3.10.6 I/O Ports
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 the drive value (if configured as output) or enabling/disabling pull-up resistors (if configured as input). Each output buffer has symmetrical drive characteristics with both, high sink and source capability.
The only exception for this rule is the RX_ACTIVE (PB7) pin when configured as supply. For details, see Alternate Port Functions. All port pins have individually selectable pull-up resistors with supply-voltage invariant resistance. All I/O pins have protection diodes to both VS and DGND, as shown in the figure Figure 3-59 and described in ESD Protection Circuits. I/O Characteristics for Ports PB0 to PB7 and PC0 to PC5 for ports PB0 to PB7 and PC0 to PC5 for a complete list of parameters. The reference pins for the ports are pin 13 (VS) and pin 21 (DGND).
All registers and bit references in this section are written in general form. A lower case “x” represents the numbering letter for the port, and a lower case “n” represents the bit number within the port. However, when using the register or bit defines in a program, the precise form must be used. For example, as a general rule PORTB3 for bit no. 3 in port B is documented as PORTxn. The physical I/O registers and bit locations are listed in I/O Ports Register Description.
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 a logic ‘1’ to a
bit in the PINx register results in a toggle in the corresponding bit in the data
register. In addition, when it is set the pull-up disable (PUD) bit in MCUCR disables
the pull-up function for all pins in all ports.
Using the I/O port as general digital I/O is described in Ports as General Digital I/O. 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. Refer to the individual module sections for a full description of the alternate functions.