ESP32 Forum

FWIW, my designs use GPIO0 and I didn't have any trouble.

GPIO0 works fine here as clock-out, even with a 30cm long programming cable that I initially feared would cause problems.
Have you an extra load connected to it that impairs the waveform?

Interesting,
@mikemoy: Do you use the GPIO0 as input with the external clock or the apll out from the ESP?
my problem (with WROOM module) the IO0 (configured as clock out) did not work, the IO16 or 17 (also clocking out the apll for ethernet) is working fine.
Strange behaviour..
@jamie: I have tried CLK line with jumper resistor or with 10 to 30R and no air wiring but no improvement, no matter what kind of resistor or other things I placed there. Only difference to other pins is the very short stub to the programming interface, but also without any caps.

Interesting,
@mikemoy: Do you use the GPIO0 as input with the external clock or the apll out from the ESP?

Input.

I'm sorry I have to correct myself. Checking the schematics again I see that IO17 is used as clock output, so not IO0. I mixed it up with the ESP32-CAM that generates 20 MHz at IO0 and that is no problem even with the 30cm programming cable.
I guess I had problems with IO0 and therefore used IO17, but don't remember the details anymore.