AT89S52
4
Pin Description
VCC
Supply voltage.
GND
Ground.
Port 0
Port 0 is an 8-bit open drain bidirectional I/O port. As an
output port, each pin can sink eight TTL inputs. When 1s
are written to port 0 pins, the pins can be used as high-
impedance inputs.
Port 0 can also be configured to be the multiplexed low-
order address/data bus during accesses to external
program and data memory. In this mode, P0 has internal
pullups.
Port 0 also receives the code bytes during Flash program-
ming and outputs the code bytes during program verifica-
tion. External pullups are required during program
verification.
Port 1
Port 1 is an 8-bit bidirectional I/O port with internal pullups.
The Port 1 output buffers can sink/source four TTL inputs.
When 1s are written to Port 1 pins, they are pulled high by
the internal pullups and can be used as inputs. As inputs,
Port 1 pins that are externally being pulled low will source
current (I
IL
) because of the internal pullups.
In addition, P1.0 and P1.1 can be configured to be the
timer/counter 2 external count input (P1.0/T2) and the
timer/counter 2 trigger input (P1.1/T2EX), respectively, as
shown in the following table.
Port 1 also receives the low-order address bytes during
Flash programming and verification.
Port 2
Port 2 is an 8-bit bidirectional I/O port with internal pullups.
The Port 2 output buffers can sink/source four TTL inputs.
When 1s are written to Port 2 pins, they are pulled high by
the internal pullups and can be used as inputs. As inputs,
Port 2 pins that are externally being pulled low will source
current (I
IL
) because of the internal pullups.
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, Port 2 uses strong internal pul-
lups when emitting 1s. During accesses to external data
memory that use 8-bit addresses (MOVX @ RI), Port 2
emits the contents of the P2 Special Function Register.
Port 2 also receives the high-order address bits and some
control signals during Flash programming and verification.
Port 3
Port 3 is an 8-bit bidirectional I/O port with internal pullups.
The Port 3 output buffers can sink/source four TTL inputs.
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 AT89S52, as shown in the following table.
Port 3 also receives some control signals for Flash pro-
gramming and verification.
RST
Reset input. A high on this pin for two machine cycles while
the oscillator is running resets the device. This pin drives
High for 96 oscillator periods after the Watchdog times out.
The DISRTO bit in SFR AUXR (address 8EH) can be used
to disable this feature. In the default state of bit DISRTO,
the RESET HIGH out feature is enabled.
ALE/PROG
Address Latch Enable (ALE) is an output pulse for latching
the low byte of the address during accesses to external
memory. This pin is also the program pulse input (PROG
)
during Flash programming.
In normal operation, ALE is emitted at a constant rate of
1/6 the oscillator frequency and may be used for external
timing or clocking purposes. Note, however, that one
ALE pulse is skipped during each access to external data
memory.
If desired, ALE operation can be disabled by setting bit 0 of
SFR location 8EH. With the bit set, ALE is active only dur-
ing a MOVX or MOVC instruction. Otherwise, the pin is
Port Pin Alternate Functions
P1.0 T2 (external count input to Timer/Counter 2),
clock-out
P1.1 T2EX (Timer/Counter 2 capture/reload trigger
and direction control)
P1.5 MOSI (used for In-System Programming)
P1.6 MISO (used for In-System Programming)
P1.7 SCK (used for In-System Programming)
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)
P3.6 WR
(external data memory write strobe)
P3.7 RD
(external data memory read strobe)
