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[[zero | Radxa Zero]] > [[zero/dev | Development]] > [[zero/dev/u-boot | U-Boot]]

=== Build U-Boot and flash it on Radxa Zero ===

This guide describes how to build U-Boot and flash it on Radxa Zero. U-Boot image can be flashed on eMMC or microSD, and it can be sideload from USB as well.

<font color="#FF0000">WARNING!</font> Flash bootloader incorrectly could result in unbootable system and loss of data. Do not replace bundled bootloader unless you understand the risk.

==== Requirement ====

* Radxa Zero
* USB A to C cable
* A computer running Ubuntu 18.04 or higher

==== Build U-Boot ====

===== Step 1: Get the source code =====

We use git to fetch source code from internet. If you do not have git installed, try following commands:

    sudo apt update
    sudo apt install git

Once you have git installed, you can download the source code:

    git clone -b radxa-zero-v2021.07 https://github.com/radxa/u-boot.git

FIP(Firmware in Package) is a collection of scripts and pre-built binaries (such as first stage loader and so on) for Radxa Zero U-Boot. Get them with following command:

    git clone https://github.com/radxa/fip.git

===== Step 2: Set up build environment =====

Execute following commands to install build dependencies and the tool chain necessary for building.

  sudo apt install bc nano mc build-essential autoconf libtool cmake pkg-config git python python-dev swig libpcre3-dev libnode-dev gawk wget diffstat bison flex device-tree-compiler libncurses5 libncurses5-dev gcc-aarch64-linux-gnu g++-aarch64-linux-gnu binfmt-support qemu-user-static dosfstools libssl-dev mtools
  wget https://developer.arm.com/-/media/Files/downloads/gnu-a/10.3-2021.07/binrel/gcc-arm-10.3-2021.07-x86_64-aarch64-none-elf.tar.xz
  sudo tar xvf gcc-arm-10.3-2021.07-x86_64-aarch64-none-elf.tar.xz -C /opt

===== Step 3: Build U-boot =====

  export CROSS_COMPILE=/opt/gcc-arm-10.2-2020.11-x86_64-aarch64-none-elf/bin/aarch64-none-elf-
  export ARCH=arm
  cd u-boot
  make radxa-zero_defconfig 
  make

You will find <code>u-boot.bin</code> under <code>u-boot</code> folder if everything runs without error.

===== Step 4: Pack U-boot =====

  cp u-boot.bin ../fip/radxa-zero/bl33.bin
  cd ../fip/radxa-zero
  make

This will generate 

    u-boot.bin u-boot.bin.sd.bin u-boot.bin.usb.bl2 u-boot.bin.usb.tpl

==== Run U-boot ====

* Sideloading from USB

When your U-Boot is broken you can sideload a working image to recover an unbootable system. This can also be used to verify the newly generated image is working before flashing it permanently on storage media.

Install the maskrom usb download tool with pip

  sudo apt install pip3
  sudo pip3 install pyamlboot

Press and hold the [https://wiki.radxa.com/mw/images/e/e2/Radxa_zero_ports.jpg USB boot button] on the back of Radxa Zero, and connect Zero to your PC.

You should see following device connected to your system:

  $ lsusb
  Bus 002 Device 030: ID 1b8e:c003 Amlogic, Inc. GX-CHIP

Sideloadng u-boot.bin:

  boot-g12.py /path/to/'''fip/radxa-zero/u-boot.bin'''

If you have serial console attached to Radxa Zero, you will see the boot message from U-Boot.

* Flash U-Boot to microSD

  bin=/path/to/'''fip/radxa-zero/u-boot.bin.sd.bin'''
  sudo dd if=$bin of=/dev/sdx conv=fsync,notrunc bs=1 count=444
  sudo dd if=$bin of=/dev/sdx conv=fsync,notrunc bs=512 skip=1 seek=1

If your Zero has built-in eMMC, we recommend [[Zero/install/eMMC_erase | erase it]] so the microSD's U-Boot will be used.

* Flash U-Boot to eMMC

We use '''fastboot''' to flash U-Boot to eMMC. Unplug microSD card from Zero and press & hold the [https://wiki.radxa.com/mw/images/e/e2/Radxa_zero_ports.jpg usb boot button], then connect Zero to your PC.

  sudo apt install fastboot
  wget https://dl.radxa.com/zero/images/loader/rz-fastboot-loader.bin
  boot-g12.py rz-fastboot-loader.bin
  sudo fastboot flash 0x200 /path/to/'''fip/radxa-zero/u-boot.bin'''
  sudo fastboot reboot