[Testing] [OLPC New Zealand] New prototype XO-1.75s in Auckland, New Zealand

James Cameron quozl at laptop.org
Thu Aug 25 01:03:46 EDT 2011

On Wed, Aug 24, 2011 at 10:48:41PM +1200, Tom Parker wrote:
> I must say I hadn't thought about where the noise might be coming from,
> I merely wanted to process it out. Is it really sensitive enough to pick
> up normal sound (not just boom sounds!)?

The device resolution is typically 72 milligravities, which would be
0.706 metres per second squared.

The device measurement range, in the default configuration, is a minimum
of plus or minus 2.0 gravities, and typically plus or minus 2.3

The device sensitivity, in the default configuration, is between 15 and
21 milligravities per binary digit, which would be between 0.147 and
0.206 metres per second squared.

At the moment, in the XO-1.75 B1, we have the sample rate set to the
default of 100 Hz, but the CPU can only read at about 30 Hz due to an
electrical design fault (#11041).  On the A3 we did have it set higher,
and it was possible to read data at about 900 Hz, which meant you got
repeat samples.

Then, we were able to configure it for 400 Hz and read it at that rate.
Have a look at /runin/runin-accelerometer for some of the code involved.

At that rate I saw noise in the signal that was consistent with ambient
sound.  My theory is that sound in the vicinity of the laptop,
especially sound at a frequency close to resonance, would cause
vibration of the circuit board, and this would be picked up by the

One could imagine it to be a low frequency microphone in three
dimensions, with an unusual frequency response.  ;-)

> I'm not really familiar enough with Turtle Blocks to know if the
> performance I am seeing is because reading the sensor is slow or
> because computing the average is.

I don't know enough about Turtle Blocks to comment, but if an
application such as Turtle Blocks tries to read the sensor, it will
probably block for as long as it takes for the transaction to complete
... roughly 33 milliseconds.

James Cameron

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