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ATmega8A [DATASHEET]
8159E–AVR–02/2013
13. I/O Ports
13.1
Overview
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/dis-
abling of pull-up resistors (if configured as 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 V
Figure 13-1. I/O Pin Equivalent Schematic
All registers and bit references in this section are written in general form. A lower case “x” represents the number-
ing 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 (i.e., PORTB3 for bit 3 in Port B, here documented generally
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. In addition, the Pull-up Disable – PUD bit in SFIOR
disables the pull-up function for all pins in all ports when set.
are multiplexed with alternate functions for the peripheral features on the device. How each alternate function inter-
for a full description of the alternate 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.
C
pin
Logic
R
pu
See Figure
"General Digital I/O" for
Details
Pxn