Re: JTAG question

Sagaert Johan wrote: 
Hi

Thanks for the info

I assume that through the JTAG some external flash can also be programmed
that has been wired to an ARM device.



The JTAG is actually a back door into the processor
core. From its standpoint the Flash chip is an
external device.

A complete JTAG cell is able to drive the external
lines of the processor chip, but the ARM implementation
saves a latch and multiplexer in the cell, and this
makes it very difficult, if not impossible, to run the
external pins from the JTAG directly.

The rescue is to use the JTAG to load Flash writer
code to the system RAM, along with the data to
write to the Flash, and start the writer using the
JTAG interface. Often the RAM is not large enough,
so the Flash image has to be loaded in pieces.

The Flash loader code is very dependent on the
memory address space layout in the system and
on the Flash chip type, so it usually has to be
tailored for the target system.

Been there - done that.

--

Tauno Voipio
tauno voipio (at) iki fi
.

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