RP2040 boots into the RPI-RP2 volume after brownout

[carledwards]

CircuitPython version

Adafruit CircuitPython 9.1.3 on 2024-08-29; Adafruit Feather RP2040 with rp2040 Board ID:adafruit_feather_rp2040

Code/REPL

import time
import board
import neopixel
import microcontroller

pixel = neopixel.NeoPixel(board.NEOPIXEL, 1)

pixel.brightness = 0.3

print('reset reason: ' + str(microcontroller.cpu.reset_reason))

while True:
    pixel.fill((255, 0, 0))
    time.sleep(0.5)
    pixel.fill((0, 255, 0))
    time.sleep(0.5)
    pixel.fill((0, 0, 255))
    time.sleep(0.5)

Behavior

During normal voltage operation the code above runs as expected. When the power on the USB drops down to ~1v (I am using a solar cell to power the RP2040), and the power is restored, the device boots into the RP2040's bootloader and has the RPI-RP2 volume.

Description

• Power the Adafruit Feather RP2040 via USB from a solar panel and USB battery setup.
• Allow the voltage to drop to approximately ~1v.
• Bring the voltage back up to 3.3v.
• Observe the device reboot into the RPI-RP2.
• Pressing Reset will bring the device back to the CIRCUITPY volume and execute code.py.

Additional information

• danh stated this could be related to: nordic: RunMode.UF2 stopped working in 9.1.0-beta.4 and later #9423
• I also noticed if the power drops to 1.8-2v, and then the voltage returns back to 3.3v, the RP2040 does not respond. It has to drop down below ~1.1 to get the above behavior to show.

I was able to reproduce this issue using a USBC breakout board, removing the input voltage from the host computer, and supply the VCC from a variable power supply. (this also allows me to watch the REPL while testing this)