MMA7455L Z axis values near 0x00

[RoboMike]

Hey All, 


My values with the MMA7455L seem a little strange to me. The MMA7455L is in 2g measurement mode, and the device is mounted top facing up. 

Having said that my X, Y, Z are as follows: 

X = 3F 
Y = 3A 
Z = 10 

I am expecting Z to be closer to 3F as per the data sheet (http://www.freescale.com/files/sensors/doc/data_sheet/MMA7455L.pdf) 

Any idea why this might occur? 

Best Regards, 

Mike

[rocco]

Gravity?

[RoboMike]

I don't understand your response.  With gravity pulling the accelerometer towards the center of the earth, the data sheet says the expected value is $3F.  Not 0x00.  

[rocco]

Hi Mike,

The numbers you show just look like gravity to me: two axes at close to the same value, and the third axis about 1g different. Although it the Z axis looks to be reading more than 1g from X and Y, I don't think it is out-of-bounds for an uncalibrated reading.

You could simply turn the board over and see if the reading reverses.

[PJH]

Depending on how your board is mounted the axis which contains g will change.  If you are using the eval board and it is flat on a table then you can expect 1g on the z-axis.  The accelerometer will have an offset on each axis which does need to be calibrated out.  Each device will have a slightly different offset on each axis without the correction.  The app notes for this device explain this.  See AN3468.

Peter