NXP Semiconductors MPC5566, Reference Manual

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Enhanced Queued Analog-to-Digital Converter (eQADC)
MPC5566 Microcontroller Reference Manual, Rev. 2
19-90 Freescale Semiconductor
• The addition of two reduces the maximum quantization error of the ADC. Refer to
Section 19.5.6.3, “Quantization Error Reduction During Calibration
.”
Calibration constants GCC and OCC are determined by taking two samples of known reference voltages
and using these samples to calculate their values. For details and example about how to calculate the
calibration constants and use them in result calibration refer to
Section 19.5.6, “ADC Result Calibration .”
After it is calculated, the GCC coefficients are stored in ADC0_GCCR and ADC1_GCCR (see
Section 19.3.3.4, “ADCn Gain Calibration Constant Registers (ADC0_GCCR and ADC1_GCCR) ”) and
the OCC coefficients in ADC0_OCCR and ADC1_OCCR (see Section 19.3.3.5, “ADCn Offset
Calibration Constant Registers (ADC0_OCCR and ADC1_OCCR) ”) from where their values are fed to
the MAC unit. Since the analog characteristics of each on-chip ADC differs, each ADC has an independent
pair of calibration constants.
A conversion result is calibrated according to the status of CAL bit in the command that initiated the
conversion. If the CAL bit is asserted, the eQADC automatically calculates the calibrated result before
sending the result to the appropriate RFIFO. If the CAL bit is negated, the result is not calibrated, it
bypasses the calibration hardware, and is directly sent to the appropriate RFIFO.
19.4.5.4.2 MAC Unit and Operand Data Format
The MAC unit diagram is shown in Figure 19-49 . Each on-chip ADC has a separate MAC unit to calibrate
its conversion results.
Figure 19-49. MAC Unit Diagram
The OCC n operand is a 14-bit signed value and it is the upper 14 bits of the value stored in ADC0_OCCR
and ADC1_OCCR. The RAW_RES operand is the raw uncalibrated result, and it is a direct output from
the on-chip ADCs.
The GCC n operand is a 15-bit fixed point unsigned value, and it is the upper 15 bits of the value stored in
ADC0_GCCR and ADC1_GCCR. The GCC is expressed in the GCC_INT.GCC_FRAC binary format.
The integer part of the GCC (GCC_INT = GCC[1]) contains a single binary digit while its fractional part
(GCC_FRAC = GCC[2:15]) contains 14 bits. Refer to
Figure 19-50 for more information. The gain
constant equivalent decimal value ranges from 0 to 1.999938..., as shown in Table 19-49 . Two is always
added to the MAC output: see Section 19.5.6.3, “Quantization Error Reduction During Calibration .
CAL_RES output is the calibrated result, and it is a 14-bit unsigned value. CAL_RES is truncated to
0x3FFF, in case of a overflow, and to 0x0000, in case of an underflow.
MAC Unit
Offset Calibration Constant (OCC n )
(14-bit signed value from ADC n _OCCR)
Raw Uncalibrated Result (RAW_RES)
(12-bit unsigned value)
Gain Calibration Constant (GCC n )
(15-bit fixed point unsigned value
from ADC n _GCCR)
Calibrated Result (CAL_RES)
(14-bit unsigned value)
2