I placed an application hint that demonstrates how to interface a hall effect sensor to an ATmega328 using interrupts. See http://wiblocks.luciani.org/docs/app-notes/pico1trc-hall-effect.html
The Honeywell SS345 is a unipolar hall-effect sensor that can be powered from a supply that ranges from 2.7V to 7V. I wired the sensor using the prototype area of my PICO1TRC (http://wiblocks.luciani.org/PICO/PICO1TRC-index.html).
When the magnetic field changes an interrupt routine is called and a flag is set. When the flag is set the main loop of the program outputs the time and state of the magnetic field.
(* jcl *)