I/o ports, Introduction, Attiny26(l) – Rainbow Electronics ATtiny26L User Manual

Page 90

ATtiny26(L)

1477B–AVR–04/02

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, except reset, has sym-
metrical 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

and Ground as indicated in Figure 53.

Figure 53. I/O Pin Equivalent Schematic

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” on page 105.

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 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” on
page 91. Most port pins are multiplexed with alternate functions for the peripheral fea-
tures on the device. How each alternate function interferes with the port pin is described
in “Alternate Port Functions” on page 95. 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.

Logic

See Figure

"General Digital I/O" for

Details

Pxn