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Microcontroller pin goes high when its Vcc is below "Vcc ON"

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Flyback

Well-Known Member
Hello,

We implemented a SAM3N00B microcontroller, and one of its pins switchs on an NPN which turns a battery charger current source OFF.

Unfortunately, when the microcontroller has just 1V on its Vcc pin, it turns this BJT ON and this unfortunately turns OFF the charger current source.
-this behaviour is not wanted.
-when the microcontroller's Vcc voltage is below the start up voltage of the microcontroller we do not want the micro to turn ON the NPN.

Anyway, its happening, so we need some component or circuit which is like a switch, and this switch must only apply volts to the Vcc of the microcontroller when the voltage is 3V. -then the micro won't be able to switch the NPN ON when the micro's Vcc is just 1V, because the micro will never have just 1V on it.

Does such a component exist? (it would probably be composed of an internal reference and a high side FET which turns ON when the rail voltage gets to 3V.

The SAM3N00B is not supposed to start up until its Vcc is 1.62V, so when the Vcc is just 1V, it must be some strange behaviour thats being exhibited....it couldnt be anything to do with the code, because the micro is programmed to start up and keep the NPN switched OFF at first

I thought a microcontroller's pins were "high impedance" until the full Vcc voltage was present on the Vcc pin?

SAM3N00B datasheet
https://www.electro-tech-online.com/custompdfs/2013/03/doc11011.pdf

Anyway, do you know of the above described "switch" component? (i could make one from a FET, a reference and a comparator, but really need a small integrated solution.)
 
Put a resistor ont he NPN transistor, between B and E. That will combine with the base resistor you already have to make a voltage divider, this will hold the NPN transistor fully OFF until the micro's PIN voltage >2.5v.

So now the transistor can only be turned on when the micro is at operating voltage and outputting a proper HI signal.
 
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