RX & TX are still connected to UART0 during the ESP32 boot-up process and will output (and accept input) during the boot-up process.
Once the board has booted and your user code is running, you are free to use them as general IO for other purposes, it’s up to you.
A few things to consider if you are going to use them for general IO etc.
– During boot-up, the RX pin will be active and changing state as console output is being sent to it, so don’t connect it to anything that might get into a bad state from that output.
– Likewise, don’t connect the TX pin to a peripheral that could be sending output at bootup, or you will be sending that to the ESP32 chip during boot, which could cause it to fail to boot.
– Don’t connect them to anything that requires you to add an external pull-up or pull-down resistor as that could affect the boot-up state of the board.
TinyPICO Nano
On the TinyPICO Nano, RX/TX are tied to the SerialUART chip and cannot be used for other purposes even after boot-up is complete and your user code is running.
They are exposed so you can read the serial output from UART0 if you don’t have a USB connector wired up to your board.