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32.4.7 Calibration function
The ADC contains a self-calibration function that is required to achieve the specified
accuracy. Calibration must be run, or valid calibration values written, after any reset and
before a conversion is initiated. The calibration function sets the offset calibration value,
the minus-side calibration values, and the plus-side calibration values. The offset
calibration value is automatically stored in the ADC offset correction register (OFS), and
the plus-side and minus-side calibration values are automatically stored in the ADC plus-
side and minus-side calibration registers, CLPx and CLMx. The user must configure the
ADC correctly prior to calibration, and must generate the plus-side and minus-side gain
calibration results and store them in the ADC plus-side gain register (PG) after the
calibration function completes.
Prior to calibration, the user must configure the ADC's clock source and frequency, low
power configuration, voltage reference selection, sample time, and high speed
configuration according to the application's clock source availability and needs. If the
application uses the ADC in a wide variety of configurations, the configuration for which
the highest accuracy is required should be selected, or multiple calibrations can be done
for the different configurations. For best calibration results:
• Set hardware averaging to maximum, that is, SC3[AVGE]=1 and SC3[AVGS]=11
for an average of 32
• Set ADC clock frequency f
ADCK
less than or equal to 4 MHz
• V
REFH
=V
DDA
• Calibrate at nominal voltage and temperature
The input channel, conversion mode continuous function, compare function, resolution
mode, and differential/single-ended mode are all ignored during the calibration function.
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.
Chapter 32 Analog-to-Digital Converter (ADC)
K20 Sub-Family Reference Manual, Rev. 1.1, Dec 2012
Freescale Semiconductor, Inc.
Preliminary
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