Does the user code contains checksum in internal flash without intervention of user?
I read the below statement in LPC-1769 user manual:
When there is no valid user code (determined by the checksum word) in the user flash or
the ISP enable pin (P2.10) is pulled low on startup, the ISP mode will be entered and the
boot code will setup the PLL with the IRC.
After the chip enters ISP mode, it will check the checksum in the image. But based on image protocol, the checksum value is computed with only first seven 32 bits words rather than all the data in binary file. So MCUXpresso tools computes it in the *.ld file automatically and insert the checksum in vector table.
If you want to compute the checksum of all data in a binary file, pls refer to the link as Mr Con Verse points out.
Hope it can help you