Lpc2000

I've been doing some more research on adding the nTRST signal to the
DIY wiggler. 

http://www.macraigor.com/downloads/pinouts.pdf
http://sourceforge.net/tracker/index.php?func=detail&aid=799377&group_id=52603&atid=469852

According to the above links, it looks like the genuine wiggler
actually already has support for driving the nTRST JTAG signal from
the parallel port's DATA4 pin. 

The existing DIY wiggler clone schematics have no connection of DATA4
to nTRST like the real wiggler appearantly has. Could this be why DIY
wiggler clones flake out on people? 

Here's the relevant section of ARM Application Note 31
(http://www.arm.com/pdfs/DAI0031C_using_eice.pdf):

"nRESET is used to reset the processor core and put it into a known
state, while nTRST is used to reset the TAP controller and the
EmbeddedICE macrocell, including the registers in the
breakpoint/watchpoint units. Both these resets must be applied before
the device will function correctly."

So, without any circuitry to drive nTRST, the TAP controller could end
up in an unusable state where the host can not communicate with the
JTAG port. Doing a nRESET, which is all the DIY wiggler has control
over, would reset the processor core but never reset the TAP
controller used for JTAG debugging.

I don't have a real wiggler, but given that the Macraigor pinouts PDF
file shows the nTRST pin as being type "i" instead of type "oc" like
the nRESET pin, I'm guessing that the nTRST signal should be connected
to the DATA4 pin using a line driver instead of an inverting open
collector transistor circuit as used for the nRESET signal. 

I've updated my schematic diagram to include the proposed change for TRST:

http://www.k9spud.com/jtag/

--- In lpc2000@yahoogroups.com, "derbaier" <dershu@s...> wrote:
>
> --- In lpc2000@yahoogroups.com, "Ed Schlunder" <zilym@y...> wrote:
> >
> > Thank you for all the suggestions. I have put together an initial
> > schematic diagram using 74VHC14 schmitt trigger inverters and RC low
> > pass filters (as suggested here):
> > 
> > http://www.k9spud.com/jtag/
> > 
> > I'm not sure how to add additional signals like RTCK and nTRST. Won't
> > these kind of changes require software support on the PC to be useful?
> > My understanding was that the Macraigor software was not open source.
> > Where should these signals be connected to on the PC side?
> > 
> > Or are you just suggesting a jumper for the user to manually switch by
> > hand?
> > 
> > What is the nTRST signal useful for? I'm new to JTAG, just trying to
> > get my first ARM project started.
> > 
> 
> Ed, your use of 'HC14 parts is exactly the same as ARM did in their
> Application Note 31.  IMHO, I think that you will find the RC time
> constant of your filters to be much too long!  With such large
> resistors, the input capacitance of the of the Schmitt triggers will
> probably be plenty of low pass filtering!  Also, if you want to save a
> few parts, you can replace the transistor inverter and it's assocoated
> base resistors with one of your left over 'HC14s with a series diode
> on it's output to convert it to an equivalent of an open drain output.
> 
> A1. The signal connections on the PC side are fixed by the closed
> source Macraigor software, so added enhancements to the hardeware will
> not have Macraigor software support. In any case, the Wiggler clock is
> probably much too slow for RTCK to be very useful unless the target
> ARM is being clocked at a much slower than normal speed. RTCK has been
> VERY useful for very high speed JTAG debuggers like TRACE32.
> 
> A2. The nTRST signal is the JTAG TAP controller reset. The other reset
> signal is for the rest of the hardware. In other words, ARM intended
> for them to be separately resetable, as they explain in section 12.3
> of Application Note 31.