|Loading Linux Images over UART|
This application note explains how to load images to the target over UART in U-Boot. Keep in mind that loading via a serial port will take quite a long time (minutes per file!) due to the low speeds (limited to 115.2 Kps). Even if Ethernet is not required in embedded product, which is frequently the case for microcontroller applications, we always recommend to our customers that the RMII interface be brought out to a header or a set of test points on embedded board to allow connecting to Ethernet during development phases of the project. That having been said, in certain situations, when Ethernet is not available for whatever reasons, loading images over UART may be the only development and software manufacturing option.
Using the STM32F4 SOM kit, you connect to the target serial console via the USB/UART interface on the SOM-BSB-EXT baseboard. Assuming you connect to a Linux PC host, on the Linux host the STM32F4 serial console will be visible as a /dev/ttyUSBx device. The U-Boot and Linux software configures the console for 115.2 Kps operation.
There are various serial communication tools available for Linux, the most popular perhaps being kermit. kermit is a very powerful tool that can be run in interactive mode or controlled by a script. There is abundant documentation on kermit available in the Internet, if you require details.
The download procedure is based on the loadb command provided by the U-Boot command interface. loadb implements a download capability over UART using the kermit protocol and has the following synopsis:
loadb [<load_address> <baud_rate>]
If you do not specify a load address, then the value will be taken from the loadaddr environment variable. On the STM32F4 SOM loadaddr is set as follows, placing the download buffer into SDRAM:
STM32F4X9-SOM> print loadaddr
If you do not specify a baud rate, then the speed the console is currently running at will be used (set to a default value of 115200 on the STM32F4 SOM).
Once the transmission using loadb finishes, the file will be in memory at the specified load address. The loadaddr environment variable will automatically be set to the address the loadb command used. The filesize environment variable will automatically be set to the number of bytes transferred during the load operation.
Then you are free to do whatever you like with the loaded image. You can boot Linux from the image (assuming it is a bootable Linux file), copy it to some other place (for instance, on-module Flash), display the memory, etc.
To automate the download procedure, you might want to put a desired sequence of interactive steps involving interactions with the U-Boot command interface on the target and kermit on the host into a shell script. For instance, here is a sample script to download a Linux bootable image (rootfs.uImage) to SDRAM and boot Linux from it:
[vlad@yota ~]$vi uartboot-stm32f429.script
Make the script file executable:
[vlad@yota ~]$ chmod a+x ./uartboot-stm32f429.script
Copy the sample Linux image (rootfs.uImage) from the Emcraft software distribution to the host directory you will be running the shell script from.
Make sure that U-Boot console is active, and waiting for the commands input:
Then run the script to download the image to the target via UART and boot Linux from it:
[vlad@yota ~]$ ./uartboot-stm32f429.script
It will take a 5+ long minutes to download the image at 115.2Kps but finally it will get to the target and Linux will boot from it:
## Total Size = 0x0049276F = 4794223 Bytes
To exit from Kermit and release your terminal, press Ctrl-\, then Q.