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Confused by FVR and ADC2

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crosland
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2020/11/21 10:25:57 (permalink)
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Confused by FVR and ADC2

PIC18F26K83 MPLABX 5.35 XC8 2.1
 
I have a design in two different PCBs. One is 5V powered, the other 3.3V. The same external circuitry has always worked fine with earlier 10-bit ADC PICs.
 
On both the ADC2 module (in basic mode) is reading much higher than it should.
 
I tried to setup a test using the internal FVR. 
 
Figure 35.1 shows FVR1 connected to the ADC and FVR 2 connected to the ADC and other modules.
 
This led me to believe I could simultaneously use the FVR as +ve reference and channel i/p, but is this really the case?
 
Register 35-1 describes FVR1 as the comparator FVR and FVR2 as the ADC FVR, in conflict with Fig 35.1
 
Figure 37.1 shows FVR1 used as +ve reference for the ADC but does not specify which FVR is connected to the input mux.
 
I setup FVR1 for 2.048V and FVR2 for 1.024V. When the ADC input is switched to FVR it reads 4095 counts. This suggests only one FVR (FVR1) is available to the ADC.
 
Does anyone know what is actually possible?
 
 
#1

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    crosland
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    [Solved] Re: Confused by FVR and ADC2 2020/11/21 11:26:27 (permalink)
    +2 (2)
    School boy error - left weak pull-ups enabled from a previous project that affected ADC readings.
     
    So now I don't care about how the FVR works :)
     
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