ATmega64A
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 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 VCC and Ground as indicated in Figure 13-1. Refer to “Electrical Char-
acteristics” on page 327 for a complete list of parameters.
Figure 13-1. I/O Pin Equivalent Schematic
Rpu
Pxn
Logic
Cpin
See Figure
"General Digital I/O" for
Details
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 no. 3 in Port B, here documented generally as PORTxn). The physical I/O Registers and
bit locations are listed in “Register Description” on page 88.
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 69.
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 73. Refer to the individual module sections for a full description of the alternate
functions.
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8160C–AVR–07/09
ATMEL [ ATMEL ]
