56
ATmega8A [DATASHEET]
8159E–AVR–02/2013
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
details about this feature.
13.3.2
Alternate Functions of Port B
The Port B pins with alternate functions are shown in
Table 13-2.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-frequency 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 calibrated 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
inverting Oscillator amplifier. In this mode, a crystal Oscillator is connected to this pin, and the pin can not be used
as an I/O pin.
If PB6 is used as a clock pin, DDB6, PORTB6 and PINB6 will all read 0.
Table 13-2.
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
ICP1 (Timer/Counter1 Input Capture Pin)