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 NXP i.MX RT1050 EVK board, you connect to the target serial console available on the J23 connector on the baseboard. Assuming you connect to a Linux PC host, on the Linux host the i.MX RT1050 serial console will be visible as a /dev/ttyACMx 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 NXP i.MX RT1050 EVK loadaddr is set as follows, placing the download buffer into SDRAM:

=> print loadaddr
loadaddr=0x80007fc0

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 NXP i.MX RT1050 EVK).

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/ttyACM0
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}=> }
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 9.0.305 OPEN SOURCE: Alpha.07, 24 Jan 2022, Ubuntu-22.04.2.myguest.virt 
ualbox.org [10.0.2.15] 
   Current Directory: /home/sasha/rt1050
Communication Device: /dev/ttyACM0
 Communication Speed: 115200 
              Parity: none 
         RTT/Timeout: 01 / 03 
             SENDING: rootfs.uImage => ROOTFS.UIMAGE 
           File Type: BINARY 
           File Size: 8489874
        Percent Done: 56  //////////////////////////// 
...10...20...30...40...50...60...70...80...90..100 
 Estimated Time Left: 00:05:32 
  Transfer Rate, CPS: 8430 
        Window Slots: 1 of 1 
         Packet Type: D 
        Packet Count: 554 
       Packet Length: 9033 
         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:

run addip; bootm 
## Booting kernel from Legacy Image at 80007fc0 ...
   Image Name:   Linux-5.15.71
   Image Type:   ARM Linux Multi-File Image (uncompressed)
   Data Size:    8277398 Bytes = 7.9 MiB
   Load Address: 80008000
   Entry Point:  80008001
   Contents:
      Image 0: 8269856 Bytes = 7.9 MiB
      Image 1: 7530 Bytes = 7.4 KiB
   Verifying Checksum ... OK
## Flattened Device Tree from multi component Image at 80007FC0
   Booting using the fdt at 0x807eb02c
   Loading Multi-File Image
   Loading Device Tree to 81e81000, end 81e85d69 ... OK

Starting kernel ...

Booting Linux on physical CPU 0x0
Linux version 5.15.71 (sasha@workbench.emcraft.com) (arm-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 
Mon Sep 25 08:03:11 UTC 2023
CPU: ARMv7-M [411fc271] revision 1 (ARMv7M), cr=00000000
CPU: PIPT / VIPT nonaliasing data cache, PIPT instruction cache
OF: fdt: Machine model: NXP IMXRT1050-evk board
... 
Run /init as init process
[71] Jan 01 00:00:01 Running in background
/ # mmc0: new high speed SDHC card at address 0007
mmcblk0: mmc0:0007 SD8GB 7.42 GiB
 mmcblk0: p1
Micrel KSZ8081 or KSZ8091 402d8000.ethernet-1:02: attached PHY 
driver (mii_bus:phy_addr=402d8000.ethernet-1:02, irq=POLL)
fec 402d8000.ethernet eth0: Link is Up - 100Mbps/Full - flow control off

/ # ls
bin       hello.ko  lib       proc      sys       var
dev       httpd     linuxrc   root      tmp
etc       init      mnt       sbin      usr 
/ #