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
low, they will source current (I
IL
) because of the internal pullups.
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
that are externally being pulled low will source current (I
IL
) because of the pullups.
Port 3 also serves the functions of various special features of the AT89C4051 as listed below:
Port Pin
P3.0
P3.1
P3.2
P3.3
P3.4
P3.5
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)
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.
3
1001F–MICRO–6/08
ATMEL [ ATMEL CORPORATION ]
