19.3.1 Alternate Functions of Port B

The Port B pins with alternate functions are shown in the table below:

Table 19-3. Port B Pins Alternate Functions
Port PinAlternate Functions
PB7

XTAL2 (Chip Clock Oscillator pin 2)

TOSC2 (Timer Oscillator pin 2)

PCINT7 (Pin Change Interrupt 7)

PB6

XTAL1 (Chip Clock Oscillator pin 1 or External clock input)

TOSC1 (Timer Oscillator pin 1)

PCINT6 (Pin Change Interrupt 6)

PB5

SCK (SPI Bus Master clock Input)

PCINT5 (Pin Change Interrupt 5)

PB4

MISO (SPI Bus Master Input/Slave Output)

PCINT4 (Pin Change Interrupt 4)

PB3

MOSI (SPI Bus Master Output/Slave Input)

OC2A (Timer/Counter2 Output Compare Match A Output)

PCINT3 (Pin Change Interrupt 3)

PB2

SS (SPI Bus Master Slave select)

OC1B (Timer/Counter1 Output Compare Match B Output)

PCINT2 (Pin Change Interrupt 2)

PB1

OC1A (Timer/Counter1 Output Compare Match A Output)

PCINT1 (Pin Change Interrupt 1)

PB0

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 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.
    • PCINT7: Pin Change Interrupt source 7. The PB7 pin can serve as an external interrupt source.

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

  • XTAL1/TOSC1/PCINT6 – 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.
    • PCINT6: Pin Change Interrupt source 6. The PB6 pin can serve as an external interrupt source.

If PB6 is used as a clock pin, DDB6, PORTB6 and PINB6 will all read 0.

  • SCK0/PCINT5 – Port B, Bit 5
    • SCK0: Master00 Clock output, Slave Clock input pin for SPI0 channel. When the SPI0 is enabled as a Slave, this pin is configured as an input regardless of the setting of DDB5. When the SPI0 is enabled as a Master, the data direction of this pin is controlled by DDB5. When the pin is forced by the SPI0 to be an input, the pull-up can still be controlled by the PORTB5 bit.
    • PCINT5: Pin Change Interrupt source 5. The PB5 pin can serve as an external interrupt source.
  • MISO/PCINT4 – Port B, Bit 4
    • MISO: Master Data input, Slave Data output pin for SPI channel. When the SPI is enabled as a Master, this pin is configured as an input regardless of the setting of DDB4. When the SPI is enabled as a Slave, the data direction of this pin is controlled by DDB4. When the pin is forced by the SPI to be an input, the pull-up can still be controlled by the PORTB4 bit.
    • PCINT4: Pin Change Interrupt source 4. The PB4 pin can serve as an external interrupt source.
  • MOSI/OC2A/PCINT3 – Port B, Bit 3
    • MOSI: SPI Master Data output, Slave Data input for SPI channel. When the SPI is enabled as a Slave, this pin is configured as an input regardless of the setting of DDB3. When the SPI is enabled as a Master, the data direction of this pin is controlled by DDB3. When the pin is forced by the SPI to be an input, the pull-up can still be controlled by the PORTB3 bit.
    • OC2A: Output Compare Match output. The PB3 pin can serve as an external output for the Timer/Counter2 Compare Match A. The PB3 pin has to be configured as an output (DDB3 set '1') to serve this function. The OC2A pin is also the output pin for the PWM mode timer function.
    • PCINT3: Pin Change Interrupt source 3. The PB3 pin can serve as an external interrupt source.
  • SS/OC1B/PCINT2 – Port B, Bit 2
    • SS: Slave Select input. When the SPI is enabled as a Slave, this pin is configured as an input regardless of the setting of DDB2. As a Slave, the SPI is activated when this pin is driven low. When the SPI is enabled as a Master, the data direction of this pin is controlled by DDB2. When the pin is forced by the SPI to be an input, the pull-up can still be controlled by the PORTB2 bit.
    • OC1B: Output Compare Match output. The PB2 pin can serve as an external output for the Timer/Counter1 Compare Match B. The PB2 pin has to be configured as an output (DDB2 set (one)) to serve this function. The OC1B pin is also the output pin for the PWM mode timer function.
    • PCINT2: Pin Change Interrupt source 2. The PB2 pin can serve as an external interrupt source.
  • OC1A/PCINT1 – Port B, Bit 1
    • OC1A: Output Compare Match output. The PB1 pin can serve as an external output for the Timer/Counter1 Compare Match A. The PB1 pin has to be configured as an output (DDB1 set (one)) to serve this function. The OC1A pin is also the output pin for the PWM mode timer function.
    • PCINT1: Pin Change Interrupt source 1. The PB1 pin can serve as an external interrupt source.

