|Loading Linux Images over UART|
This article 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). That said, for embedded targets that do not provide an Ethernet port it may be the only reasonable development and software manufacturing option using U-Boot and uClinux.
Please refer to Connecting Serial Console to the STM32F429 Discovery for information on how to connect to the UART console and use kermit for serial line communication to the target.
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 STM32F429 Discover loadaddr is set as follows, placing the download buffer into SDRAM:
STM32F429-DISCO> 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 STM32F429 Discovery).
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 Linux uImage file), copy it to some other place (for instance, embedded Flash), display the memory, etc.
To automate the download procedure, you might want to put a desired sequence of interactive steps involving intreractions 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 (uImage) to SDRAM and boot Linux from it:
$ vi uartboot-stm32f429.script
Change the file mode to make the script an executable file:
$ chmod +x uartboot-stm32f429.script
Copy the sample Linux image (networking.uImage) from the Emcraft software distribution to the host directory you will be running the shell script from.
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 3 long mintues to download the image at 115.2Kps but finally it will get to the target and Linux will boot from it:
## Total Size = 0x00184ba0 = 1592224 Bytes
To exit kermit and free up the serial console, type Ctrl-\ and then q.