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.
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].
Functional description
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
802 NXP Semiconductors
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