AT89C2051

3

Pin Description

VCC

Supply voltage.

GND

Ground.

Port 1

Port 1 is an 8-bit bi-irectional I/O port. Port pins P1.2 to

P1.7 provide internal pullups. P1.0 and P1.1 require exter-

nal pullups. P1.0 and P1.1 also serve as the positive input

(AIN0) and the negative 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 low, they will source current (I

of the internal pullups.

Port 1 also receives code data during Flash programming

and verification.

Port 3

Port 3 pins P3.0 to P3.5, P3.7 are seven bi-irectional 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 general 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 that are externally being

pulled low will source current (I

Port 3 also serves the functions of various special features

of the AT89C2051 as listed below:

Port 3 also receives some control signals for Flash pro-

gramming and verification.

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.

XTAL1

Input to the inverting oscillator amplifier and input to the

internal clock operating circuit.

XTAL2

Output from the inverting oscillator amplifier.

Oscillator Characteristics

XTAL1 and XTAL2 are the input and output, respectively,

of an inverting amplifier which can be configured for use as

an on-chip oscillator, as shown in Figure 1. Either a quartz

crystal or ceramic resonator may be used. To drive the

device from an external clock source, XTAL2 should be left

unconnected while XTAL1 is driven as shown in Figure 2.

There are no requirements on the duty cycle of the external

clock signal, since the input to the internal clocking circuitry

is through a divide-by-two flip-flop, but minimum and maxi-

mum voltage high and low time specifications must be

observed.

Figure 1. Oscillator Connections

Note:

C1, C2 = 30 pF

± 10 pF for Crystals

= 40 pF

± 10 pF for Ceramic Resonators

Figure 2. External Clock Drive Configuration

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)