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How to calibrate ADC readings of MPC5643L in Volt?

Question asked by Peter Vranken on Dec 30, 2017
Latest reply on Jan 4, 2018 by Peter Vranken

For the MPC5643L, analog to digital conversion, I don't find a clear statement about the reference voltage the ADC uses and which to relate the digital result register readings to:

I understand the MCU reference manual such that the analog supply voltage is the reference voltage at the same time and that this voltage can be chosen as 3.3 V or 5 V. If so, the reference voltage would depend on the external circuitry and for my TRK-USB-MPC5643L it would be 3.3 V.

On the other hand mentions the reference manual the internal analog source VREG_1.2V that is connected to channel 10 of boths ADCs. No further explanation is given on this signal source. The name indicates that it is a constant voltage source of 1.2 V. On my boards this signal is however measured as 1.3 V if I assume the reference voltage as said in the previous paragraph, i.e. as 3.3 V.

Is it correct that signal VREG_1.2V, ADC channel 10, should read as 1.2 V? If so, is it intended to relate all other channels to this reading, like: 


    const float factor = 1.2f / (float)(ADC_0.CDR[10].R & 0xfff0);

    float u0 = factor * (float)(ADC_0.CDR[0].R & 0xfff0)
          , u1 = factor * (float)(ADC_0.CDR[1].R & 0xfff0)

          , ...
          , u8 = factor * (float)(ADC_0.CDR[8].R & 0xfff0);

Is this the way to calibrate the ADC readings in Volt?