Mesmic 2125 accelerometer

Don French

Now that I have my compass module working fairly well, I have a problem with the Mesmic 2125 accelerometer.· I got it to work perfectly with the example code·for the BS2, but on the Javelin I get nothing back but zero values.· I am only trying to read the X axis and am only interested in the raw value.· I thought this single line of code should do the trick:

·int value =· CPU.pulseIn(20000, CPU.pin7, true);

It appears to be wired up correctly and pins 3 and 4·are tied together and go to ground, so I don't know why I am not getting any results.· Could it be something about the CPU.pulseIn() method that is different than the BASIC PULSEIN?

Peter Verkaik

I think the time units are different, so you must change the count value

to reflect the same time.

regards peter

Don French

Thanks, Peter. I came to that very realization while walking to work today. I don't have the BASIC refernence here, but I don't seem to remember having to or even being able to set the timing in the PULSEIN operation. Not knowing what to set it at for the Javelin, I guess I will write a loop to find the range at which it returns a non-zero value and pick the center point of that range.

Peter Verkaik

A return value of 0 indicates no start edge was detected.

A standard BS2 has a 2 uS unit, the javelin has a 8.68 uS unit and may not

detect pulses smaller than 8.68 uS (the BS2 would detect those).

regards peter

Don French

Are you saying that it is might not be possible to use this module with the Javelin?

diafysal

Check out this code.
But if I remember it right there is an error in the code.
"findDutyFactor" in "Accelerometer.java" have the line:

return ( (50 * onTime) / (t2Time / 2) );

It can give wrong values (if I remeber it right).

Don French

Great stuff!· Thanks.·

Don French

When I got home from work I tried your code with no success. Both gForceX and gForceY are always -408. And both duty cycles are always -1. Maybe this is related to the bug in the code?

So, next I tried running a loop that sets the timeout value to every possible value between 0 and 32000. I never got anything but 0 for output on the X pin. (I was not interested in the Y pin and didn't have it connected.) But for grins I connected the Y pin and tried the the same loop with it pin and I got a non-zero value when timeout got to 300. So, this makes me think that the X pin on the Mesmic isn't working. But there was one more thing to try before returning it (can you do that?). And that was to change the Javelin I/O pin used for the pulsing the X pin. And guess what? It worked! It seems my Javelin I/O pin 7 is dead. (Which sucks, but at least my accelerometer module now works!)

automaton

The problem in the "findDutyFactor" method "Accelerometer.java" is a divide error. If you use 32bit integers you can solve this problem.
You can find the 32bit integer class here:
http://www.parallax.com/javelin/applications.asp#AN011

So to fix the error returned by the line "return ( (50 * onTime) / (t2Time / 2) );" You coud do the following with that entire method

public int findDutyFactor( char axis )
{
Int32 dutyfactor= new Int32(0);
int onTime = 0;

switch (axis)
{
case 'x': dutyfactor.set(0,findPulseWidth( txPin )*50);
break;

case 'y': dutyfactor.set(0,findPulseWidth( tyPin )*50);
break;

default: System.out.println("Error: Invalid Axis");
break;
}

dutyfactor.divide(t2Time/2);
return dutyfactor.low;
}

You will, of course, have to have the Int32 class from the above link and the import line below in the Accelerometer class
import stamp.math.Int32;

That should fix that error