Atmel AT89C2051 Manual page 3

4. Pin Description
4.1 VCC
4.2 GND
4.3 Port 1
4.4 Port 3
4.5 RST
4.6 XTAL1
0368G–MICRO–6/05
Supply voltage.
Ground.
The Port 1 is an 8-bit bi-directional I/O port. Port pins P1.2 to P1.7 provide internal pull-ups. P1.0
and P1.1 require external pull-ups. 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 out-
put 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
Port 1 also receives code data during Flash programming and verification.
Port 3 pins P3.0 to P3.5, P3.7 are seven bi-directional I/O pins with internal pull-ups. 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 pull-ups 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 Pin
P3.0
P3.1
P3.2
P3.3
P3.4
P3.5
Port 3 also receives some control signals for Flash programming and verification.
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.
Input to the inverting oscillator amplifier and input to the internal clock operating circuit.
) because of the internal pull-ups.
IL
Alternate Functions
RXD (serial input port)
TXD (serial output port)
INT0 (external interrupt 0)
INT1 (external interrupt 1)
T0 (timer 0 external input)
T1 (timer 1 external input)
AT89C2051
) because of the pull-ups.
IL
3