As explained in Installing U-Boot to the STM32H753i-EVAL, you have to install U-Boot to the STM32H753i-EVAL in order to run Linux (uClinux) on the STM32H753i-EVAL board. U-Boot runs as the primary firmware on each power-on / reset.

U-Boot is probably the most popular firmware monitor for Linux. It is developed and maintained by DENX Software Engineering (www.denx.de). If you need detailed information on any aspects of U‑Boot operation, DENX publishes extensive U‑Boot user documentation at their web site.

As soon as the STM32H753i-EVAL board is powered on or reset, the STM32H753i-EVAL proceeds to boot the U- Boot firmware from the internal Flash printing the following output to the serial console:

U-Boot 2019.04- (Aug 09 2023 - 10:31:06 +0000)

Model: STMicroelectronics STM32H753i-EVAL board
DRAM: 32 MiB
Flash: 2 MiB
MMC: STM32 SDMMC2: 0
Loading Environment from MMC... OK
In: serial@40011000
Out: serial@40011000
Err: serial@40011000
Net: eth0: ethernet@40028000
Hit any key to stop autoboot: 0
STM32H7-EVAL U-Boot >

If you hit any key on the serial console before the number of seconds defined by the U-Boot bootdelay variable has elapsed, you will enter the U-Boot interactive command monitor. From the command monitor you can run U‑Boot commands to examine memory, load an image from Ethernet, boot Linux from a loaded image or perform any other action supported by U-Boot.

U-Boot makes use of the so-called environment variables to define various aspects of the target functionality. On the STM32H753i-EVAL board, the U-Boot environment is stored in the is stored at offset 512k in the SD card and is persistent across power or reset cycles. Parameters defined by the U-Boot environment variables include: target IP address, target MAC address, location in RAM where a Linux bootable image will be loaded, and many others.

To manipulate the U-Boot environment the following commands are used:

• printenv <var> - print the value of the variable <var>. Without arguments, prints all environment variables:

STM32H7-EVAL U-Boot > printenv ipaddr
ipaddr=172.17.0.166
STM32H7-EVAL U-Boot >

• setenv <var><val> - set the variable <var> to the value <val>:

STM32H7-EVAL U-Boot > setenv image my.rootfs.uImage
STM32H7-EVAL U-Boot >

• saveenv - save the up-to-date U-Boot environment, possibly updated using setenv commands, into the SD Card. Running saveenv makes sure that any updates you have made to the U-Boot environment are persistent across power cycles and resets.

STM32H7-EVAL U-Boot > saveenv
Saving Environment to MMC... Writing to MMC(0)... OK
STM32H7-EVAL U-Boot >