Boot Process; U-Boot Environment Variables - Congatec conga-SMX8-Mini Quick Start Manual

2 Software
2.1 Starting Up
The conga-SMX8-Mini uses U-Boot as standard bootloader. The bootloader
is GNU GPL open source software. A serial terminal connection is required
in order to display the boot process and to modify the boot behavior. The
boot behavior is controlled via environment variables.
The included RS232 adapter cable has two connectors. The RX/TX signals
are already crossed. Therefore, do NOT use a crossover-cable. Use the
connector labeled "CONSOLE" for the U-Boot console output.
To establish a terminal connection, a terminal program such as TeraTerm
or Putty can be used.
Use the following communication parameters:
Baud rate: 115200
Data: 8 bit
Parity: none
Stop: 1 bit
Flow control: none
The following console output will be displayed when the system is powered
on:
Copyright © 2019 congatec AG
2.2

Boot Process

The conga-SMX8-Mini boot process starts at Power On Reset (POR), where
the hardware reset logic forces the ARM core to begin execution. The
on-chip boot ROM loads the bootloader.
After loading, the bootloader is executed and performs basic system
initialization (serial console, etc.). Afterwards, the environment settings are
parsed and the system boot continues as specified.
Press any key during startup to stop autoboot and to get to U-Boot console.
At the U-Boot console, the environment settings can be displayed using
the "print" command. In addition, useful functionality is available (such as
memory dump, access to the SPI and the I2C system, etc.). The "help"
command will display any command supported by the U-Boot.
If autoboot is not interrupted by pressing a key, the boot process goes
ahead and the module will boot the Linux operating system that is installed
on the SD card.
2.3

U-Boot Environment Variables

The U-Boot environment is located in SPI Flash (in SD card for prototypes).
One of the benefits of the U-Boot bootloader is the possibility to specify its
run time configuration using environment variables.
The environment variables of U-Boot can be displayed using the printenv
(or the print) command.
During the boot process, the bootloader evaluates the "bootcmd" variable
and executes it. The boot command tries to load a bootscript or a kernel
from the boot device. If this is successful, the script or kernel will be started,
otherwise a fallback to network boot is performed. The variable "mmcdev"
specifies the mmc boot device. Furthermore, the variable "mmcroot" is
passed to the kernel in order to specify the location of the root filesystem.
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