| Running U-Boot |
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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. On the STM32F4 microcontrollers, U-Boot runs directly from the internal Flash. Volatile data (stack, variables, dynamic pool) are maintained in the internal SRAM. External RAM is not used unless you explicitly call a U-Boot command with an argument pointing to a memory location in SDRAM. As soon as the board is powered on or reset, the STM32F4 proceeds to boot the U-Boot firmware from the internal Flash printing the following output to the serial console: U-Boot 2010.03-cortexm-1.14.2 (Sep 04 2015 - 20:31:28) 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 STM32F4 SOM, the U-Boot environment is stored in the on-module NOR Flash 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:
STM32F4X9-SOM> printenv STM32F4X9-SOM> setenv image vlad/networking.uImage Running setenv <var> will unset (undefine) a specified U-Boot variable. STM32F4X9-SOM> saveenv The autoboot sequence in U-Boot is controlled by the two environment variables called bootdelay and bootcmd. The bootdelay variable defines a delay, in seconds, before running the autoboot command defined by bootcmd. During the bootdelay countdown, you can interrupt the autobooting by pressing any key. This will let you enter the U-Boot command line interface. The bootcmd variable defines a command executed by U-Boot automatically after the bootdelay countdown is over. Typically, this would be run netboot to boot Linux from TFTP during development or run flashboot to boot Linux from the NAND Flash on deployed units. In deployed configurations, where boot time to the service provided by your embedded device is critical, you will probably want to set bootdelay to 0: STM32F4X9-SOM> saveenv This will make sure that on each power on / reset U-Boot immediately executes the command defined by bootcmd, typically booting Linux from the on-module Flash. With bootdelay set to 0 the U-Boot countdown is disabled, so there is a question how you enter the U-Boot command monitor, should you need to enter it for some reason. To do so, push the Ctrl-C keys down and don't release them until you have hit the reset button on the baseboard. This will interrupt the U-Boot bootcmd sequence and let you enter the U-Boot command monitor: U-Boot 2010.03-cortexm-1.14.2 (Sep 04 2015 - 20:31:28)
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