microcontroller - When IO is limited on a uC, how do you move the logic away from the uC?

For instance, if you had an Arduino with 4 digital IO pins, how could you independently light > 4 LEDs, or read the state of > 4 buttons?

Answer

A shift register such as the 74595 will allow you to have many outputs with only 2 connections: a data pin and a clock pin. You set the data pin to the next value you want to move into the register, and then pulse the clock pin.