All AVR ports have true Read-Modify-Write functionality when used as general digital I/O ports. This means that the direction of one port pin can be changed without unintentionally changing the direction of any other pin with the SBI and CBI instructions. The same applies when changing drive value (if configured as an output) or enabling/disabling of pull-up resistors (if configured as an input). Each output buffer has symmetrical drive characteristics with both high sink and source capability. The pin driver is strong enough to drive LED displays directly. All port pins have individually selectable pull-up resistors with a supply voltage invariant resistance. All I/O pins have protection diodes to both VCC and ground as indicated in the following figure.

Figure 1. I/O Pin Equivalent Schematic

All registers and bit references in this section are written in general form. A lower case ā€œxā€ represents the numbering letter for the port, and a lower case ā€œnā€ represents the bit number. However, when using the register or bit defines in a program, the precise form must be used. For example, PORTB3 for bit number 3 in Port B, here documented generally as PORTxn.

Three I/O memory address locations are allocated for each port, one each for the Data Register (Portx), Data Direction Register (DDRx), and the Port Input Pins (PINx). The port input pins I/O location is read-only, while the data register and the data direction register are read/write. However, writing '1' to a bit in the PINx register will result in a toggle in the corresponding bit in the data register. In addition, the Pull-up Disable (PUD) bit in MCUCR disables the pull-up function for all pins in all ports when set.

Using the I/O port as general digital I/O is described in next section. Most port pins are multiplexed with alternate functions for the peripheral features on the device. How each alternate function interferes with the port pin is described in Alternate Port Functions section in this chapter. Refer to the individual module sections for a full description of the alternate functions.

Enabling the alternate function of some of the port pins does not affect the use of the other pins in the port as general digital I/O.