Well, I took a different approach. Microchip is having the bootloader
reprogram the reset vector and the interrupt vector table. As I recall,
the 24H series lets you write protect a boot section that starts at 0. So,
if your bootloader is going to write stuff down at 0, besides overwriting
the reset vector (sounds like a bad idea to me), it could overwrite the
bootloader itself, since you will not be able to write protect that.

What I did is to put the bootloader from 0x200 to 0x7ff. There is a goto
the start of my application at 0x800. Right above that is a jump table for
all the interrputs, whether I'm using them or not. The reset vector points
to my bootloader, which checks a switch. If the switch is not closed, we
go to 0x800, then to the start of the application. Everything below 0x800
is write protected.

The bootloader I currently have in two products using the 24H does
bootloading over Ethernet. Using an HTTP POST, the hex file for the
application (addresses 0x800 and above) is sent to the PIC. It is parsed
by the PIC and stored in external SPI flash. After the whole file is
received without error, the bootload code in the boot section is called.
Note that when the boot section is write protected, only the first few
addresses (I think it's 32) in the section can be reached by a goto from
another section. So, there's a goto in these early addresses that points
to the actual code in the boot section. Also, interrupts are strange when
executing in a secure segment, so I disable them. Anyway, once I have the
code in SPI flash, I copy it to PIC flash using the code in the boot
section of the PIC. In the unlikely event that this fails (perhaps bad
code sent to the SPI flash), the user can hold a switch on power-up that
copies the original code from SPI flash back to the PIC. During
production, the PIC code that was loaded is copied out to SPI flash.

The bootloader is just another module in the application instead of being
a separate project.

Harold



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