Zener diode voltage for Mega GPIO protection

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got quick question. i'm wanting protect gpio pins of new mega, , planning build gpio protection board using zeners bleed off over-voltage. digikey doesn't have straight 5v thru-hole zeners; closest have either 4v7 or 5v1. i'm guessing better use 4v7 ones, cause issues?

thanks!

i'd been looking "absolute maximum" voltages atmega2560; on atmel's product page lists "i/o supply class" voltage of "1.8 5.5" - i'm looking for? ( http://www.atmel.com/devices/atmega2560.aspx?tab=parameters )

i found datasheet (the link on arduino product page dead btw.) lists "voltage on pin except reset" -.5 vcc+0.5. don't see listing vcc specifically, "maximum operating voltage" of 6v. so, mean gpio pins theoretically work voltage between -0.5 , +6.5v? or accept -0.5v +6v?

sorry ignorance, i'm getting started of this, lot of terminology totally new me...


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