Unexpected reset pin behavior after flashing feather\_nrf52840\_express bootloader on SuperMini NRF52840

[linkua]

Hi,

My hardware is SuperMini NRF52840, which is hardware-compatible with nice!nano.

• When I flash nice_nano_bootloader-0.9.2_s140_6.1.1.zip from the releases, everything works fine — the board boots normally and shows up as a USB drive on my computer.
• Out of curiosity, I tried flashing feather_nrf52840_express_bootloader-0.9.2_s140_6.1.1.zip with the following command:

adafruit-nrfutil --verbose dfu serial \
  --package feather_nrf52840_express_bootloader-0.9.2_s140_6.1.1.zip \
  -p /dev/ttyACM0 -b 115200 --singlebank --touch 1200

After flashing and rebooting:

• The board did not start normally.

• Measuring with a multimeter, I found:

  • RESET pin: 0V
  • VDD: 1.8V

I then tried pulling up the RESET pin:

• With a 10k resistor to 3.3V, RESET stayed at 0V, still could not boot.

• With two 470Ω resistors in parallel, the board booted successfully, and the computer showed the USB drive.

  • INFO_UF2.TXT shows it is running the nrf52840_express_bootloader.
  • At this point, the RESET pin measured 1.8V.

I also checked the feather_nrf52840_express schematic, and it seems the RESET pin doesn’t have a pull-up resistor there either.

👉 Could anyone explain what is causing this phenomenon?
Thanks a lot! 🙏