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| Fremantle provides an accelerometer API. Currently<ref>[https://bugs.maemo.org/show_bug.cgi?id=4724 #4724] ''Lack of official documentation on how to use the accelerometer''</ref> there are two interfaces available:
| | #REDIRECT [[N900 accelerometer]] |
| # D-Bus | |
| # sysfs
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| See also the [http://talk.maemo.org/showthread.php?p=288990 related thread] at talk.maemo.org.
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| == D-Bus ==
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| Thomas Thurman ([http://twitter.com/marnanel marnanel]) has put together a simple demo of an application using accelerometers using the D-Bus interface. You can find sources and .deb up at http://people.collabora.co.uk/~tthurman/sandcastle/
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| == sysfs ==
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| Another way is to use the sysfs file information:
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| /sys/class/i2c-adapter/i2c-3/3-001d/coord
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| When reading that file you get 3 values X, Y and Z (provided on one line, separated by white space). Values are in mG (milli G). 1000 = 1 G
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| {| class="wikitable"
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| |-
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| ! Position
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| ! X
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| ! Y
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| ! Z
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| |-
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| | Lying on table (back down)
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| | 0
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| | 0
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| | -1000
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| |-
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| | Lying on table (face down)
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| | 0
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| | 0
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| | 1000
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| |-
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| | Sitting on table (bottom edge down)
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| | 0
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| | -1000
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| | 0
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| |-
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| | Sitting on table (right edge down)
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| | -1000
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| | 0
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| | 0
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| |-
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| | Sitting on table (left edge down)
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| | 1000
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| | 0
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| | 0
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| |-
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| | Bottom right corner down (approx.)
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| | -500
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| | -500
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| | 0
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| |}
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| These are theoretical values. In real life your mileage will vary.
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| An example can be found at: [http://git.zimmerle.org/?p=inclinometer.git;a=summary]
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| == Using the data ==
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| The ''X'' and ''Y'' values can be used to calculate<ref>Tom Pycke, ''[http://tom.pycke.be/mav/69/accelerometer-to-attitude Accelerometer to pitch and roll]''</ref> the roll (that is, clockwise rotation) using the ''atan2'' function (note the inverted sign of ''y''):
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| angle_in_radians = '''atan2'''(''x'', -''y'')
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| Similar, ''Y'' and ''Z'' can be used to calculate the pitch:
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| angle_in_radians = '''atan2'''(''y'', -''z'')
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| == Python ==
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| Using the sysfs interface:
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| <pre>
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| def get_rotation():
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| f = open("/sys/class/i2c-adapter/i2c-3/3-001d/coord", 'r' )
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| coords = [int(w) for w in f.readline().split()]
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| f.close()
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| return coords
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| </pre>
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| == Smoothed C interface ==
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| This is a C function which returns smooth values from the accelerometer.
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| Designed for applications running at >=25fps (otherwise it lags)
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| (GPL code extracted from libliqbase: liqaccel.c)
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| <pre>
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| static int ocnt=0;
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| static int oax=0;
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| static int oay=0;
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| static int oaz=0;
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|
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| static const char *accel_filename = "/sys/class/i2c-adapter/i2c-3/3-001d/coord";
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| int liqaccel_read(int *ax,int *ay,int *az)
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| {
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| FILE *fd;
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| int rs;
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| fd = fopen(accel_filename, "r");
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| if(fd==NULL){ liqapp_log("liqaccel, cannot open for reading"); return -1;}
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| rs=fscanf((FILE*) fd,"%i %i %i",ax,ay,az);
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| fclose(fd);
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| if(rs != 3){ liqapp_log("liqaccel, cannot read information"); return -2;}
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| int bx=*ax;
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| int by=*ay;
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| int bz=*az;
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| if(ocnt>0)
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| {
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| *ax=oax+(bx-oax)*0.1;
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| *ay=oay+(by-oay)*0.1;
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| *az=oaz+(bz-oaz)*0.1;
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| }
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| oax=*ax;
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| oay=*ay;
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| oaz=*az;
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| ocnt++;
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| return 0;
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| }
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| </pre>
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| == References ==
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| <small>
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| <references/>
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| </small>
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| [[Category:Development]][[Category:Fremantle]]
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