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,^[1][2] SPI bus, or SerDes.
• External reset
• Switch debounce
• Input pull-up (or -down) resistors
• open collector output
• Pulse counting^[3]
• Timing pulses^[4][5]

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