I am trying to use an external interrupt on INTO to detect the falling edge of a 100Hz/5V square wave. The square wave is generated from a board with its own power supply and ground which is separate from my target board. The boards are connected with a single lead (5") and a series resistor.
If I look at the output of generator on a scope with probe ground attached to the generator boards ground, the signal is clean and stable. When I connect the signal to my target board (INT0 pin of the AVR) and look at the signal at that pin with probe ground connected to the target board ground, it has alot of noise.
This noise is strong enough to cause false edge detection by the avr and generation of unexpected INTO interrupts.
Any ideas on where this noise is coming from or how to deal with it? I tried a 0.1uF capacitor to ground, but it was not enough to make edge detection work correctly.
FYI, I am a hobbyist and my electronics knowledge is self learned (i.e. limited)