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jcal54321
Joined: 06 Mar 2009
Posts: 10
Location: Salinas, CA
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| ADXL345: Convert 16-bit Twos Complement to 10-bit Signed Int |
 Posted: Sat Jun 16, 2012 5:40 am |
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Hi All:
I was hoping I could get some help converting data bytes from the ADXL345 accelerometer that I am reading via I2C. The acceleration data is formatted into two bytes, say axDataHi and axDataLo, which are concatenated to form a 16-bit twos complement value. The actual acceleration data is a 10-bit value that is right justified with sign extension. If I am clear on how this works, the sign extension means that the leading bits of the axDataHi are padded with 6 bits, either 1 or 0, depending on the sign of the data.
Here is what I have tried, but I am not sure if this gives me the right result. accelIntArray[] has the six bytes for all three axes after a multi-byte read is accomplished. accelIntArray[0,2,4] is the low byte, and accelIntArray[1,3,5] is the high byte.
| Code: |
aX = ((int16) (accelIntArray[0] | accelIntArray[1]<<8));
aY = ((int16) (accelIntArray[2] | accelIntArray[3]<<8));
aZ = ((int16) (accelIntArray[4] | accelIntArray[5]<<8)); |
I have read through some of the forums and Wikipedia dealing with twos complement. I am still not really clear on what I need to do to convert the two bytes into a signed 10-bit integer. Any help would be greatly appreciated! Thank you. |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19539
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| Posted: Sun Jun 17, 2012 1:05 am |
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| Code: |
//First thing, aX, to aZ, need to be declared as _signed_ int16
signed int16 aX,aY,aZ;
aX=make16(accelIntArray[1],accelIntArray[0]);
aY=make16(accelIntArray[3],accelIntArray[2]);
aZ=make16(accelIntArray[5],accelIntArray[4]);
//CCS, has the 'make16' function, that allows you to put two bytes together
//using single byte moves, instead of having to convert to int16 first
//and rotate.
printf("%4.3fg in X\n\r",aX/256.0);
//Just display the X axis in g as a demo - assuming unit is in +/-2g mode
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Best Wishes |
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jcal54321
Joined: 06 Mar 2009
Posts: 10
Location: Salinas, CA
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| Posted: Mon Jun 18, 2012 6:12 pm |
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Thank you for the good advice.
V/R James C. |
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