How to detect low battery in battery powered F683 circuit

[mark_s]

Hello

This might give you some ideas

http://ww1.microchip.com/downloads/e...Doc/41215C.pdf

[tekart]

Yes, I saw that but the best circuit (on p.8) uses an external comparator. Not sure if that can be implemented with the INTERNAL one?

[pedja089]

Why not? It will work.
But you can only use R3 and D1 connected to ADC pin.
Put Vdd as reference, take sample voltage, and, for larger Vdd ADC result will be lower, and for lower Vdd result will bi higher.

[tekart]

I don't see how this is any different from putting a resistor divider on the ADC input. The problem is that the divided voltage will track Vdd and the A/D reading will always represent the resistor divider ratio (or resistor/diode drop ratio) and thus will maintain a consistent reading despite changing Vdd. Am I missing something here?

[pedja089]

Hey, where I was mention voltage divider???

But you can only use R3 and D1 connected to ADC pin.

That isn't voltage divider.
Biased D1 will produce almost constant voltage on ADC input pin. By keeping constant voltage on ADC input, and changing reference voltage(Vdd)

for larger Vdd ADC result will be lower, and for lower Vdd result will bi higher.

At least try to read response when you ask for help. That is all from me...

[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?

[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!