GPS External antenna detect issue
andy at openmoko.com
Tue Apr 29 08:52:07 CEST 2008
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Somebody in the thread at some point said:
|> 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).
Yes quite likely the effect of not driving the select input hard to the
rails is attenuation or worse modulation of the RF according to noise on
the select line.
| One more thing, don't know if it's related.
| In the Antaris 4 System integration manual which you can find here:
| 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
| 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
| 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. :-)
I found this a couple of weeks ago when I started looking at raw GPS (in
fact I mentioned it to Werner yesterday on the conf call), but I was
wrongfooted because I measured the antenna DC power and saw that it was
actually not "powered down". There is a note in the data as well that
some firmware variants report they power down on short, but it can be
ignored, so I figured we must be in that category. But yesterday I saw
antenna power isn't controlled by the GPS chip but by the antenna select
mux, therefore if it decides to disable it in the GPS chip we wouldn't
see any "power down" at the antenna anyway.
So likewise I am unconvinced that these messages are in fact "harmless",
especially since I spent the last few hours yesterday unable to even see
one satellite from a window here that saw several earlier, on two GTA05
and three antennas. I forced VCONT to 0V with high hopes but it didn't
help. So figuring out how to logically disable PDoS using the
proprietary protocol is the next job here.
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