Lpc2000

> Date: Mon, 02 Jan 2006 21:01:04 -0500
> From: Sean <embeddedrelated@...>

> I have seen a lot of talk around about Wiggler-based JTAG
> debugger boards, which can be built for < $1, and I see several
> commercial JTAG debugging pods that are > $1000.  What is it that
> these things do that you can't do with a simple wiggler
> interface?

Warning: Response from JTAG vendor: See
  http://www.mpeforth.com/jtagwidget.htm
which is designed for hardwaare bring-up as well as software 
debugging.

There are three aspects to consider:
  connection to PC host
  connection to ARM target
  software

Host Connection
===============
The host connection in rough order of performance and cost is
  PC parallel port
  USB
  dedicated PCI card
Some parallel port interfaces are powered from the parallel 
port. Without great care in component selection and PC layout 
these can be unreliable. We have several, some work well, some 
don't.

USB interfaces can draw power from the USB port. In general, 
these are the most convenient for site use. But the hardware is 
more expensive and software is required, probably both on the PC 
and on the JTAG adaptor. Software costs.

PCI interfaces will give the highest performance, but are 
usually only found on dedicated testers.

ARM target connection
=====================
As the JTAG speed increases, you need to be more careful with 
drive levels, cable length and cable impedances. We do not use a 
cheap HC244, we use a real level shifter on the JTAG side. It's 
rated to below 1.8 volts. Similarly, the lifetime and cost of a 
good quality 3M connector is much higher than that of a generic 
20 pin connector.

Software
========
Once you move away from the parallel port connection, you'll 
most likely have to write software for the adaptor as well as 
for the PC.

Conclusion
==========
Better (performance, reliability ...) JTAG debuggers require 
more work, which has to be paid for somehow or other. Technical 
support costs time/money too. There's a lot more to ARM JTAG 
debuggers than at first meets the eye.

Stephen

--
Stephen Pelc, stephen@...
MicroProcessor Engineering Ltd - More Real, Less Time
133 Hill Lane, Southampton SO15 5AF, England
tel: +44 23 80 631441, fax: +44 23 80 339691
web: http://www.mpeforth.com - free VFX Forth downloads