Overview

This post documents the custom U-Boot binary used in the previous FEL mode guide — the bootloader that exposes the frame’s eMMC as a USB mass storage device on your host machine, allowing you to mount the Android filesystem and install ADB keys.

Build and install sunxi-fel sunxi-tools

Download the binary: u-boot-sunxi-with-spl.bin

What This U-Boot Does

When booted via FEL mode, this custom U-Boot:

  • Initializes the Allwinner A33 SoC and DRAM
  • Boots directly into USB mass storage mode (ums 0 mmc 2)
  • Exposes the frame’s internal eMMC as a plain block device on the host
  • Requires no serial cable, no fastboot, no manual interaction

How to Use It

See the previous post for the full procedure, but in brief:

  1. Short the UPDATE pads on the MS33-V2.0 board while powering on to force FEL mode
  2. Boot this U-Boot via FEL — no flashing required, it runs entirely in SRAM/DRAM: 
    $ sunxi-fel -v uboot u-boot-sunxi-with-spl.bin
  3. The frame’s eMMC appears on your host as a USB mass storage device

Mounting the Userdata Partition and Adding ADB Keys

Once the eMMC is exposed as USB mass storage, you can mount the Android userdata partition and plant your ADB public key:

Mount the Partition

Based on the partition layout from the previous post, the userdata partition is typically /dev/sda1. Create a mount point and mount it:

$ sudo mkdir -p /mnt/frame_userdata
$ sudo mount /dev/sda1 /mnt/frame_userdata

Plant Your ADB Public Key

First, generate an ADB keypair if you don’t have one:

$ mkdir -p ~/.android
$ adb keygen ~/.android/adbkey

Create the ADB keys directory and copy your public key:

$ sudo mkdir -p /mnt/frame_userdata/misc/adb
$ sudo cp ~/.android/adbkey.pub /mnt/frame_userdata/misc/adb/adb_keys

Unmount and Reboot

Safely unmount the partition:

$ sudo umount /mnt/frame_userdata

Power cycle the frame normally — no UPDATE pad shorting needed on reboot. The frame will boot Android and automatically trust your ADB public key.

Verify ADB Connection

After the frame boots, verify that adb devices shows the frame as authorized:

$ adb devices
List of attached devices
<frame-serial>    device

No prompt or fingerprint prompt should appear.

Binary Details

This prebuilt U-Boot binary is based on mainline U-Boot configured for the Allwinner A33 with these key settings:

  • Boot command: ums 0 mmc 2 — boots straight into USB mass storage mode with zero delay
  • No PMIC: Configured for boards without the AXP PMIC
  • USB gadget: Supports USB mass storage gadget mode for block device access
  • Device tree: Customized for the MS33-V2.0 board to properly initialize the eMMC and USB OTG controller

Why This Works Without Fastboot or Serial

  • FEL mode is baked into the Allwinner A33 boot ROM and cannot be disabled by vendor firmware
  • USB mass storage mode in U-Boot gives direct block-device access to the eMMC without needing to boot Android
  • No manufacturer restrictions apply below the Android layer — you’re talking directly to the SoC

Next Steps

With ADB now enabled, you can follow the ImmichFrame Frameo setup guide to sideload the app or continue with any other development workflows.

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