On 28 Mar 2017 at 10:21, James Wages wrote:

> Brent,
>=20
> Thank you for sharing your thoughts.
>=20
> I am curious though what input impedance (effectively, "resistance")
> you yourself model example circuits upon.  1M-ohm? 500k-ohm? 2M-ohm?
> 5M-ohm?  I've read all many of personal opinions in various online
> forums which fall within that range, but no one is basing that on
> numerical calculations from what I can see.  Obviously, if we model
> a simply connection on a breadboard or computer sim without using a
> PIC, substitution a resistance instead, it can be very helpful,
> which is why I am curious what you use and why, and whether you vary
> that based on different PICs.=20

Hi James,

The short answer is that I seldom model the inputs. If I had need to I woul=
d look=20
very closely at the data sheet for the specific parts and get the answer fr=
om there.

In practice though, here's how I look at it and the assumptions made:

The internal pull-up is non-linear, but my memory (could be wrong) tells me=
 in that=20
case it is, very roughly, something like 75k Ohms or more. Or, I'm not usin=
g the=20
internal pull-up, in which case it's a high impedance CMOS input, equivalen=
t to=20
many many meg Ohms.

The only scenarios I can think of where one may reasonably rely on the inte=
rnal=20
pull-up alone is with relatively short traces on a PCB to say, a DIP switch=
, jumper=20
pins, or an open collector device. I do/have used them in such ways.

If wiring going off board (as is often the case with a single chip micro) t=
hen I will=20
want some more current flowing to give some margin against electrical noise=
.. I may=20
also want to increase the current to meet the minimum (wetting) current=20
specifications of say some switch or relay contacts.

The input impedance (or external pull-up) I desire then ends up being quite=
 low=20
compared to the input impedance ( or internal pull-up) of the PIC - so much=
 that it=20
becomes practically insignificant in the calculations.


--=20
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