zuf2 is a standalone UF2 USB MSC bootloader for Zephyr/NCS targets.

The nRF54LM20 overlays in boards/ use CPUAPP RRAM like this:

• 0x00000000..0x0003ffff: boot_partition, zuf2
• 0x00040000..0x001f3fff: slot0_partition, chain-loaded app
• 0x001f4000..0x001fcfff: storage_partition

Applications that boot through zuf2 must be built with the same partition layout and CONFIG_USE_DT_CODE_PARTITION=y.

From a Zephyr/NCS workspace with this repository as an application:

west build --no-sysbuild -p always \
  -b <board-target> \
  -d build/zuf2 \
  path/to/zuf2

For the included nRF54LM20 DK overlay:

west build --no-sysbuild -p always \
  -b nrf54lm20dk/nrf54lm20a/cpuapp \
  -d build/zuf2 \
  path/to/zuf2

Build the sample chain-loaded app the same way:

west build --no-sysbuild -p always \
  -b nrf54lm20dk/nrf54lm20a/cpuapp \
  -d build/zuf2_hello \
  path/to/zuf2/samples/hello

Convert the sample app HEX to UF2:

uv run path/to/zuf2/scripts/zuf2conv.py \
  build/zuf2_hello/zephyr/zephyr.hex \
  -f 0x016f65e4 \
  --base 0x40000 \
  --max-size 0x1b4000 \
  -o build/zuf2_hello/zephyr/zuf2_hello.uf2

Flash the bootloader normally:

west flash -d build/zuf2

The board enumerates as a USB MSC drive named ZUF2BOOT when no valid app is present, when sw0 is held during reset on boards that define that alias, or when reset is tapped twice within the double-tap window. Copy a UF2 file to the drive. After all UF2 blocks are received, zuf2 resets and boots the application.

By default, CONFIG_ZUF2_DOUBLE_TAP_RESET=y and the double-tap window is CONFIG_ZUF2_DOUBLE_TAP_DELAY_MS=500. On a board with a reset button, tap reset twice quickly to stay in UF2 mode. Nordic overlays can enable gpregret1 for the retained double-tap marker; boards without a retained-memory device fall back to no-init RAM. If only one reset is seen, zuf2 waits for the window to expire and then boots the application.

The second reset must happen after the bootloader has started, so use a quick double-click gesture with a short beat between presses rather than holding reset or pressing both taps as fast as possible.

To program an app, copy the generated UF2 file to the mounted ZUF2BOOT drive.

For another board, provide a devicetree overlay with:

• boot_partition as /chosen/zephyr,code-partition for the bootloader build
• slot0_partition for the application
• the board USB device controller exposed as zephyr_udc0
• optional sw0 alias for forced bootloader entry

Change CONFIG_ZUF2_FAMILY_ID, CONFIG_ZUF2_MODEL, and CONFIG_ZUF2_BOARD_ID for a real board/product.