I2C and GPS

Benjamin Deering ben_deering at swissmail.org
Thu Jan 12 00:00:40 CET 2012

I promised myself that I wouldn't take my FR apart again until it was 
time to install the GTA04 board so I won't have pics of what I did this 
weekend, but I can share some other info.

BMP085 barometer install pics: 
MLX90614 thermometer install pics: 

Adding i2c devices to a Freerunner is fairly easy.  There are only 4 
wires and as long as there aren't address conflicts, you can just keep 
adding more devices.  Here is where you can find a picture of where to 
attach the wires: 

The change I made this weekend was to stand an SMD resistor up 
vertically on the SCL test pad (If you look at the pictures I link 
above, that is where the orange wire attaches to the board) in the FR 
and solder it.  I then attached the SCL wires for my 2 sensors to the 
top of the resistor.  I used 4.7k ohm which is probably more than I 
should have used, but it works.  I think 1K might make more sense, there 
was some mention of adding series resistors for long i2c circuits, 
though each of my SCL wires is 8 cm at the most.

Sorry for the lack of pictures of the current state, but the way I 
crammed the BMP085 in is fragile and I don't want to break it.  I hope 
my description along with the other pics makes it clear enough.

As for schematics, i2c is just 4 wires

VDD -> voltage source on the AUX button
GND -> ground side of the clock battery (supercap in my case)
SDA -> SDA test pad near the debug connector
SCL -> 4.7k resistor ->SCL test pad near the debug connector

The reference implementation for the 2 sensors I installed was a little 
more complicated with pullup resistors and decoupling capacitors, but 
everything seems to work without them.


On 01/11/2012 07:46 AM, David Matthews wrote:
>>   can we have a picture of the final tweak� please ?
> +1 yes please! I'm struggling to understand what you've done, but ity 
> sounds like something
> I'd be very please to have.
> and by the way that stuff I wrote earlier about sdcards was garbage - 
> it was
> a faulty card, not a problem with qtmoko or shr
> For me now first thoughts when I see an sdcard problem are that either 
> the
> card is faulty or inherantly incompatible with the freerunner (and 
> that seems
> very common)

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