PinChangeInt (PinChangeInt.h)

http://playground.arduino.cc/Main/PinChangeInt -- The PinChangeInt? library implements Pin Change interrupts for the Arduino environment. It is a drop in replacement for the PCint library. This library was designed for the Arduino Uno/Duemilanove, and has been reported to work fine on the Nano, but it has not been tested there. As mentioned above, MEGA support is included but support for that platform is weak. What are Pin Change interrupts? The ATmega processor at the heart of the Arduino has two different kinds of interrupts: “external”, and “pin change”. There are only two external interrupt pins, INT0 and INT1, and they are mapped to Arduino pins 2 and 3. These interrupts can be set to trigger on RISING or FALLING signal edges, or on low level. The triggers are interpreted by hardware, and the interrupt is very fast. On the other hand there are only 2 such pins on the ATmega328p in the Arduino Uno and Duemilanove. On the other hand the pin change interrupts can be enabled on any or all of the Arduino's signal pins (Uno/Duemilanove/Nano). They are triggered equally on RISING or FALLING signal edges, so it is up to the interrupt code to set the proper pins to receive interrupts, to determine what happened (which pin? ...did the signal rise, or fall?), and to handle it properly. Furthermore, the pin change interrupts are grouped into 3 “port”s on the MCU, so there are only 3 interrupt vectors (subroutines) for the entire body of 20 pins. This makes the job of resolving the action on a single interrupt even more complicated. The interrupt routine should be fast, but complication is the enemy of speed. The PinChangeInt? library is designed to handle the Arduino's pin change interrupts as quickly and reasonably as possible.