I2C and GPS

Benjamin Deering ben_deering at swissmail.org
Wed Jan 11 03:52:00 CET 2012

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 
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.


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>> 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>>  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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