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weak pull-ups in PIC µC ... ???

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donkey41

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hello there,

can anyone explain me, what "weak pullups" on PIC µC do mean ?
i have very basic understanding of pullups, but i cant understand its use
in PIC. what will happen if i leave them disabled or waht will happen if i enable them ?

thanks
 

The datasheet of a 16F877 says weak pullup current 50 to 400 uA at 5V which means the weak pullup can range from 12K5 to 100K.

I often use them if I have to connect keys to a pic so I do not need any external resistor to +5V.

If you do not enable the weak pullups the pin is floating if you use it as an input which means that you will have to use an external pullup.

best regards
 

it means, if i leave a pin's pull-up disable and use it as an input without and external resistor .... it can damage that pin/port ?
 

No nothing gets damaged but you can not read the correct input level as the pin is floating.

It means that you could read a wrong pin level if you do not use a pullup whenever you want to use a pin as an input.

best regards
 

You can use weak pullups enabled to read from keys. You will get lower noise pickup on long leads if you use lower external values, around 2k2 - 4k7 is about right. Choice is yours, will probably be ok on internal.
 

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