CMPS11 - Tilt Compensated Compass Module 

Serial mode

Connections

3.6v-5v
Tx
Rx
Mode
Factory use
0v Ground

To use the serial mode of operation the mode pin must be connected to ground.

Communication settings
The Serial mode operates over a link with a default baud rate of 9600 bps (no parity, 2 stop bits) and 3.3v-5v signal levels. 
This is not RS232. Do not connect RS232 to the module, the high RS232 voltages will irreversibly damage the module.

Commands
Below is a table describing commands that can be sent to the CMPS11 and the data it will respond with.

Commands for Serial

 Command 

Name

 Bytes returned  

Returned data description

0x11

GET VERSION 1 Software version

0x12

GET ANGLE 8 BIT 1 Angle as a single byte 0-255

0x13

GET ANGLE 16 BIT 2 Angle as two bytes, high byte first 0-3599

0x14

GET PITCH 1 Pitch angle +/- 0-85 Kalman filtered

0x15

GET ROLL 1 Roll angle +/- 0-85 Kalman filtered
0x16 GET PITCH NO KAL 1 Pitch angle +/- 0-85 no Kalman filter
0x17 GET ROLL NO KAL 1 Roll angle +/- 0-85 no Kalman filter

0x19

GET MAG RAW 6 Raw magnetic data, 16 bit signed: X high, X low, Y high, Y low, Z high, Z low 
0x20 GET ACCEL RAW 6  Raw accelerometer data, 16 bit signed: X high, X low, Y high, Y low, Z high, Z low 
0x21 GET GYRO RAW 6 Raw gyro data, 16 bit signed: X high, X low, Y high, Y low, Z high, Z low
0x22 GET TEMP 2 Temperature as two bytes, high byte first 0. Typical resolution 8 LSB/C

0x23

GET ALL 4 angle high, angle low (0-3599), pitch (+/- 0-85), roll  (+/- 0-85)
0xF0 CALIBRATE BYTE 1 1 returns ok (0x55)
0xF5 CALIBRATE BYTE 2 1 returns ok (0x55)
0xF6 CALIBRATE BYTE 3 FULL 1 returns ok (0x55)
0xF7  CALIBRATE BYTE 3 FLAT   1 returns ok (0x55)
0xF8 CALIBRATE EXIT 1 returns ok (0x55)
0x6A RESTORE 1 1 returns ok (0x55)
0x7C RESTORE 2 1 returns ok (0x55)
0x81 RESTORE 3 1 returns ok (0x55)

0xA0

BAUD 19200 1 returns ok (0x55)
0xA1 BAUD 38400 1 returns ok (0x55)

Calibration of the CMPS11 
Please do not do this until you have serial communication fully working. I would recommend evaluating the CMPS11 performance first before implementing this function. Its purpose is to remove sensor gain and offset of both magnetometer and accelerometer and achieves this by looking for maximum sensor outputs. First of all you need to enter the calibration mode by sending a 3 byte sequence of 0xF0,0xF5 and then 0xF6 (reading the acknowledge byte after each one).  The LED will then extinguish and the CMPS11 should now be rotated in all directions in 3 dimensions, if a new maximum for any of the sensors is detected then the LED will flash, when you cannot get any further LED flashes in any direction then exit the calibration mode with a command of 0xF8. Please make sure that the CMPS11 is not located near to ferrous objects as this will distort the magnetic field and induce errors in the reading. While calibrating rotate the compass slowly. Remember the axis of the magnetic field is unlikely to be horizontal, it dips into the earth at an angle which varies depending on your location. At our offices in the UK it dips into the earth at 67 degrees and that is the orientation each axis of the compass needs to be to find the maximums. You need to find both positive and negative maximums for each axis so there are 6 points to calibrate. The accelerometer is also calibrated at the same time, so the module should also be positioned horizontal, inverted, and on all 4 sides to calibrate the 6 accelerometer points. Each accelerometer point needs to be stable for 200mS for its reading to be used for calibration. This delay is deliberate so that light taps to the module do not produces disruptive accelerometer readings which would mess up the pitch and roll angles. There is no delay for the magnetic points. The performance of the module is directly related to how well you perform calibration so do this slowly and carefully. 

Calibration of the CMPS11 for horizontal only operation
If the compass does not require the tilt compensation then a simple calibration may be used that can be implemented by a rotation on the horizontal plane only. First of all you need to enter the calibration mode by sending a 3 byte sequence of 0xF0,0xF5 and then 0xF7 (reading the acknowledge byte after each one).  The LED will then extinguish and the CMPS11 should now be rotated in all directions on a horizontal plane, if a new maximum for any of the sensors is detected then the LED will flash, when you cannot get any further LED flashes in any direction then exit the calibration mode with a command of 0xF8. Please make sure that the CMPS11 is not located near to ferrous objects as this will distort the magnetic field and induce errors in the reading. While calibrating rotate the compass slowly. Only the X and Y magnetometer axis are calibrated in this mode.

Restore of factory calibration of the CMPS11
To perform a restore of the factory calibration write a sequence of 3 commands in the correct order. The sequence is 0x6A,0x7C,0x81 (reading the acknowledge byte after each one).

Changing the baud rate
The default serial baud rate of 9600 can be changed. There are two other baud rates that can be used, for 19200 just send 0xA0 or alternatively for 38400 send 0xA1. Please note that the CMPS11 will always default to its 9600 bps rate after power cycling and after setting a new baud rate the ok response (0x55) will be sent at the newly selected speed.