Quick Start Summary - Texas Instruments UCD3138 Technical Reference Manual

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17.1 Quick Start Summary

This section is a quick summary of the issues and solutions in Flash security. More detailed explanations
are available in the sections which follow.
17.1.1 ROM Bootstrap and Program Flash Checksum
When the UCD3138 is reset or powered up, control goes first to the ROM bootstrap program. The ROM
bootstrap initializes the device and then performs a checksum on the Program Flash. If the checksum
matches, the ROM turns control over to the customer supplied program in the Program Flash.
If the checksum does not match, control stays in the ROM, and the flash can be reprogrammed.
17.1.2 Firmware Development Setup
The best firmware development setup is to never program the checksum into the program flash.
The disadvantage of this approach is that it requires the use of a PMBus interface every time the
UCD3138 is reset. The program won't start on its own, so the start has to be commanded through the
PMBus interface.
The second best setup permits auto startup. It is:
1. Have an I/O line based backdoor as the first element in the main program.
2. Have an additional backdoor from whatever communication port is in use (PMBus, or serial typically)
3. Only program the checksum into a new program version after the backdoors have been tested and
verified.
PMBus backdoors are convenient, but unreliable. One bug can kill the PMBus function and the backdoor.
A simple backdoor at the very beginning of the program is much more reliable.
17.1.3 Production Setup
The best production setup is:
1. Have a well-tested, secure backdoor which erases program flash. It can be communications port based
only, if desired.
2. Don't program the program flash checksum until all other flash is programmed, including calibration
and manufacturing data.
The next sections cover these issues in much more detail.
17.2 Flash Memory Operations
17.2.1 UCD3138 Memory Maps
The UCD3138 has only one memory bus used for both program and data.
Memories are called "Program" Flash" and "Data" Flash, but all memories can be used for both.
The names just show the main use for each memory.
After reset, the UCD3138 starts executing in ROM. In this mode, the ROM is mapped to fill up the entire
first 64 Kbytes of memory. The 4 Kbytes of ROM are repeated 16 times.
Here is the memory map for ROM mode:
0x00000 => 0x0FFFF => Boot ROM - 4 Kbytes, repeated 16 times.
0x10000 => 0x17FFF => Program Flash - 32 Kbytes
0x18800 => 0x18FFF => Data Flash - 2 Kbytes
0x19000 => 0x19FFF => Data RAM - 4 Kbytes
If the ROM finds a valid checksum for the Program Flash, or if a PMBus command is sent telling the ROM
to transfer control to the flash, the memory map is changed a little bit.
SNIU028A – February 2016 – Revised April 2016
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Flash Memory Programming, Integrity, and Security
Quick Start Summary
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