memsic 2125 accelerometer help

Forum 2005-2010 (read only) Hardware Interfacing
1

I am interfacing the Memsic 2125 accelerometer to the Arduino Pro Mini 5 V. I need to distinguish between the accelerometer being in the upright position and being upside down. As I rotated the accelerometer 180 degrees, the PulseIn values increased but then decreased back to the original upright value. I don't understand why this is happening and if it is right.

This is the x-axis output as I rotated it 180 degrees along the axis:

4972 //upright position
4976
5260
5531
5691
5864
6149
6208
6175
6134
6011
5885
5729
5608
5482
5402
5252
5087
4979
4972 //upside down

If this seems right, how can I distinguish between the accelerometer being upright and upside down?
Thank you.

2

An accelerometer (when rotated) is measuring the force of gravity. The choice of coordinates is arbitrary. If this accelerometer only measures one axis and you want to determine "down" vs "up" then call the one of the sides down (ie mount it rotated 90deg). The way you've got it setup now X is normal to gravity so up or down gravity won't effect the measurement.