Best Answer Mario Vernari, 25 August 2013 - 05:33 AM
Michael, that's not surprising at all!
Any Netduino (Plus) 2 input does not require any current, or -better- that's unreadable with a normal multimeter. Now, since the Ohm's law says that V=R*I, a "zero" current yields a "zero" voltage drop, even when the resistance is high.
A resistor across two pins is always a good bet, not just because -as explained- an input will suck current, but you may wrongly set the pin as an output, and that could lead to an improper working state and a lot of stress for the outputs.
To be clear, I'd say that a 180 Ohms resistor is the *minimum*, but I would avoid to consider a borderline point.
Go to the shop and buy a couple dozen of 10k Ohms resistors: that's a midrange value which adapts to a lot of contexts. You'll never feel to have trashed your money!
BTW, when you switch a multimeter for current measurements, it does expose a pretty low resistance across its leads: it could span from some Ohms to some hundreds/thousand, depending on the actual range displayed/switched. The lower is the full-scale range, the higher is the resistor. If you use the "special" 10A current loop, the resistor is probably below 1 Ohm.
So, no wonder on the equivalent effect of the multimeter in place of a "real" resistor: you probably wired an equivalent resistor lower than 180 Ohms!
Bear in mind that, yes, a resistor will protect the circuit/chip from erroneous settings/wiring, but even in case of short the chip should be designed to resist for a indefinite time. In other words, your errors hardly kill the Netduino, but won't lead it happy as well.
Finally, a word about the inputs.
It's a very good practice avoiding leaving inputs floating, that is without any "strong reference". That's because an input without some "hook" require a very little energy to swing high-and-low, keeps strange levels (between low and high), and excessive current consumption as well.
When an input is not used (I mean wired), you should choose among one of these:
- activate the internal pull-up, if that's an available feature;
- place your own pull-up externally, by using a "famous" 10k Ohms resistor (of course, one resistor for each pin);
- set the pin as an output, and leave it open.
Note that the options are ordered by preference, whereas the latest is the most undesirable.
Hope it helps.
Cheers
Go to the full post