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Atmel ATmega16/32/64/M1/C1 [DATASHEET]
7647K–AVR–12/13
9.
I/O-Ports
9.1
Introduction
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 output) or enabling/disabling of pull-up resistors (if configured as
input). Each output buffer has symmetrical drive characteristics with both high sink and source capability. 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
parameters.
Figure 9-1.
I/O Pin Equivalent Schematic
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. However, when using the register or bit defines in a program, the
precise form must be used. For example, PORTB3 for bit no. 3 in Port B, here documented generally as PORTxn. The physical
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 one 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.
alternate functions for the peripheral features on the device. How each alternate function interferes with the port pin is described
functions.
Note that 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.
Cpin
Rpu
Pxn
Logic
See Figure
”General Digital I/O”
for Details