I2C and GPS

Benjamin Deering ben_deering at swissmail.org
Wed Nov 23 03:20:16 CET 2011

For months now I have had poor performance with my Freerunner's GPS.  
The GPS problems did not seem to coincide with the two times I have 
opened my Freerunner to add I2C sensors.  I have an infrared thermometer 
and a barometer.

Lately I have missed the GPS enough to look into the issue more.  After 
trying various things in software and messing with the GPS antenna, I 
started to look at my sensors.  The reference schematics for both 
sensors call for a 100nf decoupling capacitor between ground and VDD.  I 
looked around in my junk bin for some.  All I could find were some huge 
(for the inside of a FR case) ceramic ones.  They didn't seem to help.  
Using frameworkd.log with the log level turned up, I was able to watch 
the UBX packets as they came down and disconnecting SDA and SCL for both 
sensors gave an immediate improvement in GPS reception.

I didn't notice a problem with reception when I first added the 
sensors.  Was anything done in software that would make I2C cause more 
interference?  Should I pursue the decoupling capacitors?  Does it 
matter what kind if they are 100nf?

Thanks for any ideas,

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