Editing AVR microcontrollers (section)

==== General-purpose input/output (GPIO) ports ====
Each [[GPIO]] port on a tiny or mega AVR drives up to eight pins and is controlled by three 8-bit registers: DDR''x'', PORT''x'' and PIN''x'', where ''x'' is the port identifier.

* DDR''x'': Data Direction Register, configures the pins as either inputs or outputs.
* PORT''x'': Output port register. Sets the output value on pins configured as outputs. Enables or disables the [[pull-up resistor]] on pins configured as inputs.
* PIN''x'': Input register, used to read an input signal. On some devices, this register can be used for pin toggling: writing a logic one to a PIN''x'' bit toggles the corresponding bit in PORT''x'', irrespective of the setting of the DDR''x'' bit.<ref>[http://www.atmel.com/Images/Atmel-2586-AVR-8-bit-Microcontroller-ATtiny25-ATtiny45-ATtiny85_Datasheet.pdf atmel.com]</ref>

Newer ATtiny AVRs, like ATtiny817 and its siblings, have their port control registers somewhat differently defined.
xmegaAVR have additional registers for push/pull, totem-pole and pullup configurations.