Re:[avr-gcc-list] JTAG interface

[hanzl]

>> I was just wondering if it's possible to use the JTAG interface of the
>> AVRs without any special ICE.. I mean, using just some cables and resistors,
>> even diodes, but nothing complicated..

> uisp -dprog=xil

That's nice (misusing JTAG cable for MISO/MOSI etc.), downloading
programs via cheap JTAG cable really connected to ATmega's JTAG is
also nice (using AVRSVF) - BUT can we also have gdb connected to
ATmega16+ via cheap printer-port JTAG ???

That would be very very nice (having great debug ability for nearly free).

I mean something like this:

 gdb debuger (with ddd)
    |
 AVaRICE patched to talk to thing below instead of serial line
    |
 'AVR JTAG ICE' in software on PC
    |
 Printer Port
    |
 Cheap JTAG dongle (*)
    |
 ATmega16 or 32 or 64 or 128


Is there any principal problem with this? Or does it work for somebody
somewhere?

I can imagine just the following problems:

1) Debugging via ATmega's JTAG requires signals too quick to
generate/percieve on the PC's printer port - I know no details here
but I hope this is not the case.

2) Protocol normally found on RS232 to AVR JTAG ICE is unknown - I
guess it is now known because AVaRICE exists.

3) Some commands of protocol normally found on JTAG between AVR JTAG
ICE and ATmega16+ are not known - I guess at least they are known to
those who make cheaper AVR JTAG ICE clones so future authors of 'AVR
JTAG ICE in software on PC' could find out as well.

All together, it seems to me that super-cheap super-debug capability
is possible yet I do not see this described anywhere - so what is the
problem? Just nobody did it yet? Most likely I am missing some very
basic thing...

Thanks for any comments.

Vaclav Hanzl

(*) For example this one:
  http://www.lart.tudelft.nl/projects/jtag/
  http://www.tuxscreen.net/wiki/view/JTAG

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