ATMEGA8_08 Datasheet, PDF(56/308 Page) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

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ATMEGA8_08 Datasheet, PDF (56/308 Pages) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

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Unconnected pins

Alternate Port

Functions

If some pins are unused, it is recommended to ensure that these pins have a defined level. Even

though most of the digital inputs are disabled in the deep sleep modes as described above, float-

ing inputs should be avoided to reduce current consumption in all other modes where the digital

inputs are enabled (Reset, Active mode and Idle mode).

The simplest method to ensure a defined level of an unused pin, is to enable the internal pull-up.

In this case, the pull-up will be disabled during reset. If low power consumption during reset is

important, it is recommended to use an external pull-up or pull-down. Connecting unused pins

directly to VCC or GND is not recommended, since this may cause excessive currents if the pin is

accidentally configured as an output.

Most port pins have alternate functions in addition to being general digital I/Os. Figure 25 shows

how the port pin control signals from the simplified Figure 22 can be overridden by alternate

functions. The overriding signals may not be present in all port pins, but the figure serves as a

generic description applicable to all port pins in the AVR microcontroller family.

Figure 25. Alternate Port Functions(1)

PUOExn

PUOVxn

1

0

PUD

DDOExn

DDOVxn

1

0

PVOExn

PVOVxn

QD

DDxn

Q CLR

RESET

WDx

RDx

1

Pxn

0

QD

PORTxn

DIEOExn

Q CLR

WPx

DIEOVxn

1

0

SLEEP

RESET

RRx

SYNCHRONIZER

D SET Q

L CLR Q

DQ

PINxn

CLR Q

RPx

clk I/O

DIxn

AIOxn

PUOExn:

PUOVxn:

DDOExn:

DDOVxn:

PVOExn:

PVOVxn:

DIEOExn:

DIEOVxn:

SLEEP:

Pxn PULL-UP OVERRIDE ENABLE

Pxn PULL-UP OVERRIDE VALUE

Pxn DATA DIRECTION OVERRIDE ENABLE

Pxn DATA DIRECTION OVERRIDE VALUE

Pxn PORT VALUE OVERRIDE ENABLE

Pxn PORT VALUE OVERRIDE VALUE

Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE

Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE

SLEEP CONTROL

PUD:

WDx:

RDx:

RRx:

WPx:

RPx:

clkI/O:

DIxn:

AIOxn:

PULLUP DISABLE

WRITE DDRx

READ DDRx

READ PORTx REGISTER

WRITE PORTx

READ PORTx PIN

I/O CLOCK

DIGITAL INPUT PIN n ON PORTx

ANALOG INPUT/OUTPUT PIN n ON PORTx

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. All other signals are unique for each pin.

56 ATmega8(L)

2486TâAVRâ05/08

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