To initiate calibration, the user sets SC3[CAL] and the calibration will automatically
begin if the SC2[ADTRG] is 0. If SC2[ADTRG] is 1, SC3[CAL] will not get set and
SC3[CALF] will be set. While calibration is active, no ADC register can be written and
no stop mode may be entered, or the calibration routine will be aborted causing
SC3[CAL] to clear and SC3[CALF] to set. At the end of a calibration sequence,
SC1n[COCO] will be set. SC1n[AIEN] can be used to allow an interrupt to occur at the
end of a calibration sequence. At the end of the calibration routine, if SC3[CALF] is not
set, the automatic calibration routine is completed successfully.
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.
7. Repeat the procedure for the minus-side gain calibration value.
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 and minus-side gain, and plus-side and minus-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.
Chapter 28 Analog-to-Digital Converter (ADC)
KL25 Sub-Family Reference Manual, Rev. 3, September 2012
Freescale Semiconductor, Inc. 495
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