How to detect low battery in battery powered F683 circuit

[tekart]

I do a lot of battery powered designs with li-po (3.7v) batteries - mostly using 8 pin chips like the F683. I am trying to figure out if there is a way to use an AD input - or comparator configuration to determine if the battery that is powering the chip is running low. A threshold of 3.2 V would be good. A resistor divider connected to the A/D input will not work because it will always read the resistor ratio voltage as a constant value as the battery voltage changes. I have been fooling around with a zener diode/resistor combination with no luck. I don't have any experience using the comparators inside PICs and wonder if there is some way to configure a comparator to detect a low battery threshold. Does anyone have an elegant solution for detecting a low battery condition? In my current design I only have AN0 available for battery sensing on the F683.

I really need a very low part count/low cost solution! No external comparators or circuits, is this practical?

[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.