GPIO behaviour when shut down

[dloeckx]

What is the behaviour of the GPIO pins when the device is shut down (by keeping the EN pin low and/or providing insufficient power)?

Use case: I want to use an ESP32 powered by two supercaps in series, charged by a solar sell. The ESP32 is powered by a buck convertor converting the supercaps voltage down to 3.3V, and the EN pin is used to ensure the ESP32 is turned on only when there is enough power in the supercaps. ADC1_CHANNEL_7 (GPIO 35) and ADC1_CHANNEL_6 (GPIO 34) are used to monitor the solar cell and supercaps voltage (with a voltage divider of two 100kΩ resistors). When the ESP32 is in deep sleep, everything works as expected, after adding rtc_gpio_isolate to isolate the GPIO pins during deep sleep. However, when trying to start from a cold start, I have the impression there is a significant leakage from the supercaps and/or solar cell to ground through the ADC convertors.

I couldn't find information about the behaviour of the GPIO pins when the ESP32 is powered down. Or, more specifically, when a voltage between 0V and 3.3V is fed to the ESP32 and the EN pin is kept low.