For completeness, host board circuitry is required to maintain the JTAG
port lines, although chip manufacturers may provide internal pull-ups /
downs.

For devices with TRST ( I know it's active low, but that's it's name in
the spec :) pull all other lines high, pull TRST low. (There are
exceptions to this - the Intel PXA series has specific *ordinary* reset
timing requirements applied to TRST).

For devices that do not have TRST:

Pull TCK ***LOW***, TDI, TMS high. The reason is the JTAG chain will
enter test mode with TMS high on a rising edge of TCK. At system
startup, depending on the board capacitances and the resistor sizes (to
say nothing of chip innards), if both TCK and TMS are pulled to power,
it is possible that normal startup can put the device in test mode (not
a good thing [tm]).

Cheers

PeteS

thanks for the answers , I am a software guy, I went thru the docs but
could not find a answers, I am using Au1550 processor and DM642.

"Arie de Muynck" ...
> "dspwhiz" ...
> > How to detect from the CPU wheather JTAG Is connected or not
>
> Use a pulldown on nTRST. That line will be driven high by the JTAG dongle
> when communicating. When the JTAG is not connected the line will be low
(and
> the JTAG logic kept in reset, the prefered state).

P.S.: Connect the nTRST line also to a GPIO pin or other input of your
system so it can be tested by the SW.

Regards,
Arie de Muynck

Scott Moore <samiamsansspam@Sun.COM> writes:

> John Devereux wrote On 10/21/05 05:26,:
> > Tauno Voipio <tauno.voipio@iki.fi.NOSPAM.invalid> writes:
> > 
> > 
> >>dspwhiz wrote:
> >>
> >>>How to detect from the CPU wheather JTAG Is connected or not
> >>
> >>No way.
> >>
> >>Why?
> > 
> > 
> > Don't know the OP's reason, but I wanted to do this too, for ARM7.
> > 
> > The reason was when trying to use the ARM debug communications
> > channel, the program hangs if the JTAG is not connected. I was unable
> > to find a way around this :(
> > 
> 
> You've answered your own question.
> 
> Timeouts.


I am pretty sure that even an attempted write to the dcc locked up the
program. There was no opportunity to do a "Timeout". Having said that,
it was difficult to tell exactly what was going on without the
debugger(!), and it was a few weeks ago now so I could be
mistaken. Has anyone actually implemented this successfully? (I.e., a
timeout on the debug communications channel that recovers from a
missing debugger?)
 

-- 

John Devereux

John Devereux wrote On 10/21/05 05:26,:
> Tauno Voipio <tauno.voipio@iki.fi.NOSPAM.invalid> writes:
> 
> 
>>dspwhiz wrote:
>>
>>>How to detect from the CPU wheather JTAG Is connected or not
>>
>>No way.
>>
>>Why?
> 
> 
> Don't know the OP's reason, but I wanted to do this too, for ARM7.
> 
> The reason was when trying to use the ARM debug communications
> channel, the program hangs if the JTAG is not connected. I was unable
> to find a way around this :(
> 

You've answered your own question.

Timeouts.

"dspwhiz" ...
> How to detect from the CPU wheather JTAG Is connected or not

Use a pulldown on nTRST. That line will be driven high by the JTAG dongle
when communicating. When the JTAG is not connected the line will be low (and
the JTAG logic kept in reset, the prefered state).

Regards,
Arie de Muynck

I am by no means an expert on JTAG, but every time I have implemented a
JTAG port on a circuit, I had to add pull-up and pull-down resistors.
It isn't a software solution, which I gather is what you are after, but
perhaps it is possible to monitor the state of these signal lines to
determine if a device is connected.

Tauno Voipio <tauno.voipio@iki.fi.NOSPAM.invalid> writes:

> dspwhiz wrote:
> > How to detect from the CPU wheather JTAG Is connected or not
> 
> No way.
> 
> Why?

Don't know the OP's reason, but I wanted to do this too, for ARM7.

The reason was when trying to use the ARM debug communications
channel, the program hangs if the JTAG is not connected. I was unable
to find a way around this :(

-- 

John Devereux

dspwhiz wrote:
> How to detect from the CPU wheather JTAG Is connected or not
> Thanks 
> SM
> 
If you are a whiz, how come you need to ask such elementary questions? :-\

Steve

dspwhiz wrote:
> How to detect from the CPU wheather JTAG Is connected or not

No way.

Why?

-- 

Tauno Voipio
tauno voipio (at) iki fi