AT90CAN128
I/O-Ports
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 uninten-
tionally 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 and Ground as indicated in Figure 30. Refer to “Electrical
Characteristics(1)” on page 355 for a complete list of parameters.
Figure 30. 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” 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 I/O Registers and bit locations are listed in “Register Descrip-
tion for I/O-Ports”.
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.
Using the I/O port as General Digital I/O is described in “Ports as General Digital I/O”.
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 67. Refer to the individual module sections 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.
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4250E–CAN–12/04
ATMEL [ ATMEL ]
