-----Original Message-----
>From: IVP <joecolquitt@clear.net.nz>
>Sent: Apr 3, 2013 4:40 PM
>To: "Microcontroller discussion list - Public." <piclist@mit.edu>
>Subject: Re: [PIC] Pull Up Resistors
>
>> Is there a way to compute the optimum value of pull up (or
>> pull down) resistors on PIC digital inputs?
>
>Hi Allen,
>
>yes and no, selection is going to depend a bit on the circuit. In theory
>the value will have to be between a value which overcomes the pin
>leakage current (1uA) by some margin and a value which does not
>let the pin exceed Imax (20 or 25mA)
>
>In practice though, a pull-up of several hundred k could leave the
>pin susceptible to noise and a pull-up of a few hundred is going to
>be wasteful of power
>
>However, there is a guide in the datasheets
>
>PortB pull-ups are often defined as 400uA when Vpin =3D Vss.
>Assuming Vcc =3D 5V, this equates to a resistance of
>

This was an interesting way of explaining PortB pull-ups:
   http://www.microchip.com/forums/m251782.aspx

"don't think of it as a resistor, it's more likely to be a small current so=
urce. "

Microchip never refers to it as a resistor.
    http://ww1.microchip.com/downloads/en/devicedoc/31009a.pdf

"Each of the PORTB pins has a weak internal pull-up."



>V/I =3D R or 5,000,000uV/400uA =3D 12500 ohms
>
>So that's what Microchip has decided is a generally useable pull-up
>
>Joe
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