• Before starting calibration, the calibration registers (CLPS, CLP3, CLP2, CLP1,
CLP0, CLPX, and CLP9) must be cleared by writing 0x0 into them.
• Start ADC calibration by writing ADC_SC3 register with: CAL=1, AVGE=1,
AVGS=11.
• Wait for calibration to finish. This will be indicated by conversion complete flag
(COCO in ADC_SC1n).
• Now you can run ADC conversions with high accuracy in your application. Please
make sure to reconfigure the ADCK clock speed and reconfigure AVGE and AVGS
to your desired settings. (Maximum clock speed and no use of hardware averaging is
possible.)
The total calibration conversion time is: 12 * ( # of AVERAGE * [Sample time (sample
+1) + 1 cycle for hold + 34 cycles for compare phase]) + 1st conversion synchronization
(~5 ADC 5 IPG clocks).
For high accuracy of the ADC (as specified in data sheet) on your application board
(PCB), the following requirements should be met:
• Bypass caps between VREFH and VREFL. Suggested cap sizes: 1 nF, 100 nF, 10
μF.
• Place caps on PCB as close as possible to the device pins VREFH and VREFL.
• Bypass caps between VDDA and VSSA. Suggested cap sizes: 1 nF, 100 nF, 10 μF.
• Place caps on PCB as close as possible to the device pins VDDA and VSSA.
• Routing of VDDA, VSSA, VREFH, and VREFL on PCB:
• Low impedance between the bypass caps and the MCU pins.
• Keep routing distant from noisy signal routes like switching I/Os.
36.5.7 User-defined offset function
OFS is a two's-complement, left-justified register that contains the calibration-generated
offset error correction value.
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 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 36 Analog-to-Digital Converter (ADC)
Kinetis KE1xZ256 Sub-Family Reference Manual, Rev. 3, 07/2018
NXP Semiconductors
771
Содержание Kinetis KE1xZ256
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