ATMEGA8_08 Datasheet, PDF (51/308 Pages) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

I/O Ports
Introduction
ATmega8(L)
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 chang-
ing 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. The pin driver is strong enough to drive LED displays directly. All port pins have indi-
vidually selectable pull-up resistors with a supply-voltage invariant resistance. All I/O pins have
protection diodes to both V_CC and Ground as indicated in Figure 21. Refer to “Electrical Charac-
teristics” on page 242 for a complete list of parameters.
Figure 21. I/O Pin Equivalent Schematic
Pxn
C_pin
R_pu
Logic
See Figure
"General Digital I/O" for
Details
Ports as General
Digital I/O
All registers and bit references in this section are written in general form. A lower case “x” repre-
sents 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 (i.e., PORTB3
for bit 3 in Port B, here documented generally as PORTxn). The physical I/O Registers and bit
locations are listed in “Register Description for I/O Ports” on page 65.
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.
Using the I/O port as General Digital I/O is described in “Ports as General Digital I/O” on page
51. 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” on page 56. Refer to the individual module sections for a full description of the alter-
nate 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.
The ports are bi-directional I/O ports with optional internal pull-ups. Figure 22 shows a functional
description of one I/O port pin, here generically called Pxn.
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