ATMEGA88 datasheet(5/349 Pages) ATMEL

5 / 349 page

ATmega48/88/168

2545D–AVR–07/04

Pin Descriptions

VCC

Digital supply voltage.

GND

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)

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.

PC6/RESET

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.

CC is the supply voltage pin for the A/D Converter, PC3..0, and ADC7..6. It should be

externally connected to V

CC, even if the ADC is not used. If the ADC is used, it should be

connected to V

CC through a low-pass filter. Note that PC6..4 use digital supply voltage,

CC.

AREF

AREF is the analog reference pin for the A/D Converter.

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