Special input/output

Special input/output (Special I/O or SIO) are inputs and/or outputs of a microcontroller designated to perform specialized functions or have specialized features.

Specialized functions can include:
 * Hardware interrupts,
 * analog input or output
 * PWM output
 * Serial communication, such as UART, USART, SPI bus, or SerDes.
 * External reset
 * Switch debounce
 * Input pull-up (or -down) resistors
 * open collector output
 * Pulse counting
 * Timing pulses

Some kinds of special I/O functions can sometimes be emulated with general-purpose input/output and bit banging software.