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To confirm that the board is in UART boot mode, open the UART terminal and confirm that you see the character 'C' getting printed on the console every 2-3 seconds.
The AM263x-LP has default SBL NULL present on the flash. This secondary bootloader is loaded by ROM after identifying a valid x509 certificate. The SBL_NULL boots all the cores and executes WFI. This allows users to connects the debugger to any core and program an application via CCS.
As you have removed the SBL, the ROM is not able to recognize any valid x509 certificate, the cores are not correctly initialized. This is the reason, why you are not able to connect the debugger properly. This is also the reason you are seeing XMODEM prints from ROM with "C" because the X509 parsing has failed in QSPI and ROM could not boot any image hence it is waiting for a valid application via UART.
I suggest you to add a valid SBL (preferably SBL_NULL) if you intend to load the application via CCS.
