3

1001E–MICRO–6/05

AT89C4051

4.

Pin Description

4.1

VCC

Supply voltage.

4.2

GND

Ground.

4.3

Port 1

Port 1 is an 8-bit bi-directional I/O port. Port pins P1.2 to P1.7 provide internal pullups. P1.0 and

P1.1 require external pullups. P1.0 and P1.1 also serve as the positive input (AIN0) and the neg-

ative input (AIN1), respectively, of the on-chip precision analog comparator. The Port 1 output

buffers can sink 20 mA and can drive LED displays directly. When 1s are written to Port 1 pins,

they can be used as inputs. When pins P1.2 to P1.7 are used as inputs and are externally pulled

Port 1 also receives code data during Flash programming and verification.

4.4

Port 3

Port 3 pins P3.0 to P3.5, P3.7 are seven bi-directional I/O pins with internal pullups. P3.6 is

hard-wired as an input to the output of the on-chip comparator and is not accessible as a gen-

eral-purpose I/O pin. The Port 3 output buffers can sink 20 mA. When 1s are written to Port 3

pins they are pulled high by the internal pullups and can be used as inputs. As inputs, Port 3 pins

Port 3 also serves the functions of various special features of the AT89C4051 as listed below:

Port 3 also receives some control signals for Flash programming and verification.

4.5

RST

Reset input. All I/O pins are reset to 1s as soon as RST goes high. Holding the RST pin high for

two machine cycles while the oscillator is running resets the device.

Each machine cycle takes 12 oscillator or clock cycles.

4.6

XTAL1

Input to the inverting oscillator amplifier and input to the internal clock operating circuit.

Port Pin

Alternate Functions

P3.0

RXD (serial input port)

P3.1

TXD (serial output port)

P3.2

INT0 (external interrupt 0)

P3.3

INT1 (external interrupt 1)

P3.4

T0 (timer 0 external input)

P3.5

T1 (timer 1 external input)