Warranty after fix for GTA02 GPS rework for SD card interferenceissue

Andy Green andy at openmoko.com
Fri Aug 1 08:27:35 CEST 2008

Hash: SHA1

Somebody in the thread at some point said:

| Actually I based my observation on the fact that i saw an entry of
| 10/x/y as the endmost entry on the TangoGPS status line.
| Isn't that supposed to mean that it still has fixes from 10 satellites.

Our GPS chip is quite capable to report garbage as fact, for a while
anyway.  I also saw things like "12 satellites" each with individual SNR
reported, when I inserted or removed internal antenna while powered
(something you aren't meant to do BTW).  After some seconds these
disappeared from the reporting back to the virtual reality they came
from.  So madness under extreme circumstances is normal for it I believe.

| So I thought it must be a SNR issue and the capacitor fix
| is supposed to help on that ... right?

At the time the cap workaround was conceived, we ran SD_CLK all the time
even with the card idle and did not test for what it does during card
transactions.  Since then, the way we treat the card has gotten
seriously more optimized:

~ - SD_CLK is stopped unless we are making an access
~ - Voltage to shipping SD Card is reduced to 2.7V from 3.3V
~ - Edges on all SD signals have reduced slew rate
~ - SD_CLK rate is reduced if GPS unit is powered

The reason I avoid recommending the cap is there are 6 signals going to
the card, not just SD_CLK, and during transfer 5 of them have a lot of
rapid transitions.  The reasoning for cherrypicking SD_CLK is no longer
entirely valid.

| This was all guesswork on my part. If somebody could help with how to
| make observations on GPS performance, I'd be glad to
| do testing and provide input.

I'm afraid I don't know the userspace apps.  I literally cat the serial
port and read the NMEA.  Maybe someone else can advise, this Tango GPS
thing seems to be widely admired.

| The kernel update is going on as I type this. So will test again today
| if possible. And don't worry ... will be doing the hardware
| fix only after OM finally, after all software fixes, decides it will
| still help / otherwise not.

Great, more info the better we can understand where we are at.

- -Andy
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Comment: Using GnuPG with Fedora - http://enigmail.mozdev.org


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