The AT90S8515

This particular MCU can perform many interesting input and output functions. Just to give you an idea:

1. input: detect whether the electrical potential at a pin is ``high'' or ``low''
2. output: change (drive) the electrical potential at a pin so that it is either high or low
3. intput/output: utilize changing electrical potential as a means to communicate with other MCUs or computers (Universal Asynchronous Receiver Transmitter)
4. output: generate square wave of a certain frequency but with a varying duty cycle (on time)
5. input: measure the amount of time when the electrical potential of a pin goes high or low
6. input: notifies software when the electrical potential of a pin changes
7. input: quantifies (measures) the electrical potential at a certain pin

Obviously, we will not get into most of these items. We will, however, talk about the first two items. These are commonly called ``general purpose input/output'', or GPIO in short.