AVR-Chat

Background:

I am considering driving LEDs directly from the ATMega16 I/O pin (current
sink mode) to avoid having to use an additional driver chip, however, the
LEDs are located as much as 800 feet from the ATMega16 on an outdoor buried
CAT-5 cable (long story).

The max current rating of the I/O port is 40 ma so I'm planning on putting a
125 ohm resistor between the I/O pin and the cable to limit fault current
(wire shorted to Gnd) to 40 ma and protect the I/O pin.  

This means that, to deliver 10 ma @ 5v to the LED (assuming for the moment a
2v forward drop in the LED) I need a total R value of ~300 ohm.  The
additional ~175 ohm will be placed at the LED location as part of surge
protection at the LED (R + Transorb).  


Question:

The ATMega16's datasheet shows that the I/O pins have diodes from the pin to
Gnd and Vcc.  These would seem to protect the pin from surge voltages less
than Gnd or greater than Vcc but I do not see any specification as to what
the max current capability of these diodes is.

Knowing this will tell me what over/under voltages I can stand on the
outboard side of the 125 ohm resistor before having to consider placing
additional external surge protection on the pin (Transorb).