Nothing custom except the CPU?
Okay, get yourself an ARM development board WITH jtag, hook up the memories, and figure out how to get the code to run on your board (you may have to write some test code for the board being examined to puzzle out on-chip peripherals). Then poke and prod with the jtag to your heart's content.
If this board is a consumer device (which, judging by other posts in this thread, it is), you may want to consider that even if you found the JTAG port it would probably be useless.
Most "secure" chips include a phase in the manufacturing process during which the JTAG fuses are blown.
If you want to fix that, you'll need an electron microscope and time.... :)
ttyl,
--buddy
Amazingly, this is often not used. Witness the huge number of people selling JTAG dongles for DirectTV decoders :)
The OP can already read the external flash. What would they be protecting?
Sometimes, parallel and/or high voltage programmings can override the fuse.
Something that's not in the _external_ flash...
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