Dave Platt wrote:

In article ,
Michael A. Terrell wrote:

How do you propose using the programming interface to troubleshoot
something? Are you going to write custom software and reprogram the
processor? What software and interface do you have, and is it
appropriate for the CPU involved? I programmed various embedded
controller boards for four years with JTAG, but there was no way to
troubleshoot the board from that port. Boundary scan is for testing the
CPU, not the rest of the board.

Hunh?

JTAG was originally designed to be a board-checkout feature! It
allows you to chain together any number of digital devices and probe
them, read their input pins, write values to their output pins, etc.

As the Great Source of Dubious Knowledge (Wikipedia) states, "It was
initially devised for testing printed circuit boards using boundary
scan and is still widely used for this application."

Its use as a method of programming the on-board (or external) flash
memory for a microcontroller is only one of its functions... a very
common one but not necessarily its most important.

With many processors, it's possible to access the on-chip debug-and-
trace engine - stop the processor, single-step it, read out the
registers or modify them, etc.

It's very much up to a board designer (and to the designers of the
chips being used) to decide how much functionality is going to be
exposed via JTAG. On some boards, accessing/programming the flash may
be all that's possible. On others, you may be able to individually
interrogate and exercise every single I/O pin on every outward-facing
chip on the whole board.


How is it going to help troubleshoot the analog portions of a board?
Or a faulty on board voltage regulator? Have you personally ever used
it to fix a bad circuit board?


--
You can't fix stupid. You can't even put a Band-Aid™ on it, because it's
Teflon coated.