AN2295 Freescale Semiconductor / Motorola, AN2295 Datasheet - Page 18

• Manufacturer Part Number: AN2295
• Description: Developers Serial Bootloader for M68HC08 and HCS08 MCUs
• Manufacturer: Freescale Semiconductor / Motorola

FC Protocol, Version 1, M68HC908 Implementation
completely dedicated to bootloader code protection, it is unavailable to the user application code. If the
value for FLPBR appears in the user S19 code, a warning is displayed. Such an occurrence should be
omitted from user S19 code.
Some families have the FLASH block protection register stored in RAM instead (the MC68HC908JK/JL
Families are like this). The bootloader sets the correct value at the beginning of its execution to protect
itself. However, user code can modify this register and protect its own memory areas as needed. This also
implies that the bootloader is not 100% protected from user code.
See the specific MCU data sheet for a detailed explanation.
SRSR Register Unusable
The bootloader uses an SRSR register (as described in
User Code
Start) to recognize the source of reset
to determine whether the user code will run. Because the SRSR register is one-time readable (i.e., it is
reset after first read), the user code does not have access to the SRSR value (if the bootloader is present
in the memory and makes the first read after each reset). There’s no simple remedy for this situation. After
the SRSR register is read by the bootloader, it is stored in one RAM location. Unfortunately, its memory
location may differ from one implementation to another. If the application requires the SRSR register and
bootloader, the user must redirect the SRSR reading to this specific RAM location. This location can be
obtained from the bootloader’s MAP file.
Developer’s Serial Bootloader for M68HC08 and HCS08 MCUs, Rev. 6
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