GPS External antenna detect issue

[Daniel Willmann]

Hi,

On Mon, 28 Apr 2008 19:53:34 +0100
Andy Green <andy at openmoko.com> wrote:

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> Hi folks -
> 
> While trying to make DM2 GPS work good, I became annoyed at the range
> of behaviours the GPS unit and finally spent the whole latter part of
> the day meddling.  I don't think I found where the problems came
> from, but I found something that is less good than it should be.  GPS
> stopped working altogether on my boards at the moment (unrelated to
> ripping the cans off the GPS section I believe) so I don't know where
> I got to.

ah, that shield is quite stubborn.

> specifies low level input selecting "OUTPUT" 2 is min -200mV, typ 0
> and max 200mV.  The other active level is specified min 2.7V, typ
> 2.8V, max 3.0V.  So you have to drive that chip hard to 0V or the
> power rail to select either input.
> 
> But at our normal situation with internal antenna and no load on
> external antenna, we drive VCONT to 810mV (26% Vdd).

That might explain some of the things I've been seeing here, bad
reception with the internal GPS antenna and maybe the failure of the
antenna switch on my v4 was a result of this floating (or the tolerance
just changed resulting in really bad reception).

One more thing, don't know if it's related.
In the Antaris 4 System integration manual which you can find here:
http://www.u-blox.com/customersupport/antaris4_doc.html

Near page 77, ff. is the antenna short circuit protection described.
Now when I power up my Neos GPS it spits out the following messages:

$GPTXT,01,01,02,ANTSUPERV=AC SD OD PDoS *0A
$GPTXT,01,01,02,ANTSTATUS=SHORT*6D

If you read the manual you will notice, that
AC: Antenna Control, antenna will be switched on/off by the GPS
SD: Short circuit control enabled
PDoS: Power down antenna if short circuit is detected

And now of course ANTSTATUS=SHORT, antenna short circuit has been
detected.

Okay, I just had a look in the ATR635 Manual and it says to just leave
pins ANTON, ANTSHORT_N and AADET_N unconnected if we're not using it
which is what we do. I guess we can ignore the messages, though I'm not
100% convinced yet. :-)


Regards,
Daniel Willmann
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