Boot Rom Function - Texas Instruments UCD3138 Technical Reference Manual

Boot ROM Function

13.1 Boot ROM Function
The Boot ROM performs several functions:
• initializes the UCD3138
• examines checksums for boot flash and program flash, and automatically executes either if the
checksum is valid
• If the checksum is not valid, the Boot ROM will permit examination and modification of memory and
peripherals.
13.1.1 Initializing UCD3138
The Boot ROM performs several initializations on the UCD3138. The most important one is memory map
initialization. This is described in
13.1.2 Verifying Checksums
In the UCD3138, the Boot ROM supports 2 checksums in the 32KB Program Flash:
• Boot Flash Checksum at 0x7fc-0x7ff (2KB)
• Program Flash Checksum at 0x7ffc to 0x7fff (32KB)
If either checksum is valid, the ROM will move the Program Flash to location 0, and jump to that location
to start execution of the customer provided program in the Program Flash. The 32K checksum verification
takes about 10 milliseconds to complete. The checksum is a simple additive checksum. Each byte in
memory is treated as a positive 8 bit number and is added to the checksum. A 4 byte checksum is used,
so no truncation occurs. Even if the entire memory is full of 0xff, the checksum value will only be
0x7F7C04. The checksum calculation includes all the bytes in the block except for the last 4, since the
expected checksum is located there.
Before verifying the checksums, the Boot ROM first checks to see if there is a 0xEA in the first location.
This represents a jump opcode. This is a jump to the start of the program. If there is not an EA, the Boot
ROM doesn't check checksums, but goes straight to BOOT ROM communication mode with the PMBus.
This prevents the system from locking up if all of Program Flash is filled with 0.
13.1.3 Uses for 2 Different Checksums
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.
438
Boot ROM and Boot Flash
Chapter 5 and Chapter 6.
Copyright © 2016, Texas Instruments Incorporated
SNIU028A – February 2016 – Revised April 2016
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