IIRC you push reset while holding down the BOOTSEL button to get into BOOTSEL mode.
In this mode I would imagine that the bootloader only sets up the pins it needs, with the reset remaining in their reset state. That should be input mode or similar.
If there is a spare pin, within your application set it to open drain output at startup, and drive it low.
Create a small circuit comprising an LED driven by an NPN transistor or N-channel FET to this pin; connect a resistor (something around 1K should do) between base/gate and +3.3V so that the LED is normally on. Then connect your Pico pin to the base/gate.
In BOOTSEL mode the LED circuit is essentially disconnected from the Pico, so turns on. In your application, taking the base/gate low turns off the transistor/FET.
In this mode I would imagine that the bootloader only sets up the pins it needs, with the reset remaining in their reset state. That should be input mode or similar.
If there is a spare pin, within your application set it to open drain output at startup, and drive it low.
Create a small circuit comprising an LED driven by an NPN transistor or N-channel FET to this pin; connect a resistor (something around 1K should do) between base/gate and +3.3V so that the LED is normally on. Then connect your Pico pin to the base/gate.
In BOOTSEL mode the LED circuit is essentially disconnected from the Pico, so turns on. In your application, taking the base/gate low turns off the transistor/FET.
Statistics: Posted by stevend — Fri Nov 21, 2025 6:07 pm