ATmega48/88/168
10.3.1
MCU Control Register – MCUCR
Bit
7
–
6
–
5
–
4
3
–
2
–
1
IVSEL
R/W
0
0
IVCE
R/W
0
PUD
R/W
0
MCUCR
Read/Write
Initial Value
R
0
R
0
R
0
R
0
R
0
• Bit 4 – 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 ”Con-
figuring the Pin” on page 65 for more details about this feature.
10.3.2
Alternate Functions of Port B
The Port B pins with alternate functions are shown in Table 10-3.
Table 10-3. Port B Pins Alternate Functions
Port Pin
Alternate Functions
XTAL2 (Chip Clock Oscillator pin 2)
TOSC2 (Timer Oscillator pin 2)
PCINT7 (Pin Change Interrupt 7)
PB7
XTAL1 (Chip Clock Oscillator pin 1 or External clock input)
TOSC1 (Timer Oscillator pin 1)
PB6
PCINT6 (Pin Change Interrupt 6)
SCK (SPI Bus Master clock Input)
PCINT5 (Pin Change Interrupt 5)
PB5
PB4
MISO (SPI Bus Master Input/Slave Output)
PCINT4 (Pin Change Interrupt 4)
MOSI (SPI Bus Master Output/Slave Input)
OC2A (Timer/Counter2 Output Compare Match A Output)
PCINT3 (Pin Change Interrupt 3)
PB3
SS (SPI Bus Master Slave select)
PB2
PB1
PB0
OC1B (Timer/Counter1 Output Compare Match B Output)
PCINT2 (Pin Change Interrupt 2)
OC1A (Timer/Counter1 Output Compare Match A Output)
PCINT1 (Pin Change Interrupt 1)
ICP1 (Timer/Counter1 Input Capture Input)
CLKO (Divided System Clock Output)
PCINT0 (Pin Change Interrupt 0)
The alternate pin configuration is as follows:
• XTAL2/TOSC2/PCINT7 – 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) and the EXCLK bit is cleared (zero) to enable asynchronous clock-
ing of Timer/Counter2 using the Crystal Oscillator, pin PB7 is disconnected from the port, and
71
2545E–AVR–02/05
MICROCHIP [ MICROCHIP ]
