I have configured GPIO39 (SENSOR_VN) to function as a digital input by calling gpio_config() from the IDF.
When I pull GPIO39 of the ESP-WROOM-32 module low with a 3.3V LVTTL gate it seems to oscillate. Attaching a scope shows a (pure) sine wave of 3Vp-p, 15ns period. Touching the scope lead changes the amplitude. When driven high there is no oscillation.
Clearly it's picking up external noise of some kind. Does this have to do with the 270pf capacitor attached to GPIO39 in the WROOM module?
Testing some other pins shows that they work as normal. It's just GPIO39 that seems unusual. Does it need additional configuration to be used as a digital input?