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What disadvantages can I run into when using weak internal pullups (100k) on microcontrollers? I'm wondering how susceptible lines (with only parasitic trace/component capacitance) become to EMI transients with these weak pullups.

Digital filtering with a 3-4ms window may decrease the odds of transients having an effect, but is there anything else to keep in mind on a professional PCB?

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You can always add an external pullup in parallel to decrease the impedance. –  Dave Tweed Feb 20 '13 at 17:08
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Sounds like you need a clamping circuit too, to prevent the controller inputs from getting damaged. A bit like this: zone.ni.com/images/reference/en-XX/help/370520K-01/… Long wires will inevitably introduce over- and undershoot, and your controller will not be too pleased with them. The internal pull up's are usually very weak and don't act like normal resistors (at the outside of the controller) anyway. –  jippie Feb 20 '13 at 17:21
    
The internal pullups would be used to skip external ones due to board size restrictions so puting externals defeats the original purpose. If the lines aren't very long then clamping would only benefit against really large transients capable of coupling damaging voltages or currents into the trace leading to the input pin. –  Steinar Feb 21 '13 at 10:08
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1 Answer

If this is a concern to you, then the only real answer is to model the end result. You have to understand how much coupling capacitance there is to an aggressor signal, and you have to understand how much swing is acceptable on the input pin before it causes problems.

While this sounds hard to do, usually if you use simple capacitor formulaes and ensure that you over estimate the coupling most rules of thumb indicate that there isn't anything that needs to be done. This is what probably informed the manufacturer to choice the size of PU that they did.

Kudos to you for thinking about these things, it's attention to details like this, that when you work it through give you a good background to do good designs.

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protected by W5VO Feb 24 '13 at 16:34

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