The most easy and straightforward way of installing a uClinux image from U-Boot is downloading it over the Ethernet interface. If Ethernet is not available for any reason then the UART interface can be used instead. To install uClinux from U-Boot over the UART Interface, please see the steps below:

1. Linux host preparation steps. Install the following packages on your Linux host:
   • picocom - a terminal emulation application;
   • lrzsz - a communication package providing YMODEM file transfer protocols.
2. U-Boot environment settings.
   The default U-Boot environment contains a useful variable update that downloads a Linux kernel image via TFTP and puts it to Flash. We will define a similar variable updatey to download an image using the loady YMODEM U-Boot command:
   • Connect to the board with picocom (use your terminal device instead of ttyS2):

   picocom --send-cmd "sb -vv" -l -b 115200 /dev/ttyS2

   • Define the updatey environment variable:

   SF2-DEV-KIT> setenv updatey loady \${loadaddr}\;run spiprobe\;sf erase \${spiaddr} 0x3F0000\;sf write \${loadaddr} \${spiaddr} \${filesize}\;setenv spisize 0x\${filesize}\;saveenv

   • Verify the new variable:

   SF2-DEV-KIT> print updatey
   updatey=loady ${loadaddr};run spiprobe;sf erase ${spiaddr} 0x3F0000;sf write ${loadaddr} ${spiaddr} ${filesize};setenv spisize 0x${filesize};saveenv
   SF2-DEV-KIT>

   • Define the bootcmd environment variable to boot from Flash:

   SF2-DEV-KIT> setenv bootcmd run flashboot

   • Save the environment to the persistent storage:

   SF2-DEV-KIT> saveenv
   Saving Environment to SPI Flash...
   Erasing SPI flash...Writing to SPI flash...done
   SF2-DEV-KIT>

3. Installing the uClinux image to Flash over the UART interface.
   Before proceeding with the uClinux installation, place the uClinux image on the Linux host where picocom is running. After that:
   • Run the updatey command:

   SF2-DEV-KIT> run updatey
   ## Ready for binary (ymodem) download to 0x0A007FC0 at 115200 bps...

   • At this point, type ^-A and then ^-S. The prompt will appear:

   *** file:

   • Enter the full path to the uClinux image file to start downloading. It will take a few minutes to complete:

   *** file: /tmp/networking.uImage
   sb -vv /tmp/networking.uImage
   Sending: /tmp/networking.uImage
   Ymodem sectors/kbytes sent: 0/ 0kRetry 0: NAK on sector
   Bytes Sent:1345792 BPS:9363
   Sending:
   Ymodem sectors/kbytes sent: 0/ 0k
   Transfer complete
   
   *** exit status: 0
   xyzModem - CRC mode, 10516(SOH)/0(STX)/0(CAN) packets, 4 retries
   ## Total Size = 0x001488c0 = 1345728 Bytes
   
   8192 KiB AT25DF641 at 0:0 is now current device
   Saving Environment to SPI Flash...
   Erasing SPI flash...Writing to SPI flash...done
   SF2-DEV-KIT>

4. Next reset will load the newly installed image from Flash:

SF2-DEV-KIT> reset
resetting ...