SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
If either checksum is programmed correctly, the Boot ROM will turn control directly over to the Program Flash. This is necessary for product startup in production units. It also provides security for the program, because the Boot ROM cannot be used to read from the Program Flash.
Even in production programs a “backdoor” is often added to the program to enable clearing the checksum. This permits firmware update. Normally on production programs, clearing of the checksum is protected by a customer defined password. Or instead of clearing the checksum, the entire flash is cleared, preventing a third party from examining the program.
This approach works fine for production, since the backdoor is thoroughly tested before the production program is released.
In development, however, untested programs are often installed into the program flash. If the backdoor is not at the very beginning of the program, a program bug could prevent it from working. So there are 2 options to avoid Program Flash Lockup:
More detailed information is available in Chapter 17.