I2C and GPS

[Sylvain Paré]

can we have a picture of the final tweak  please ?


2012/1/11 Benjamin Deering <ben_deering at swissmail.org>

> Dusting off a very old thread...
>
> I have both of my sensors working and good gps performance now.
>
> This weekend I opened the position settings->satellite details dialog in
> SHR as well as my phone and tried a few things.  I was seeing serious
> signal loss as soon as I connected the SCL line from the sensor to the test
> pad near the debug connector.  I tried wrapping the SCL wire around a
> ferrite bead, and putting a decoupling cap between power and ground near
> the sensor.  What ended up working was adding a series resistor on the SCL
> line.  I grabbed a largeish SMD resistor from some volkswagen parts I had
> laying around (http://www.jeepingben.net/**plog-content/thumbs/2010/**
> volkswagen/large/995-dscf2246.**jpg<http://www.jeepingben.net/plog-content/thumbs/2010/volkswagen/large/995-dscf2246.jpg>).
>  I thought I was grabbing a 470 ohm, but I guess it was 4.7k (marked 472).
>  With this resistor in series everything works.  I don't know if anyone
> else is adding i2c sensors to their freerunner, but I thought this might be
> helpful.
>
> Ben
>
> On 11/24/2011 11:36 AM, Alastair Johnson wrote:
>
>> Given the performance degradation in GPS lock cause by high SD drive
>> strength and the i2c being on flying leads right next to the antenna I
>> certainly wouldn't rule it out.
>>
>> On 11/24/2011 01:49 PM, Lars Poulsen wrote:
>>
>>> Hi
>>> You can try to add a series resistor to the signals or reducing the
>>> drive strength of the driver.
>>> The frequency of the signals are not that important but rather the rise
>>> time.
>>> This is usually not a problem om i2c but  who knows.
>>>
>>> On 24/11/2011, at 14.17, Dave <dave.tv at gmail.com
>>> <mailto:dave.tv at gmail.com>> wrote:
>>>
>>>  Hi,
>>>>
>>>> Ferrite beads aid with removing VHF-SHF feedback and/or uncontrolled
>>>> oscillation mainly. I cannot see why they would be of use on a
>>>> (500khz?) I2C bus. At that frequency they would have minimal effect.
>>>>
>>>> On Thu, Nov 24, 2011 at 9:44 PM, Benjamin Deering
>>>> <ben_deering at swissmail.org <mailto:ben_deering at swissmail.**org<ben_deering at swissmail.org>>>
>>>> wrote:
>>>>
>>>>
>>>>    With the i2c devices removed, I get TTFF of around 1 minute in
>>>>    shr-core.  It sounds like putting ferrite beads on SDA and SCL
>>>>    might help reduce EMI, so I will try that when I get a chance.
>>>>
>>>>    Ben
>>>>
>>>>
>>>>    On 11/23/2011 05:06 PM, dmatthews.org <http://dmatthews.org> wrote:
>>>>
>>>>        On Tue, 22 Nov 2011 21:20:16 -0500 <tel:16%20-0500>
>>>>        Benjamin Deering<ben_deering at swissmail.**__org
>>>> <mailto:ben_deering at swissmail.**org <ben_deering at swissmail.org>>>
>>>>  wrote:
>>>>
>>>>        Hi Ben
>>>>
>>>>        Not sure this is relevent to you, but I now have the fastest
>>>>        GPS fix I've ever had on the freerunner.
>>>>
>>>>        QTMoko v35 and I put this in /etc/default/gpsd:-
>>>>
>>>>        START_DAEMON="true"
>>>>        GPSD_OPTIONS=""
>>>>        DEVICES="/dev/ttySAC1"
>>>>        USBAUTO="false"
>>>>        GPSD_SOCKET="/var/run/gpsd.__**sock"
>>>>
>>>>        Before doing this it was pretty poor - worse than earlier
>>>>        versions of qtmoko and much worse than every SHR I've tried.
>>>>        On a reasonably clear day I now reliably get a fix in under a
>>>>        minute, sometimes within a few seconds.
>>>>
>>>>        The only other varying factor (doubtful relevence) is that I
>>>>        got pissed with QTMoko and to a lesser extent SHR foobarring
>>>>        the SD card, so I'm now running from the card instead of NAND
>>>>        and everything is pretty good
>>>>
>>>>
>>>>
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