ATmega48/88/168
Pin Descriptions
VCC
GND
Digital supply voltage.
Ground.
Port B (PB7..0) XTAL1/
XTAL2/TOSC1/TOSC2
Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each
bit). The Port B output buffers have symmetrical drive characteristics with both high sink
and source capability. As inputs, Port B pins that are externally pulled low will source
current if the pull-up resistors are activated. The Port B pins are tri-stated when a reset
condition becomes active, even if the clock is not running.
Depending on the clock selection fuse settings, PB6 can be used as input to the invert-
ing Oscillator amplifier and input to the internal clock operating circuit.
Depending on the clock selection fuse settings, PB7 can be used as output from the
inverting Oscillator amplifier.
If the Internal Calibrated RC Oscillator is used as chip clock source, PB7..6 is used as
TOSC2..1 input for the Asynchronous Timer/Counter2 if the AS2 bit in ASSR is set.
The various special features of Port B are elaborated in “Alternate Functions of Port B”
on page 69 and “System Clock and Clock Options” on page 24.
Port C (PC5..0)
PC6/RESET
Port C is a 7-bit bi-directional I/O port with internal pull-up resistors (selected for each
bit). The PC5..0 output buffers have symmetrical drive characteristics with both high
sink and source capability. As inputs, Port C pins that are externally pulled low will
source current if the pull-up resistors are activated. The Port C pins are tri-stated when a
reset condition becomes active, even if the clock is not running.
If the RSTDISBL Fuse is programmed, PC6 is used as an I/O pin. Note that the electri-
cal characteristics of PC6 differ from those of the other pins of Port C.
If the RSTDISBL Fuse is unprogrammed, PC6 is used as a Reset input. A low level on
this pin for longer than the minimum pulse length will generate a Reset, even if the clock
is not running. The minimum pulse length is given in Table 20 on page 41. Shorter
pulses are not guaranteed to generate a Reset.
The various special features of Port C are elaborated in “Alternate Functions of Port C”
on page 73.
Port D (PD7..0)
Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each
bit). The Port D output buffers have symmetrical drive characteristics with both high sink
and source capability. As inputs, Port D pins that are externally pulled low will source
current if the pull-up resistors are activated. The Port D pins are tri-stated when a reset
condition becomes active, even if the clock is not running.
The various special features of Port D are elaborated in “Alternate Functions of Port D”
on page 75.
AVCC
AVCC is the supply voltage pin for the A/D Converter, PC3..0, and ADC7..6. It should be
externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be
connected to VCC through a low-pass filter. Note that PC6..4 use digital supply voltage,
VCC
.
AREF
AREF is the analog reference pin for the A/D Converter.
5
2545D–AVR–07/04
ATMEL [ ATMEL ]
