
3
80C32E
4149M–AERO–06/04
Pin Description
Mnemonic
Type
Name and Function
V
SS
I
Ground:
 0V reference
V
CC
I
Power Supply:
 This is the power supply voltage for normal, idle and 
power-down operation
P0.0-P0.7
I/O
Port 0
: Port 0 is an open-drain, bidirectional I/O port. Port 0 pins that 
have 1s written to them float and can be used as high impedance inputs. 
Port 0 pins must be polarized to Vcc or Vss in order to prevent any 
parasitic current consumption. Port 0 is also the multiplexed low-order 
address and data bus during access to external program and data 
memory. In this application, it uses strong internal pull-up when emitting 
1s.
P1.0-P1.7
I/O
Port 1
: Port 1 is an 8-bit bidirectional I/O port with internal pull-ups. Port 1 
pins that have 1s written to them are pulled high by the internal pull-ups 
and can be used as inputs. As inputs, Port 1 pins that are externally 
pulled low will source current because of the internal pull-ups. 
P2.0-P2.7
I/O
Port 2
: Port 2 is an 8-bit bidirectional I/O port with internal pull-ups. Port 2 
pins that have 1s written to them are pulled high by the internal pull-ups 
and can be used as inputs. As inputs, Port 2 pins that are externally 
pulled low will source current because of the internal pull-ups. Port 2 
emits the high-order address byte during fetches from external program 
memory and during accesses to external data memory that use 16-bit 
addresses (MOVX @DPTR).In this application, it uses strong internal 
pull-ups emitting 1s. During accesses to external data memory that use 
8-bit addresses (MOVX @Ri), port 2 emits the contents of the P2 SFR. 
P3.0-P3.7
I/O
Port 3
: Port 3 is an 8-bit bidirectional I/O port with internal pull-ups. Port 3 
pins that have 1s written to them are pulled high by the internal pull-ups 
and can be used as inputs. As inputs, Port 3 pins that are externally 
pulled low will source current because of the internal pull-ups. Port 3 also 
serves the special features of the 80C51 family, as listed below.
I
RXD (P3.0):
 Serial input port
O
TXD (P3.1):
 Serial output port
I
INT0 (P3.2):
 External interrupt 0
I
INT1 (P3.3):
 External interrupt 1
I
T0 (P3.4):
 Timer 0 external input
I
T1 (P3.5):
 Timer 1 external input
O
WR (P3.6):
 External data memory write strobe
O
RD (P3.7):
 External data memory read strobe
RST
I
Reset:
 A high on this pin for two machine cycles while the oscillator is 
running, resets the device. An internal diffused resistor to V
SS 
permits a 
power-on reset using only an external capacitor to V
CC.