>
>My electronics experience is strictly through ham radio, and we usually use
>just whatever happens to be in the junkbox  and seems to work.  But I would
>like to be a little bit more careful with the stuff I am designing now.  So
>could someone just tell me the rule of thumb on that?

Mostly, it depends on what you need to protect against.
For an empty input pin, you can go as low as zero ohms.
This will get you into trouble though if that pin is ever enabled as an output in the opposite direction.
Weak pullups are usually used where low power consumption is needed.
Strong pullups where high speed is needed.


>  And what is a "weak pull up" as described for some of the I/O pins in the datasheet?  I would assume "weak" means a fairly high resistance, but maybe not.

Yes, think of pullups in terms of current.
EMI can induce currents into your circuit, so the pullups need to be strong enough to overcome that induced current.

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