SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
The Program Flash Checksum at address 0x7FFC is used for cases where field upgradeability of the Program Flash is not desired, or where field upgradeability can be done with simple PMBus interface using the ROM. It is checked after reset to verify that there is a valid program in the flash before flash execution is started.
The Boot Flash Checksum at address 0x07FC provides more sophisticated field upgradeability. It is useful for cases which require an upgrade:
All of these tasks can actually be accomplished with the Program Flash checksum as well – so long as the upgrade is not interrupted. If the upgrade is interrupted before it is complete, the Program Flash checksum will not be correct, and the Boot ROM will not pass control to the Program Flash.
With the Boot ROM, only the first 2K of the program flash must be correct. The customer program in the Boot Flash area has the responsibility for verifying the integrity of the other 30K of the Program Flash, which can contain the operating program for the power supply.
If the upgrade is interrupted, the Boot Flash area will still have a valid checksum, and the Boot Flash program must be written to recover and request a new download.
Since the Boot Flash program is in Program Flash, the user of the chip is free to program it as desired.