87C51/80C51BH/80C31BH
PIN DESCRIPTION
Port 2 also receives some control signals and the
high-order address bits during EPROM programming
and program verification.
V : Supply voltage during normal, Idle and Power
CC
Down operations.
Port 3: Port 3 is an 8-bit bidirectional I/O port with
internal pullups. The Port 3 output buffers can drive
LS TTL inputs. Port 3 pins that have 1’s written to
them are pulled high by the internal pullups, and in
that state can be used as inputs. As inputs, Port 3
pins that are externally pulled low will source current
V
: Circuit ground.
SS
Port 0: Port 0 is an 8-bit open drain bidirectional I/O
port. As an output port each pin can sink several LS
TTL inputs. Port 0 pins that have 1’s written to them
float, and in that state can be used as high-imped-
ance inputs.
(I , on the data sheet) because of the pullups.
IL
Port 3 also serves the functions of various special
features of the MCS-51 Family, as listed below:
Port 0 is also the multiplexed low-order address and
data bus during accesses to external memory. In this
application it uses strong internal pullups when emit-
ting 1’s.
Pin Name
Alternate Function
P3.0 RXD Serial input line
P3.1 TXD Serial output line
P3.2 INT0 External Interrupt 0
P3.3 INT1 External Interrupt 1
Port 0 also receives the code bytes during EPROM
programming, and outputs the code bytes during
program verification. External pullups are required
during program verification.
P3.4
P3.5
P3.6
P3.7
T0
T1
Timer 0 external input
Timer 1 external input
Port 1: Port 1 is an 8-bit bidirectional I/O port with
internal pullups. The Port 1 output buffers can drive
LS TTL inputs. Port 1 pins that have 1’s written to
them are pulled high by the internal pullups, and in
that state can be used as inputs. As inputs, Port 1
pins that are externally pulled low will source current
WR
RD
External Data Memory Write strobe
External Data Memory Read strobe
Port 3 also receives some control signals for
EPROM programming and program verification.
(I , on the data sheet) because of the internal pull-
IL
ups.
RST: Reset input. A high on this pin for two machine
cycles while the oscillator is running resets the de-
vice. The port pins will be driven to their reset condi-
Port 1 also receives the low-order address bytes
during EPROM programming and program verifica-
tion.
tion when a minimum V
voltage is applied wheth-
IH1
er the oscillator is running or not. An internal pull-
down resistor permits a power-on reset with only a
Port 2: Port 2 is an 8-bit bidirectional I/O port with
internal pullups. Port 2 pins that have 1’s written to
them are pulled high by the internal pullups, and in
that state can be used as inputs. As inputs, Port 2
pins that are externally pulled low will source current
capacitor connected to V
.
CC
ALE/PROG: Address Latch Enable output signal for
latching the low byte of the address during accesses
to external memory. This pin is also the program
pulse input (PROG) during EPROM programming for
the 87C51.
(I , on the data sheet) because of the internal pull-
IL
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
If desired, ALE operation can be disabled by setting
bit 0 of SFR location 8EH. With this bit set, the pin is
weakly pulled high. However, the ALE disable fea-
ture will be suspended during a MOVX or MOVC in-
struction, idle mode, power down mode and ICE
mode. The ALE disable feature will be terminated by
reset. When the ALE disable feature is suspended or
terminated, the ALE pin will no longer be pulled up
weakly. Setting the ALE-disable bit has no effect if
the microcontroller is in external execution mode.
@
address (MOVX DPTR). In this application it uses
strong internal pullups when emitting 1’s.
During accesses to external Data Memory that use
@
8-bit addresses (MOVX Ri), Port 2 emits the con-
tents of the P2 Special Function Register.
4
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