ATmega48/88/168
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 “Configuring the Pin” on page 63 for more details about this feature.
Alternate Functions of Port B The Port B pins with alternate functions are shown in Table 33.
Table 33. 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)
PCINT6 (Pin Change Interrupt 6)
PB6
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-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) and the EXCLK bit is cleared (zero) to
enable asynchronous clocking of Timer/Counter2 using the Crystal Oscillator, pin PB7 is
disconnected from the port, and becomes the inverting output 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.
69
2545D–AVR–07/04
ATMEL [ ATMEL ]
