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
(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>> 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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>>
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