How to detect low battery in battery powered F683 circuit

[tekart]

Sorry pedja089,
I am having trouble understanding your very terse sentence. I would appreciate it if you could elaborate what you mean by this

Put Vdd as reference, take sample voltage, and, for larger Vdd ADC result will be lower, and for lower Vdd result will bi higher.

As I understand it, my A/D reading will always be defined by the voltage drop of the diode forward voltage drop and so will not change. Remember I only have the one A/D input to work with in my very simple circuit.

[mark_s]

Remember I only have the one A/D input to work with in my very simple circuit.

In that case you need a pic12f1822 or similar with an internal vref. The internal vref will work with the adc or comparator module.

[pedja089]

Connect one end of diode to ground, other to pic input and pull up resistor. Just like in schematic on page 8.
They suggesting to connect diode to ground thru mosfet to minimize current consumption. When mosfet is turned on, there is about 0,6V on pic input, and that voltage won't change.
You do not need to use mosfet, you can connect diode to ground permanently, or connect to output pin.

[tekart]

Connect one end of diode to ground, other to pic input and pull up resistor. Just like in schematic on page 8.
They suggesting to connect diode to ground thru mosfet to minimize current consumption. When mosfet is turned on, there is about 0,6V on pic input, and that voltage won't change.
You do not need to use mosfet, you can connect diode to ground permanently, or connect to output pin.

Yeah, I get the hardware side now - but how to implement this on the F683 with no Vref and one A/D? Am I missing something?

[pedja089]

You are missing...
There is Vref on every pic with ADC.
http://ww1.microchip.com/downloads/e...oc/41211d_.pdf Page 63 Bit VCFG(bit 6 of ADCON0)

[tekart]

OK, I see that now. So how do I select between the IO pin and the Vref source in code? No idea how that is done...
Still don't see how the resistor - diod junction will provide me with a reading that represents the changing Vbatt...

[mark_s]

I think the confusion between you and pedja089. The first way you need two external pins from the ADC or comparator.
One of the pins is tied to an external vref source and the other is your analog input. There is no voltage reference in the 683
VCFG bit or ADCON0.6 either connects the ADC vref(input) to VDD internally or connects it to and external (pin) reference you provide. .
The second way only one external pin is needed. (pic12F1822) has an internal reference diode which can be configured to be the vref source for the adc or comparator.

[tekart]

In that case you need a pic12f1822 or similar with an internal vref. The internal vref will work with the adc or comparator module.

OK, now that is helpful. I was not aware of an 8-pin chip with all those features and at a good price too! Now I'll just have to learn how to configure the internal Vref to get the results I need. Another slog through data sheets that can take hours!