JTAG/Boundary Scan

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.

I doubt you will find anyone using a *raw* JTAG i/f in this way. You would need far too much information about the devices involved, their interconnects, etc.

When *designing* a product, careful consideration to the JTAG abilities of the device(s) you use can help bring *up* a system (virgin hardware) by giving you control over nodes that would otherwise be difficult to manipulate. Many MCU's also use JTAG to program onboard memory and/or a crude software debugging interface.

But, unless the manufacturer has specifically designed a tool that hides all of this detail from you and reduces it to a set of indicators (or, perhaps, an interface to software running on a laptop), chances are, it won't be of much practical use (though it is often used to "hack" or "repurpose" existing devices despite the manufacturer's original intent).

Some products will bring JTAG pins out to an (undocumented) connector. Or, they'll be microprobed in production (to save the cost of the connector :> ). Similarly, many products have "free" serial ports that are routed (and often have interesting capabilities! :> )

But, JTAG isn't as nice and semi-standardized as OBD, OBD-II, etc. for automobiles... too many variables to account for.

Sorry.

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?

I see that no one bit on your duplicate post on news:sci.electronics.design

Most designs don't have enough spare flash for any diagnostics. It isn't worth the trouble for anything outside a factory where an automated test fixture can test the entire board without being reprogrammed. Not what you want to hear, but it's the truth. The 'boundary' stops at the I/O pins where it was intended to. It was a way to test a complex processor with limited tools, not an entire board.