ATMEGA8 datasheet(56/303 Pages) ATMEL

56 / 303 page

ATmega8(L)

2486M–AVR–12/03

Special Function IO Register –

SFIOR

• Bit 2 – PUD: Pull-up Disable

When this bit is written to one, the pull-ups in the I/O ports are disabled even if the DDxn

and PORTxn Registers are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01).

See “Configuring the Pin” on page 50 for more details about this feature.

Alternate Functions of Port B

The Port B pins with alternate functions are shown in Table 22.

The alternate pin configuration is as follows:

• XTAL2/TOSC2 – Port B, Bit 7

XTAL2: Chip clock Oscillator pin 2. Used as clock pin for crystal Oscillator or Low-fre-

quency crystal Oscillator. When used as a clock pin, the pin can not be used as an I/O

pin.

TOSC2: Timer Oscillator pin 2. Used only if internal calibrated RC Oscillator is selected

as chip clock source, and the asynchronous timer is enabled by the correct setting in

ASSR. When the AS2 bit in ASSR is set (one) to enable asynchronous clocking of

Timer/Counter2, pin PB7 is disconnected from the port, and becomes the inverting out-

put of the Oscillator amplifier. In this mode, a crystal Oscillator is connected to this pin,

and the pin cannot be used as an I/O pin.

If PB7 is used as a clock pin, DDB7, PORTB7 and PINB7 will all read 0.

• XTAL1/TOSC1 – Port B, Bit 6

XTAL1: Chip clock Oscillator pin 1. Used for all chip clock sources except internal cali-

brated RC Oscillator. When used as a clock pin, the pin can not be used as an I/O pin.

TOSC1: Timer Oscillator pin 1. Used only if internal calibrated RC Oscillator is selected

as chip clock source, and the asynchronous timer is enabled by the correct setting in

ASSR. When the AS2 bit in ASSR is set (one) to enable asynchronous clocking of

Timer/Counter2, pin PB6 is disconnected from the port, and becomes the input of the

Bit

ACME

PUD

PSR2

PSR10

SFIOR

Read/Write

R/W

Initial Value

Table 22. Port B Pins Alternate Functions

Port Pin

Alternate Functions

PB7

XTAL2 (

Chip Clock Oscillator pin 2)

TOSC2 (

Timer Oscillator pin 2)

PB6

XTAL1 (

Chip Clock Oscillator pin 1 or External clock input)

TOSC1 (

Timer Oscillator pin 1)

PB5

SCK (SPI Bus Master clock Input)

PB4

MISO (SPI Bus Master Input/Slave Output)

PB3

MOSI (SPI Bus Master Output/Slave Input)

OC2 (Timer/Counter2 Output Compare Match Output)

PB2

SS (SPI Bus Master Slave select)

OC1B (Timer/Counter1 Output Compare Match B Output)

PB1

OC1A (Timer/Counter1 Output Compare Match A Output)

PB0

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