internal pullups vs. external pullups

[davt]

Hi all

Is it unwise to use the weak internal pullups to detect switch changes when designing equipment to be used in an industrial environment?
Would external stronger pullup resistors be better guard against noise?
Or am I being to cautious!
Thanks.

Dave

[valemike]

Personally, i never have used internal pullups. Microchip and Atmel have similiar appnotes where they use internal pullups for keypad applications.

I always use external pullups anyways. That way, if a board change is needed, then i won't be confined to port_b.

It's not the pullups that will protect you against noise either. You'll need EMI filters on your inputs in some harsh cases. For example, UL has recommended us to use a 100pF capacitor, 56uH inductor, to act as an LC filter at the input. That blocks high frequency noise. Of course, you'll still need your current limiting resistor, pullup, and typical .01-.1uf capacitor for switch noise. You might not need so many discrete parts nowadays, since they do have ESD/EMI filter chips (e.g. Onsemi, Fairchild, etc.)

[Steve H]

Maxim makes some really nice fully protected switch debouncers. I have used these in the past and they are really great.

MAX6816, MAX6817, MAX6818
±15kV ESD-Protected, Single/Dual/Octal, CMOS Switch Debouncers

www.maxim-ic.com

Regards,

Steve H.

[SherpaDoug]

What sort of switch are you using? If you have a grounded metal box with a grounded metal switch handle going through it to say a toggle switch, you should not have any EMI problems.
On the other hand if you have twenty feet of wire going to a switch on the opposite side of an arc welder you are in EMI hell. Going with one of those Maxim chips is pretty safe, but may be overkill.
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[valemike]