People, see this AN from ATMEL :
[urlhttp://www.atmel.com/images/doc2508.pdf[/url]
Atmel uses only a resistor to limit the current in inputs. And in page 7 is told that we must only observes that the maximum current flowing in the clamp diodes can't pass 1 mA ....
From datasheet you see that the AVR line has input protection. Sure, we can help to better this protection, because the high impedance in this input any noise can be induced and can make false level change. This was resolved lowering the input impedance with one resistor to ground.
I have used this method in my motocycle, an Yamaha Dragstar, from more than one year. And evey day I use whitout any problem.
There is no reason for use optocouplers, or 78L05, or zeners, or fast diodes. Only a resistor. I put one small capacitor too to resolve interference problem with my alarm, that uses RF and is very close to my circuit. This can happens in the car too.
Excuse my bad English.
Paulo
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