The i.MX8M family of processors can boot from multiple sources: eMMC, SD card, etc. Our board lacks an SD card slot so this method can be discarded right off the bat. Although booting from eMMC is possible, it would require us to overwrite its persistent storage with a disk image every single time. We will do this at some point in the following sessions but for now the most convenient solution is using SDP to download flash.bin via a serial connection, over USB.

In order to select Serial Download as the preferred method of boot, you will need to set two jumpers on the board. Find J1 and J2 and configure them according to the figure below.

The Universal Update Utility (UUU) is an image deployment tool created by NXP for it's i.MX architecture. For more information about its usage, scripting interface and supported protocols, check out the documentation.

Grab the source code from here and compile *uuu*. The project uses the cmake build system for the sake of portability. If you haven't encountered it yet, follow these setup steps:

# currently in the mfgtools repo root
 
$ mkdir build && cd build
$ cmake ..
$ make -j $(nproc)

First things first, connect the Micro USB and USB-C cables. While the latter will be used to power on the board, the former will expose two Serial Devices to your computer. The one used for console I/O by the bootloaders should appear to you as /dev/ttyUSB0.

Connect to this device using a serial terminal emulation tool of your choice. We recommend picocom. The default baud rate of i.MX devices is 115200 by convention.

Note that nothing will be printed on the console yet, but you need to stay connected to receive the messages that will follow!

Ever since the USB-C cable was plugged in, BL1 has been waiting for the FIP data over serial, thanks to our jumper configuration. Now we can finally provide this data using uuu and it's SDP implementation:

# paths for uuu and flash.bin truncated
$ uuu -b spl flash.bin

About now you should see lots of debug messages via picocom (or whatever serial console tool you prefer). If the last log line is Run fastboot …, send it a Ctrl + C and you should get the u-boot=> prompt.

You might be wondering: but we don't have an OS installed yet… is this all we can do now?

Don't worry, we'll now get to see why u-boot is the most popular choice for embedded devices (here's another fact: most Android phones also use it)!

Now that we finally have access to the interactive shell (we've stopped at BL33), try to run bdinfo for some generic board information. Run help to see what other commands are available to you, and help <command> for detailed information of said command. Note that this may not be an exhaustive list of commands; some may not have been compiled into the final binary, depending on your .config.

Try to perform the following:

• Get the vendor and System on Chip (SOC) name from the environment variables.
• Print the available eMMC devices, as well as their partition tables (if any are available).
• Perform a memory test on the first GB of DRAM. Note that U-Boot relocates itself toward the end of the DRAM bank during its initialization phase (check bdinfo for the exact address). Stay away from that region unless you want to overwrite BL33 itself with your test pattern.