Hi,

As some other threads around the forum about battery monitoring, I want to setup a battery monitoring system based on MICROCHIP's AN1072 "Measuring VDD Using the 0.6V Reference".

In short, it says:
"To measure VDD, VDD should be selected as the reference to the ADC via VCFG, and the 0.6V reference selected as the input using the channel select bits, CHS<3:0>. A measurement of the 0.6V input is taken with the ADC, and the result represents 0.6 Volts as a percentage of VDD. As VDD increases, the resulting number will decrease and vice versa. This yields a direct “1/x” relationship between V DD and the produced digital value as seen in Equation 1. In short, given a specific VDD, the digital value is always the same. Working backwards, if the digital value is known, VDD may be calculated."

Name: 2015-09-19 08_51_49-MICROCHIP_AN1072 Measuring VDD using Vref.pdf - [01072A.book] - SumatraPDF.png Views: 1865 Size: 50.5 KB

It tells you what you have to do and even how. Now guess: I can't make it work

I have a circuit using a 16F690 (great! same one as used in the AN!). For the test, the circuit is connected to my variable power-supply so I can simulate a low battery condition (i.e. from 6VDC, I go down to 5,5VDC*).

According to the AN1072, the ADCON0 register has been set to %10110101.

Now when I vary the voltage on the power-supply, I get always the same ADC reading (no value change at all).

What am I doing wrong?

NB*: Yes, I'm using a LDO voltage regulator so I could use another way to check the battery's condition (i.e. comparator or resistor-divider)...but I want to have a go with the AN1072