accelerometer jitter

[Al Johnson]

On Saturday 20 September 2008, Joel Newkirk wrote:
> I wrote a short shell script to repeatedly take the raw output from one of
> the accelerometers (/sys/devices/platform/spi_s3c24xx_gpio.1/spi0.1/dump)
> and massage the data to output X,Y,and Z-axis readings in decimal, -128 to
> 128.
>
> With my Freerunner sitting flat, face up, on the concrete slab of my house,
> the 'Z' reading (positive Z is toward the back of the handset, BTW)
> fluctuates between 51 and 56. (X and Y range from -1 to 2)  That's a
> variance of +/- 5%, and I'm fairly confident gravity isn't fluctuating
> significantly... ;)
>
> Is that to be expected, is my Freerunner defective, or am I doing something
> wrong?

What does the datasheet say about noise levels? You're looking at +/-5% of 
reading but +/-1% of full scale which isn't implausible for a cheap 
accelerometer.

> I'm trying to develop a simple tool that will determine 'down' based on
> output from the accelerometer, and indicate deviations from various
> 90-degree orientations - will I need to massage the data further to ignore
> variations, should I be averaging several readings, or what?

Underspecified ;-) How accurate does it need to be? Is the phone expected to 
be stationary or moving? Is power consumption important?