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Enhanced Queued Analog-to-Digital Converter (EQADC)
MPC5644A Microcontroller Reference Manual, Rev. 6
Freescale Semiconductor
1165
25.7.6.1
MAC Configuration Procedure
The following steps illustrate how to configure the calibration hardware, namely, determining the values
of the gain and offset calibration constants, and the writing of these constants to the calibration registers.
The procedure below should be performed for ADC0 and for ADC1.
1. Convert channel 44 with a command that has its CAL bit negated and obtain the raw, uncalibrated
result for 25%VREF (RAW_RES
25%VREF
).
2. Convert channel 43 with a command that has its CAL bit negated and obtain the raw, uncalibrated
result for 75%VREF (RAW_RES
75%VREF
).
3. Since the expected values for the conversion of these voltages are known (CAL_RES
25%VREF
and
CAL_RES
75%VREF
), GCC and OCC values can be calculated from equations
using these values, and the ones determined in steps 1 and 2.
4. Reformat GCC and OCC to the proper data formats as specified in
. GCC is an unsigned 15-bit fixed point value and OCC is a signed 14-bit
value.
5. Write GCC value to
Section 25.5.3.4, ADC0/1 Gain Calibration Constant Registers
Section 25.5.3.5, ADC0/1 Offset
Calibration Constant Registers (ADC0_OCCR and ADC1_OCCR)
, using write configuration
commands.
25.7.6.2
Example
The raw results obtained when sampling reference voltages 25%VREF and 75%VREF were, respectively,
3798 and 11592. The results that should have been obtained from the conversion of these reference
voltages are, respectively, 4096 and 12288. Therefore, using equations
,
the gain and offset calibration constants are:
GCC=(12288-4096)/(11592-3798) = 1.05106492-> 1.05102539 = 0x4388
OCC=12288 - 1.05106492*11592 - 2 = 102.06-> 102 = 0x0066
shows, for this particular case, examples of how the result values change according to GCC
and OCC when result calibration is executed (CAL=1) and when it is not (CAL=0).
Table 25-77. Calibration example
Input Voltage
Raw result (CAL=0)
Calibrated result (CAL=1)
Hexadecimal
Decimal
Hexadecimal
Decimal
25% VREF
1
1
For accurate calibration, the 25% VREF channel must be converted using the Long Sample Time (LST)
setting for either 64 or 128 ADC sample cycles in the ADC Conversion Command Message (LST = 0b10
or 0b11).
0x0ED6
3798
0x1000
4095.794
75% VREF
0x2D48
11592
0x3000
12287.486
Summary of Contents for MPC5644A
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