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LIS331DLH accelerometer noise

jccraig

My board uses a PIC to interface using I2C with an ST accelerometer chip, LIS331DLH. I can read and write to registers okay, at least I can consistently R/W to the control registers for sure. The 8 bytes of X,Y,Z data are extremely noisy though. The low byte in each case is more or less random, the high byte appears to be approximately right, but with some jitter. I've tried different G ranges (2, 4, 8) and different sample rates (50, 100, 400 Hz) and I'm starting to pull my hair out. This is the second prototype board layout, using suggested capacitors etc. All boards act identical. It feels like I'm overlooking some detail in the programming, setup, initialization of the chip, perhaps a detail in my I2C routines, some sort of delay timing issue?  I monitor for data ready in the control register before reading. I'm rather at a loss right now.  Has anyone encountered similar issues with this chip, or can you offer any suggestions for things to check or to be aware of?  Thanks in advance for any help.

  • 0

    Its time to drag out the O'scope and look at what the output of the sensor is.. Ie find the noise.

    enjoy..

    Cris H.

     

    BTW I missed the part about you using a PIC.. check the specs. I think your cpu is tooo slow, and you are getting flooded.

  • 0

    John,

    I am wondering if you have found your noise problem? I am having a similar problem. I don't think that it is noise on my power because I have added caps, removed the debugger, etc. and the problem stays the same.

    I have used this part on a different board and it works fine, so although my code is slightly different, I am setting up the device the same. The design that works is a four layer board, while the broken one is 2. One thing

    I am considering is bad ground on the smt connections on the part. This is a hard thing to test, as there is no access to the under chip pads. Another possibility might be over temp during reflow.

  • 0

    Bruce, it turned out I needed to parse the bits better. I wasn't doing it right, but once I got it working right the "noise" went away.  On a related note, just before we went to our first manufacturing run we learned that ST Micro was replacing the LIS331DLH with a new chip, the LIS3DH.  The footprint is the same, but the internal registers vary some. I didn't have to change my board at all, but I did have to change my program to work with bits that were relocated to some different register locations.  They added some features, and a couple of the pins that weren't used before do have a purpose now, although they can be left tied to either low or high inputs.  The new chip is working well in my first product, which you can see at LucidBrake dot com if you want to check it out.

  • 0 in reply to jccraig

    John,

    Thanks for the information, I just started on this site, but I am thinking you should mark this as answered (by yourself). I checked out your product, great idea, simple, but it could save a life.

  • 0 in reply to jccraig

    John

    Very nice product.

     

    Its great to see technology helping to solve problems.

     

    Mark

  • 0 in reply to jccraig

    I am having noise problems with the LIS331DLH accelerometer also.  You state that you resolved the issue by parsing the bits better.  Could you explain your resolution how you parsed the acceleration data to reduce the noisy values? Do you have a code sample of correctly reading the 6 acceleration registers and turning them into valid acceleration data?

     

    Thanks