28.4.7 User-defined offset function
OFS contains the user-selected or calibration-generated offset error correction value. This
register is a 2’s complement, left-justified. The value in OFS is subtracted from the
conversion and the result is transferred into the result registers, Rn. If the result is greater
than the maximum or less than the minimum result value, it is forced to the appropriate
limit for the current mode of operation.
The formatting of the OFS is different from the data result register, Rn, to preserve the
resolution of the calibration value regardless of the conversion mode selected. Lower
order bits are ignored in lower resolution modes. For example, in 8-bit single-ended
mode, OFS[14:7] are subtracted from D[7:0]; OFS[15] indicates the sign (negative
numbers are effectively added to the result) and OFS[6:0] are ignored. The same bits are
used in 9-bit differential mode because OFS[15] indicates the sign bit, which maps to
D[8]. For 16-bit differential mode, OFS[15:0] are directly subtracted from the conversion
result data D[15:0]. In 16-bit single-ended mode, there is no field in the OFS
corresponding to the least significant result D[0], so odd values, such as -1 or +1, cannot
be subtracted from the result.
OFS is automatically set according to calibration requirements once the self-calibration
sequence is done, that is, SC3[CAL] is cleared. The user may write to OFS to override
the calibration result if desired. If the OFS is written by the user to a value that is
different from the calibration value, the ADC error specifications may not be met. Storing
the value generated by the calibration function in memory before overwriting with a user-
specified value is recommended.
Note
There is an effective limit to the values of offset that can be set
by the user. If the magnitude of the offset is too high, the results
of the conversions will cap off at the limits.
The offset calibration function may be employed by the user to remove application
offsets or DC bias values. OFS may be written with a number in 2's complement format
and this offset will be subtracted from the result, or hardware averaged value. To add an
offset, store the negative offset in 2's complement format and the effect will be an
addition. An offset correction that results in an out-of-range value will be forced to the
minimum or maximum value. The minimum value for single-ended conversions is
0x0000; for a differential conversion it is 0x8000.
To preserve accuracy, the calibrated offset value initially stored in OFS must be added to
the user-defined offset. For applications that may change the offset repeatedly during
operation, store the initial offset calibration value in flash so it can be recovered and
added to any user offset adjustment value and the sum stored in OFS.
Functional description
KL25 Sub-Family Reference Manual, Rev. 3, September 2012
496 Freescale Semiconductor, Inc.
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