about PIC used for detecting low and high supply voltages

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I need to use the PIC microcontroller for detecting the low voltage and high voltage levels, i.e. low voltage means less 2.5v (from the supply voltage for PIC ) and high volt means greater the supply voltage. So is it possible to use PIC for detecting such voltage level since I need to use this voltage as a supply voltage also ? I thought that the reference level changes if the suppy voltage changes, so I cannot be able to use.
In short, I need to activate an o/p if voltage less than 2.5v from, supply and another o/p if voltage is greater than 5v where the controller uses this as main supply.

If yes can anybody explain wtih example program.

[/dev/human]

I think you can do that if you have a stable reference voltage in your hardware. Then you need to measure your supply voltage using an appropriate voltage divider and check for high enough or too low voltage.

There are other principles working as well but all I can think off right now are requiring a reference voltage. This is key in my oppinion.

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is it possible to set the reference in PIC itself? is the reference affect, if the supply voltage drops?


how can i make reference standard in PIC ie if supply volt changes it does not affect the reference

[/dev/human]

Which PIC are you using (but I don't know whether there is a PIC having a stable reference)?

Depending on your required accuracy and the supply voltage variation you can either choose simple (zener-) diodes with a current limiting resistor or a so-called "voltage reference" like e.g. LT1009
http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1154,C1002,C1223,P1218

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eight PIN PIC if the supply voltage falls , the reference in PIC affect? or not


i need to detech the low voltage (supply voltage to the PIC) without affecting the circuit

[/dev/human]

Without the number I can't tell for sure. But 8 pin, I don't think so.

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PIC 16F675 and PIC 16F729

[Ttelmah]

Not really possible without some external parts (Vref and divider). What about considering one of the 8 pin power supply monitoring IC's instead?.
Not obvious from the description, how the PIC is powered?. If it has it's own 5v supply, then this can also be used as the 'reference'. It sounds as if the PIC supply itself is not guaranteed, in which case, you cannot have it reliably operating outputs...
You can get very 'close', with something like the PIC12F636, since this has two internal comparators, and the addition of a single external voltage reference would make most of this fairly simple. It also does have a PLVD module, that would allow an output to be operated at 2.3v, but not 2.5v.
Remember also, that a low power PIC, running at low speed, can be run for days, off a simple capacitor supply.
Simplest thing really, would just be two hardware comparators, one small zener, and a few resistors.... Even this though won't work without power.

Best Wishes

[/dev/human]

Could you pleas check on the number again? 16F675 is not existing on the Microchip website. Neither is 16F729.

Should it be 12F675 (which sounds reasonable) it does not offer a reference but you may set the internal reference input to you voltage supply and check with one ADC an external reference. Because then you will see the external reference voltage rise with falling supply voltage. The limits are up to you.