Meaning of "LSB/Unit" and "Unit/LSB"
At the moment I'm playing with the LSM303DLHC accelerometer/magnetometer/thermometer.
This is its datasheet: http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/DM00027543.pdf
Everything is working quite well, but I don't know how to interpret the output values. The datasheet (page 9) says something like "1 mg/LSB" (no, it's not milligramm :D) about the linear acceleration sensitivity in my configuration. What the hell should that mean? Same with temperature sensor output change (8 LSB/°C) and magnetic gain setting (1100 LSB/gauss), only the other way around.
For example, what to do with this accelerometer output: 16384? That is my measured gravitational acceleration.
Answer
Now I got the trick. There are several things on this MEMS you have to know, but which are not mentioned in the datasheet:
- The accelerometer's output register is just 12 bits and not 16 bits, so you need to right-shift the value by 4 and multiply it with 0,001 G. Furthermore it's little-endian.
- The magnetometer's output register is 16 bits, but big-endian. Furthermore the vector order is (X|Z|Y) not (X|Y|Z). To calculate the correct value you need to devide X and Y by 980 gauss⁻¹, while it's 1100 gauss⁻¹ for Z.
- The temperature sensor works, but it's not calibrated. So you can use it to measure temperature change, but no absolute temperatures. It's also just 12 bits, but big-endian and you have to devide the output by 8 C⁻¹.
With that Information it's possible to use the LSM303DLHC. But who the hell invented this? "Let's build a new accelerometer, magnetometer and thermometer in one package and screw the user up by mixing word length and endianness up without mentioning it in the datasheet."