AN3745
Rev 0, 07/2008
Freescale Semiconductor
Application Note
How to Calibrate 0g Offset for the
MMA745xL Family
The offset can be calibrated by storing the offset values in
the designated offset drift registers $10 to $15 in the
accelerometer. These values will be stored here until the part
loses power. It is a very simple to store these values written to
the registers in the memory of a microcontroller, if used in
conjunction with the sensor. This will provide automatic
calibration of the sensor each time the sensor is turned back
on.
In order to calibrate the MMA745xL 0g offset, the
predetermined digital offset values should be subtracted from
the reading of the actual digital sensing values. The following
procedure is a recommendation for how this can be
accomplished:
1. After power up, set up the “Mode Control Register”
(Register $16) to be in “measurement mode” by writing
$05 into Register $16. Then read the X, Y and Z offset
values from the Registers $00-$08. The first 6 registers
of the 9 are 10-bit XYZ output values: LSB, first; MSB,
second. Please verify with the data sheet for detailed
register information.
2. In this step, the offset compensation is calculated to
shift the offset to zero.
For example, if the 0g offset is calibrated sitting flat (where
X = 0g, Y = 0g and Z = +1g) in 2g mode (64LSB/g sensitivity),
the outputs from Registers $00-$08 are the following: X = -30,
Y = -20, Z = +20. In this case,
•
X must be shifted by +30 to get X back to zero
•
Y must be shifted by +20 to get to zero
•
Z must be shifted by (+64-20) = +44 to get to +64
(since Z is in the +1g orientation)
3. These compensation values can be written in
hexadecimal into the “Offset Drift Registers” $10-$15.
The Offset Drift Registers require each value to be
½ LSB, therefore the calibration values calculated in
Step 2 must be multiplied by two. Note that there will still
be a bit of offset shift, and you may need to multiply by
a bit more than two to exactly subtract the offset.
© Freescale Semiconductor, Inc., 2008. All rights reserved.
Therefore, for this example, you should:
•
Write +60 (3C Hex) into the X drift Register $10
•
Write +40 (28 Hex) into the Y drift Register $12
•
Write +88 (2C Hex) into the Z drift Register, $14
In the example used above, the compensation values were
all positive values. If the compensation requires negative
values, remember that 2’s complement is always used in
hexadecimal for storing the signed value. If you are using
10-bit mode, and the calibrated values are greater than 8 bits,
then there is another register for up to 3 more bits:
•
Register $10 and $11 are for X
•
Register $12 and $13 are for Y
•
Register $14 and $15 are for Z
These registers follow signed byte data using 2’s
complement. Reading or writing low byte (X, Y or Z) OUTL
latches high byte data (X, Y, or Z) OUTH to allow coherent
10-bit reads or writes. (X, Y, or Z) OUTH should be
read/written directly following (X, Y or Z) OUTL.
4. After this compensation process is complete, you can
continue to modify the values by overcompensating
until you get the final output to be right at 0 for X and Y
and at 64 for Z. Note this is an iterative process.
5. Now the calibrated values are stored in the “Offset Drift
Registers”. To avoid the values being erased when the
power is turned off, it is recommended to store these
values in flash or a nonvolatile memory in the main
processor or external to the processor. It is also
recommended to include a short start-up sequence to
write the compensation values stored in flash or
nonvolatile memory to the registers $10-$15 in the
accelerometer on start-up. If a microcontroller is used,
a recursive program is available to autocalibrate the
device.
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AN3745
Rev. 0
07/2008
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