AVR-Chat

I have the Atmel JTAG ICE mkII and I've read the documentation on it
(including as I discovered going back to v1 doc's for more complete info).
It's clear from the documentation that when the JTAG interface is enabled,
"the JTAG pins cannot be used for alternative pin functions", eg, general
I/O.  However, I do want to use those pins for I/O in normal, non-debug
operation.  So...

1. Does that mean that any external I/O connected to the JTAG pins is
(somehow) both ignored and isolated when in debug mode?  In other words, if
I have one of the JTAG pins connected to an external input signal and then I
go into an on-chip debugging session does this mean (a) I'll never see that
input and (b) that input will not mess up the JTAG signal on that same line?


2. Or, conversely, if that JTAG pin is normally used as an output pin, will
any attempt by my software to control that output be ignored (ie, my desired
output event not happen) and moreover it will not mess up the JTAG
communication?

3. If the above is, as I'm guessing, correct, then the corollary seems to be
that one should design things so that whatever I/O is normally done with the
JTAG pins should be something that (hopefully) will never need to be
debugged using the OCD.  Is that so?

4. I am presuming this is all connected with the series resistors that are
recommended be placed in the JTAG lines to external circuitry (ie, my normal
operation I/O). The documentation states that "the value of the resistor
should not exceed the maximum ratings and sink or source too much current."
So to compute the actual value of these series resistors, do I compute V/I
for both ends (one the uC and one the external circuitry) and then use the
greater of the two?  For example, if one end is the AVR running at 3.0 V,
with a spec'd max current per pin of 40 mA, then I compute a 75 ohm series
resistor insofar as the uC end is concerned.  Then do a similar calculation
for the external end and use the larger of the two.  Have I got it correct?

5. Is there just a typical "cover-all-cases" value of these series resistors
that everyone uses?

Thanks, Steve

At 08:16 AM 7/01/2008, you wrote:
>5. Is there just a typical "cover-all-cases" value of these series resistors
>that everyone uses?

I wish I knew the answers to all those questions!!

Basically, I try not to use the JTAG pins for anything else
if I can help it. Very wasteful!

Remember that JTAG needs to be disabled if you want to use
those pins, either via the fuse (permanent) or by writing
to the JTAG disable register in your code.

I would only use the pins for something like reading a dip switch
(with series resistors) or indication leds which could be simulated
during debug.


Regards

John Samperi

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