Here is my problem.  Using the ARM's hardware debugger that is accessed
through JTAG is very useful and worth demonstrating.  Historically
the JTAG hardware needed cost thousands of dollars.  Way beyond many
companys and certainly most individuals hobby projects.  But that
changed with openocd and parallel port wigglers like the macraigor
wiggler.  Well parallel ports went away but ftdi makes all kinds of
usb to whatever chips.  There are a number of jtag boards that are
under $100, but I feel that is too pricey, not something I can talk
most folks into after spending halft that on the development board.

The amontek jtag-tiny is good, problem with it is the shipping cost
to the USA if you live in the usa.  The signalyzer lite was good but
I may have bought the last one from microcontrollershop.com and was
told they are discontinued.  I am still on the hunt for a good solution
with decent shipping prices, etc.

Now I am a fan of the msp430, and with the launchpad (ti.com/launchpad)
at $4.30 and free shipping you almost have to just get a few to try.
So it is the first solution here.  But there will be others, although it
is $4.30 you need a serial interface which is around $15.  Arduinos
will work if you get a 3.3v one (NOT THE TYPICAL ARDUINO WHICH IS 5V)
and can do serial through the usb port.  The mbed and stm32f4 discovery
and many others will work as well.  Just needs a serial port, needs to
be 3.3v and needs 6 I/O lines (5 out, 1 in).

On the raspberry pi side unfortunately some soldering is required, they
didnt put all the signals we need on the P1 jumper.  See the armjtag
directory in this repository.  And not trivial soldering, so that may
end your curiosity with jtag and the raspberry pi.  It is only one
signal though, all of the rest you can do with jumper wires that
require no soldering.

So openocd http://openocd.sf.net is the software that knows the ARM
jtag protocol for getting at the hardware debugger.  It is open source.
There are/were parallel port solutions basically bit banging the jtag
through the parallel port.  It is very easy to take over one file
in the openocd sources and do whatever you want so long as you can
find something to bit bang a number of signals with.  What I did is
very simple.  Instead of writing from the openocd software directly
to some gpio/parallel port on something, I take the bits being
written and send them out a serial port (to some microcontroller for
example that takes the byte and hits a port with it.  For reading the
one input (tdo), I write a 0xFF and that tells the microcontroller to
read and send back a character with the bit of interest.  It is that
simple.  And it works, really slow but it is functional.

Until I find the ultimate solution I may from time to time add boards
to this directory.  The first board is the msp430 launchpad
http://ti.com/launchpad.  See the msplaunchpad directory.

BEWARE, you are taking chances with your raspberry pi, I will not be
responsible for the damage done.  You should use the same computer for
all the things connected via usb so that you are sure they grounds are
all the same, going from one computer to another you might have a
problem.  Also, real jtag boards have some electronics that protect
you from these differences as well as protect you from say using 3.3v
on 1.8v I/O lines or allow 5V systems to use 5Volts, etc.

UPDATE, I have a couple of lower cost solutions, first for generic
serial or serial to the msp430

http://jim.sh/ftx/

Looking at the documentation on that page I took the ftdi_sio kernel
sources and added the ftx part manually, was pretty easy, the built
the kernel image and used it.  Basically my linux is not the latest
greatest-cutting edge ubuntu, so it doesnt have the ftdi mods.  All you
are really doing when you modify the driver is adding yet another
ftdi part to the list of devices supported by that driver, many places
where it says if this ftdi part or that ftdi part, or that ftdi part,
you are just adding another to the list.

The possibly more interesting thing is that I have taken an ftdi ft232rl
board

http://www.sparkfun.com/products/718

And using the abitility to bit bang the pins on that device.  It is no
longer a serial port, while being bitbanged, the ft232rl does not have
enough stuff in it to do both.  So dont hook up the rxd and txd to
your raspi serial port, if you have a second one sure, but the jtag
one is only jtag.

Now it is not super fast, pretty slow in fact, but it is $15 plus some
wire.

Stay tuned, when I solder some pins on an ftx board I am going to try
to bit bang it and use it for jtag, resulting in a sub $10 jtag board.

Also I ordered a $18, supposedly jlink compatible board from china, it
may take a while to arrive.

Eventually you are going to want a faster and designed for jtag board
like the amontec boards or the flyswatter from tin can tools (do not
hook that serial port up to your raspberry pi you will fry the rasberry
pi) or others.