I2C Comm with Digital Compass HMC6352

[jihlein]

This is my first post to the group.

I'm designing a heading hold autopilot for a free flight glider. I'm basing it on a PIC16F88, and using an HMC6352 digital compass (http://www.sparkfun.com/datasheets/C...ts/HMC6352.pdf).

I've never played with I2C before, and figured that I would try something simple first, like reading an EEPROM register in the compass module. My problem is that no matter what I do, I seem to always read back a value that is exactly 1/2 of what it should be.

So then I tried writting a value to a RAM address and reading it back. Again, same issue, the value read back is 1/2 the value written.

I'm using PIC BASIC PRO, on a LABX18 development board. Here's the code I'm using for the EEPROM read, which is modified from some of the example programs:

Define  LCD_DREG        PORTA
Define  LCD_DBIT        0
Define  LCD_RSREG       PORTB
Define  LCD_RSBIT       6
Define  LCD_EREG        PORTB
Define  LCD_EBIT        3

' Define I2C Interface

SDA    Var    PORTB.1                    ' Data pin
SCL    Var    PORTB.4                    ' Clock pin

cmd  Var Byte
addr Var Byte
cont Var Byte

    ANSEL = 0                      ' PORTA is digital
    CMCON = 7                      ' PORTA is digital

    Pause 100                    ' Wait for LCD to start up
        
loop1:
    pauseus 1000
    cmd = $72                    ' Read from compass EEPROM
    addr= $00                    ' EPROM address $00
    I2CWRITE SDA, SCL, $42, [cmd, addr], nowrite

    pauseus 1000
     I2CREAD SDA, SCL, $42, [cont], noread
    Lcdout $fe, 1, "Data: ", cont
    
    goto loop1
    
    End

nowrite:
    Lcdout $fe, 1, "Write Err"
    goto nowrite

noread:
    Lcdout $fe, 1, "Read Err"
    goto noread

If anyone can point me to the error of my ways, it would be greatly appreciated.

John Ihlein
Albuqueruqe, NM

[skimask]

I'm fairly you don't have the I2C command syntax's correct.
Double check the PBP manual and see what you come up with.

[jihlein]

Skimask,

The manual shows the addr parameter as option. Since this isn't an EEPROM, and from looking at the HMC6352 data sheet, I figured that the commands should be sent as data arguements.

I can try sending what I'm calling cmd as the address argument:

I2CWRITE SDA, SCL, $42, [cmd, addr], nowrite

becomes

I2CWRITE, SDA, SCL, $42, cmd, [addr], nowrite

later today. It does seem like my write commands are somewhat working, however. If I change the commands to write and read RAM rather than just read EEPROM, I can see the RAM data change with the read command, it's just off by 1/2.

Looking at how the module responds to commands, I'm not sure how I would restructure the I2CREAD command. Maybe it needs a dummy addr argument??

At the moment I can't tell if the issue is on the write or the read side, but since I've tried to read known values from EEPROM and they are off by 1/2, I tend to think the read side is where my problem lies. I'm wondering if it's a data type issue, although the documentation says the data will be transmitted in bytes.

Thanks,

John

[skimask]

The manual shows the addr parameter as option. Since this isn't an EEPROM, and from looking at the HMC6352 data sheet, I figured that the commands should be sent as data arguements.

Yep, I got that.
But I still don't think they're right for what you want.
And is the module getting the 70uSec delay it requires after requesting the data from the eeprom?

[JD123]

Values that come in as 1/2 what they should have been shifted right one bit. You are loosing the LSB somewhere? I don't know the in's and out's of exactly what PBP does when shifting in I2C, but it might be that PBP is expecting one more bit. Does that compass send out 8 bits... or does it send out 7? Maybe it's sending out 9?

Anywho, it's a byte shifted right... some where, some how.

Does this from the datsheet have anything to do with your problem?

"Negative binary values will be in two’s complement form"

[skimask]

Values that come in as 1/2 what they should have been shifted right one bit.

Or maybe...
North 360 / 2 = 180 South ..... 1/2 the value
Are those values always exactly 1/2? Or are they just close to 1/2...

[jihlein]

It sure does look like a byte shift. For reading EEPROM/RAM locations, I believe it is just sending back an 8 bit byte. If you choose to read raw magnetometer data rather than computed heading, that's where the 2's complement comes into play. I haven't got that far yet, and FWIW, plan to read computed heading. Thought it would be a good simple test to just try to read an write some simple values to the unit before trying to take measurements....

The delay I put in is consdierbaly longer than the delays mentioned in the data sheet. From my understanding og the I2C protocol, the extra wait time should not be a factor. Whether or not it "bugs" the compass processing I can't see. Can reduce the wait time to the recommended 70 usecs and see if that changes anything.

If else fails, I'll take it to work and watch the transaction on a digital storage scope and see what's occuring.