PIC16F1503 Fixed Voltage Reference

[Aussie Barry]

Hi Richard,

Yes, I see that now but contradictory to this diagram, the datasheet also states that the output of the FVR can be configured to supply a reference voltage to the ADC input channel:



and this is supported in the FRVCON register (bits 1-0) regarding the relevant ADC FVR buffer gain:



Which one is to be believed?

Cheers
Barry
VK2XBP

[richard]

it could be worded better. the hight-lighted word "reference" could be removed to better reflect reality

The Fixed Voltage Reference (FVR) is a stable voltage
reference, independent of VDD, with a nominal output
level (VFVR) of 1.024V. The output of the FVR can be
configured to supply a reference voltage to the
following:
• ADC input channel
• Comparator positive input
• Comparator negative input
The FVR can be enabled by setting the FVREN bit of
the FVRCON register

[Aussie Barry]

the exact same wording is used in the PIC16F1829 datasheet yet that device DOES have the option to set the ADC Vref+ to the internal FVR module (ADPREF<1:0> = 11)

Some consistency would be a good thing....

Cheers
Barry
VK2XBP

[towlerg]

Come on guy, Microchip can't even decide what to call basic registers. Hay hoo.

[mpgmike]

I ran into this exact issue with this exact PIC! I swapped over to a PIC16F18323. I wanted to monitor battery voltage. If input voltage were to start dropping with ADC referencing Vdd, I may never see a dying battery until it was too late. Referencing ADC to 2.048 volts with a voltage divider on the input made it more precise. I think that's similar to what you are trying to do.

[Aussie Barry]

That is EXACTLY what I am trying to do!
I have looked at PIC16F1703, PIC16F1823 and the one you recommended, PIC16F18323 - all are pin compatible and have the ability to set the FVR as the reference voltage for the ADC.

Cheers
Barry
VK2XBP

[richard]

if all you need to do is calculate vdd then
connect adc to fvr and measure fvr count

vdd = 1024*fvr_const/fvr_count

fvr_const would be very close to 1.000 and could be determined using a calibrated power supply if required