ATmega128(L)
Ports as General
Digital I/O
The ports are bi-directional I/O ports with optional internal pull-ups. Figure 30 shows a functional
description of one I/O port pin, here generically called Pxn.
Figure 30. General Digital I/O(1)
PUD
Q
D
DDxn
Q CLR
WDx
RDx
RESET
Q
D
Pxn
PORTxn
Q CLR
WPx
RRx
RESET
SLEEP
SYNCHRONIZER
RPx
D
Q
D
L
Q
Q
PINxn
Q
clk I/O
WDx:
RDx:
WPx:
RRx:
RPx:
WRITE DDRx
PUD:
PULLUP DISABLE
SLEEP CONTROL
I/O CLOCK
READ DDRx
SLEEP:
WRITE PORTx
clkI/O
:
READ PORTx REGISTER
READ PORTx PIN
Note:
1. WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O, SLEEP,
and PUD are common to all ports.
Configuring the Pin
Each port pin consists of three Register bits: DDxn, PORTxn, and PINxn. As shown in “Register
Description for I/O Ports” on page 87, the DDxn bits are accessed at the DDRx I/O address, the
PORTxn bits at the PORTx I/O address, and the PINxn bits at the PINx I/O address.
The DDxn bit in the DDRx Register selects the direction of this pin. If DDxn is written logic one,
Pxn is configured as an output pin. If DDxn is written logic zero, Pxn is configured as an input
pin.
If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is
activated. To switch the pull-up resistor off, PORTxn has to be written logic zero or the pin has to
be configured as an output pin. The port pins are tri-stated when a Reset condition becomes
active, even if no clocks are running.
If PORTxn is written logic one when the pin is configured as an output pin, the port pin is driven
high (one). If PORTxn is written logic zero when the pin is configured as an output pin, the port
pin is driven low (zero).
When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn, PORTxn}
= 0b11), an intermediate state with either pull-up enabled ({DDxn, PORTxn} = 0b01) or output
67
2467P–AVR–08/07
ATMEL [ ATMEL ]
