To complete calibration, the user must generate the gain calibration values using the
following procedure:
1. Initialize or clear a 16-bit variable in RAM.
2. Add the plus-side calibration results CLP0, CLP1, CLP2, CLP3, CLP4, and CLPS to
the variable.
3. Divide the variable by two.
4. Set the MSB of the variable.
5. The previous two steps can be achieved by setting the carry bit, rotating to the right
through the carry bit on the high byte and again on the low byte.
6. Store the value in the plus-side gain calibration register PG.
When calibration is complete, the user may reconfigure and use the ADC as desired. A
second calibration may also be performed, if desired, by clearing and again setting
SC3[CAL].
Overall, the calibration routine may take as many as 14k ADCK cycles and 100 bus
cycles, depending on the results and the clock source chosen. For an 8 MHz clock source,
this length amounts to about 1.7 ms. To reduce this latency, the calibration values, which
are offset, plus-side gain, and plus-side calibration values, may be stored in flash memory
after an initial calibration and recovered prior to the first ADC conversion. This method
can reduce the calibration latency to 20 register store operations on all subsequent power,
reset, or Low-Power Stop mode recoveries.
25.4.7 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.
Chapter 25 Analog-to-Digital Converter (ADC)
KL02 Sub-Family Reference Manual, Rev. 2.1, July 2013
Freescale Semiconductor, Inc.
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