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 designs that do not provide an Ethernet port it may be the only reasonable development and software manufacturing option using U-Boot and uClinux.

Using the STM32H7 SOM board, you connect to the target serial console available on the P1 connector on the baseboard. Assuming you connect to a Linux PC host, on the Linux host the STM32H7 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 STM32H7 SOM loadaddr is set as follows, placing the download buffer into SDRAM:

STM32H7-SOM U-Boot >print loadaddr
loadaddr=0xD0C00000

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 STM32H7 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 Linux uImage 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:

$ vi uartboot.script
#!/usr/bin/kermit

set port /dev/ttyUSB0
set speed 115200
set carrier-watch off
set flow-control none
set prefixing all

echo {loading uImage}
PAUSE 1

OUTPUT loadb ${loadaddr} 115200\{13}
send rootfs.uImage
INPUT 180 {\{13}\{10}STM32H7-SOM U-Boot > }
IF FAIL STOP 1 INPUT timeout

echo {running kernel}
PAUSE 1
OUTPUT run addip; bootm\{13}
c

Copy the sample Linux image (rootfs.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:

$ chmod a+x uartboot.script
$ ./uartboot.script
C-Kermit 9.0.302 OPEN SOURCE:, 20 Aug 2011, workbench.emcraft.com [172.17.1.237]

   Current Directory: /home/sasha/2.5.0/linux-cm-stm32h7-2.5.0/projects/rootfs
Communication Device: /dev/ttyUSB0
 Communication Speed: 115200
              Parity: none
         RTT/Timeout: 01 / 03
             SENDING: rootfs.uImage => ROOTFS.UIMAGE
           File Type: BINARY
           File Size: 6447194
        Percent Done: 2   /
                          ...10...20...30...40...50...60...70...80...90..100
 Estimated Time Left: 00:11:53
  Transfer Rate, CPS: 8786
        Window Slots: 1 of 1
         Packet Type: D
        Packet Count: 31
       Packet Length: 9024
         Error Count: 0
          Last Error:
        Last Message:
X to cancel file, Z to cancel group,  <CR> to resend last packet,
E to send Error packet, ^C to quit immediately, ^L to refresh screen. 

It will take 15+ long minutes to download the image at 115.2Kps but finally it will get to the target and Linux will boot from it:

loading uImage
## Total Size      = 0x0062605a = 6447194 Bytes
## Start Addr      = 0xD0C00000
STM32H7-SOM U-Boot > running kernel
Connecting to /dev/ttyUSB0, speed 115200
 Escape character: Ctrl-\ (ASCII 28, FS): enabled
Type the escape character followed by C to get back,
or followed by ? to see other options.
----------------------------------------------------
run addip; bootm
## Booting kernel from Legacy Image at d0c00000 ...
   Image Name:   Linux-5.15.67
   Image Type:   ARM Linux Multi-File Image (uncompressed)
   Data Size:    6447130 Bytes = 6.1 MiB
   Load Address: d0008000
   Entry Point:  d0008001
   Contents:
      Image 0: 6432192 Bytes = 6.1 MiB
      Image 1: 14926 Bytes = 14.6 KiB
   Verifying Checksum ... ge at D0C00000
   Booting using the fdt at 0xd122260c
   Loading Multi-File Image ... -none-eabi-gcc (GNU Arm Embedded Toolchain 
10.3-2021.10) 10.3.1 20210824 (release), GNU ld (GNU Arm Embedded Toolchain 
10.3-2021.10) 2.36.1.20210621) #2 PREEMPT Fri Jul 7 11:02:11 UTC 2023
[    0.000000] CPU: ARMv7-M [411fc271] revision 1 oring loglevel setting.
.....
[    2.281231]     ip=172.17.0.66:172.17.0.1::::eth0:off
init started: BusyBox v1.24.2 (2023-07-07 11:01:57 UTC)
/ # HY [stmmac-0:01] driver [SMSC LAN8710/LAN8720] (irq=POLL)
[    2.535692] stm32-dwmac 40028000.ethernet eth0: Register MEM_TYPE_PAGE_POOL 
RxQ-0
[    2.555368] stm3208 Advanced Timestamp supported
[    2.570812] stm32-dwmac 40028000.ethernet eth0: registered PTP clock
[    2.578373] stm32-dwmac 40028000.ethernet eth0: configuring for phy/rmii link 
mode
[    4.648819] stm32-dwmac 40028000.ethernet eth0: Link is Up - 100Mbps/Full
 - flow control rx/tx
[    4.656248] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
/ # ls
bin       etc       httpd     lib       proc      sbin      tmp       var
dev       hello.ko  init      mnt       root      sys       usr
/ #