Re: [iPAQ] JTAG who?

On Wed, 27 Feb 2002 09:37:20 +0100
"jrlpop@mail.portland.co.uk" <jrlpop@mail.portland.co.uk> wrote:
> Sorry for this slight OT question: I'm not a newbie and I have lots of
> wierd knowledge I'll never get any use of, but I never heard of JTAG.
> Anyone with the knowledge please give a short summary for the curious.
> Is it a bus? A protocol? On what level?

It is something of both ;)
JTAG is some kind of a standard for so called boundary scan interfaces,
i.e. hardware debugging tools. Concerning a CPU you can while the system
is up and running halt the CPU and inspect mostly everything, i.e. all
registers and memory. You can even modify them.
For the iPaq and some other systems, the JTAG interface can be used to
reprogram the onboard flash chips. They are only accessible via the system
bus and using JTAG you can halt the system, allocate the bus and toggle
the appropriate pins the flash is connected to. After that you can cause a
reset via JTAG and the system should reboot with its new flash contents.
So far for the theory ;)
JTAG is not THAT standardised. Every CPU and chip manufacturer adds or
removes features and so the protocol has to be adapted for almost all
chips.
That's also the reason why you have to buy very expensive additional
libraries for expensive JTAG hardware.
For the SA1100 the case is twofold, one side better the other worse. The
worse thing is that the SA1100 does not support proper boundary scan, only
a very limited subset. That's the reason why most commercial JTAG
interfaces you can buy do not support the SA1100.
The good thing is it still has the JTAG and it can be used for flash
programming. But you have to fiddle that out by hand. There are tools to
reprogram the flashes via a parallel port JTAG interface on the Assabet
board, the LART and the Screenphone. And if you accidentially burn rubbish
into your iPaq's flashes then JTAG can also be used here. But the
interface is not accesible from outside the iPaq and even only hard to
find inside. The soft- and hardwaretools for that are also yet unknown to
the public. And for another example I have another StrongARM110 board here
which I was also not yet able to program or readout using the existing
tools :(

Buttomline: JTAG is not for beginners "Don't try this at home, kids!"
It's for very low level development involving some hardware knowledge and
skills.

CU
  nils faerber

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