@shexplorer, connect it to the internal serial port using a USB-to-serial adapter. If you don’t have one, they’re very cheap—search for CH340G on Amazon.
Quick safety guide on serial adapters:
1 - Your device has 3.3v serial , check the jumper on the CH340G to make sure its on 3.3v and not 5v
2 - DO NOT CONNECT VCC/3.3v/5v Pin!!!
Only connect:
ground
RX
TX
3 - Connect ground from to ground from device
Connect RX from usb to TX from device
Connect TX from usb to RX from device
Let us know if you are having issues with the bound rate or the pinout on your serial header in your device (send us a picture, in that case)
Once you get serial, you should see the boot looping endlessly, holding control + c should stop the loop.
Then you get an uboot console.
there you can chainload your usb or external mmc uboot.
With something similar to(ask chatgpt to help you in case of issues):
usb start # starts usb devices
usb tree # lists devices and partitions
usb info # shows USB device info
ls usb 0:1 / # lists files on partition 1 of usb 0
fatload usb 0:1 0x82000000 u-boot.bin
# 0x82000000 is an example that may work. if it does not, ask chatgpt for other values
# u-boot.bin is the usb uboot you want to chanload, might have another name.... ls usb 0:1 will help you find other files
#
go 0x82000000 # chainloads what you loaded into 0x82000000 memory address. if you change this on the previous line, change it here also.
Besides trying to chainload uboot, you can also load linux kernel + initrd + dbt files directly. Again, chatgpt will help you with those commands. But i found chainloading usb uboot easier.
It you reach linux console, remember that once you reboot, everything will loop again. So you need to fix what you did to the emmc before rebooting.
You have 3 options to fix:
1 - Restore your full backup. -> you get manufactorer android back
2 - Restore 10MB of your backup or johlnx's backup -> you should be able to boot again from usb or external mmc
3 - You can calculate exacly what you need to copy, to maintain your emmc linux but also recover uboot.
There should be some space between the partition table data and the 1st partition.
Lets imagine, GPT uses 1MB and 1st partition starts at 10MB, you can copy the data between 1MB and 10MB from your backup into the space between 1MB and 10MB of your EMMC. (this is not trivial to do, but again chatgpt can help)
This way, you still have a linux on the EMMC, but you have recovered the Uboot env.