  • ICP1/CLKO/PCINT0 – Port B, Bit 0
    • ICP1: Input Capture Pin. The PB0 pin can act as an Input Capture Pin for Timer/Counter1.
    • CLKO: Divided System Clock. The divided system clock can be output on the PB0 pin. The divided system clock will be output if the CKOUT Fuse is programmed, regardless of the PORTB0 and DDB0 settings. It will also be output during reset.
    • PCINT0: Pin Change Interrupt source 0. The PB0 pin can serve as an external interrupt source.

Table 19-4 and Table 19-5 relate the alternate functions of Port B to the overriding signals shown in Figure 19-5. SPI MSTR INPUT and SPI SLAVE OUTPUT constitute the MISO signal, while MOSI is divided into SPI MSTR OUTPUT and SPI SLAVE INPUT.

Table 19-4. Overriding Signals for Alternate Functions in PB7...PB4
Signal
NamePB7/XTAL2/TOSC2/PCINT7(1)PB6/XTAL1/TOSC1/PCINT6(1)PB5/SCK/PCINT5PB4/MISO/PCINT4
PUOEINTRCEXTCK+ AS2INTRC + AS2SPE • MSTRSPE • MSTR
PUOV00PORTB5 • PUDPORTB4 • PUD
DDOEINTRCEXTCK+ AS2INTRC + AS2SPE • MSTRSPE • MSTR
DDOV0000
PVOE00SPE • MSTRSPE • MSTR
PVOV00SCK OUTPUTSPI0SLAVE
OUTPUT
DIEOEINTRCEXTCK + AS2 + PCINT7 • PCIE0INTRC + AS2 + PCINT6 • PCIE0PCINT5 • PCIE0PCINT4 • PCIE0
DIEOV(INTRC + EXTCK) • AS2INTRC • AS211
DIPCINT7 INPUTPCINT6 INPUT

PCINT5 INPUT

SCK INPUT

PCINT4 INPUT
SPI MSTR INPUT

AIOOscillator OutputOscillator/Clock Input
Note:
  1. INTRC means that one of the internal RC Oscillators are selected (by the CKSEL fuses), EXTCK means that external clock is selected (by the CKSEL fuses).
Table 19-5. Overriding Signals for Alternate Functions in PB3...PB0
Signal 
NamePB3/MOSI/OC2A/PCINT3PB2/SS/OC1B/PCINT2PB1/OC1A/PCINT1PB0/ICP1/CLKO/PCINT0
PUOESPE • MSTR SPE • MSTR00
PUOVPORTB3 • PUDPORTB2 • PUD00
DDOESPE • MSTR SPE • MSTR00
DDOV0000
PVOESPE • MSTR + OC2A ENABLEOC1B ENABLEOC1A ENABLE0
PVOVSPI MSTR OUTPUT + OC2A OC1BOC1A0
DIEOEPCINT3 • PCIEPCINT2 • PCIE0PCINT1 • PCIE0PCINT0 • PCIE0
DIEOV1111
DI

PCINT3 INPUT
SPI SLAVE INPUT

PCINT2 INPUT

SPI SS

PCINT1 INPUT

PCINT0 INPUT

ICP1 INPUT

AIO