven in a way that could damage it.

  • I agree, that is " a normal error response" // Mikael
    – Carlmikael
    Apr 13, 2020 at 20:35

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