## Before you start

Armbian runs from a USB drive. The factory OpenWrt stays on the eMMC and starts when no USB drive is present.

- Replace only the eMMC `fip` partition with the Armbian U-Boot. Do not open the case.
- Get `u-boot_sdmmc.fip` from the `linux-u-boot-gl-mt2500-*` package.
- Do not overwrite BL2. Do not erase `u-boot-env` or `rf`. They hold data unique to your unit.
- Use stable external power while you write the FIP.

## Installation

1. Write the Armbian image to a USB drive of 8 GB or more.
2. In the factory OpenWrt, back up BL2, `u-boot-env`, `rf` and `fip`. Keep the copies off the device.
3. Copy `u-boot_sdmmc.fip` to the device with SSH. Check its size and SHA256.
4. Write the file to the `fip` partition only. On a v1 unit this is `/dev/mmcblk0p4` (2 MiB). Stop if the layout is different.
5. Run `sync`. Read back the same byte count and compare the SHA256.
6. Insert the USB drive and reboot.

## Serial console

A 3.3 V TTL UART is necessary only for low-level recovery.