Connecting the HMC6352 compass module to a Raspi using I2C

Question

I am trying to connect a HMC6352 compass module to my Raspberry Pi.

According to the datasheet I need to send an "A" for a read command. However, I am somewhat new to I2C and do not know how to accomplish this. I would like to do the coding in Python.

I know I have set up I2C correctly, and that the compass is correctly connected to the Pi, because I can see it when I run i2cdetect:

pi@raspberrypi ~ $ sudo i2cdetect -y 0
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: -- 21 -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --    

You can see the compass on address 21, although the documentation says the default address is 42.

I am running Adafruit's Occidentals V2 for my OS.

Could anyone show me some example Python code that returns the current reading from the compass?

Answer 1

I figured out how to do this using the quick2wire python library. Note it must be run using python3

#!/usr/bin/env python3                                                                          

from quick2wire.i2c import I2CMaster, writing_bytes, reading                  
import time                                                                   

address = 0x21  # found using `sudo i2cdetect -y 0`                                                         
cmd = 0x41  # 'A' means read                                                     

with I2CMaster() as master:                                                   

    while True:                                                               
        master.transaction(writing_bytes(address, cmd))                       
        raw_bytes = master.transaction(reading(address, 2))[0]                

        #Shift the first byte up by 8 bits and add it to the second
        #The result is in 10ths of degrees, so divide by 10 to get
        #degrees.
        heading = (raw_bytes[0] * 2**8 + raw_bytes[1]) / 10                   
        print(heading)                                                        

        time.sleep(.2)