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Urgent - Need 3.3V tolerant Compass

screamingtiger

Cliffs:  Anyone know of a magnetic compass that works with the raspberry Pi, I2C and is 3.3V tolerant on the SDA/SCL?  It may need to be tilt compensated.  Please have first hand experience using it, I need it to work I cannot afford any more trial and error.  Thanks!

 

Details:

I've had some serious issues with my quadcop_project in terms of getting heading.  I've tried a mems sensor from xtrinsic but its not compatible via I2C with the Raspberry Pi 2.  It works fine with RPi B+. I digress.

 

I have an HCM5883L breakout and realized its not 5V tolerant, I think I burned it up and the data doesn't make any sense to me.  It is suppose to return a value between 0 and 255 but the registers are 16 bit with MSB and LSB being read independently.

https://www.sparkfun.com/products/10530

 

The examples from the Arduino don't make any sense.  Its really simple to use:

https://www.sparkfun.com/tutorials/301

 

You can see the values they are getting there.  A couple of odd things, the datasheet says it returns the data in 2 compliment form, which to me says I need to flip the bits but the sample code does not such thing. 

 

Regardless, I am not sure how to use this because it has 3 axis!  There is very little information on this board and no idea why I would want a 3 axis reading and how I would use that to calculate north.

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  • 0

    Hi Joey,

     

    Don't you need it to have 3.3V logic levels (the RPI does not have 5V logic levels). What do you mean by 5V tolerant, do you expect the pull-up resistors on the I2C bus to be switched between 3.3V and 5V on the fly?

    or did you mean you wanted it to work with 5Vlogic levels?

    For level shifting, this link may be useful.

    However, I'm not sure that's what you need, since I don't understand the reason why 5V is mentioned, since both the RPI and the sensor you mention operate at 3.3V logic levels. For a better answer, could you explain this requirement in a little more detail?

  • 0 in reply to shabaz

    I forgot the pins are 3.3V.  I thought they were 5V so any device it is using would need to be 5V tolerant, you can drop that requirement.  I mixed it up with my arduino.


    With so what I need is a digital compass that points north, gives degrees in 0-360 and fractional degrees.

  • 0 in reply to screamingtiger

    I don't know of any sensor IC that will provide you with this single output, usually they will provide raw magnetic flux density values that need to be post-processed into the output that you need.

    The example the URL that you mention provides three such values (three axes). These three values can be converted (your microcontroller or raspberry pi etc., would be used for that).

    There is a honeywell document with the relevant calculations (PDF document).

    There may already be RPI implementations since that sensor seems quite popular, but if there isn't, there is an Adafruit guide for the HCM5883 that (according to the screenshots)

    shows that their code translates to a heading. It is for Arduino but the code can be examined (and compared to the honeywell doc).

  • 0 in reply to shabaz

    A couple things:

     

    I bought 2 of these off ebay, one is bad!!!  That keeps happening to me!

    The other thing is that it does not work with the raspberry PI.  The arduino I am using now is 5V and it works with that, so apparently the pins need to be 5V?

     

    I used the adafruit example and it is giving me heading information as expected.  Thanks!  I am going to buy a real one from sparkfun and stop buy8ing knockoffs from ebay.

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  • 0 in reply to shabaz

    A couple things:

     

    I bought 2 of these off ebay, one is bad!!!  That keeps happening to me!

    The other thing is that it does not work with the raspberry PI.  The arduino I am using now is 5V and it works with that, so apparently the pins need to be 5V?

     

    I used the adafruit example and it is giving me heading information as expected.  Thanks!  I am going to buy a real one from sparkfun and stop buy8ing knockoffs from ebay.

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