87C51 查看數據表（PDF） - Intel
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87C51 Datasheet PDF : 21 Pages
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87C51 80C51BH 80C31BH
PIN DESCRIPTION
VCC Supply voltage during normal Idle and Power
Down operations
VSS Circuit ground
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
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
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
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
(IIL on the data sheet) because of the internal pull-
ups
Port 1 also receives the low-order address bytes
during EPROM programming and program verifica-
tion
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
(IIL on the data sheet) 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
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
Port 2 also receives some control signals and the
high-order address bits during EPROM programming
and program verification
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
(IIL on the data sheet) because of the pullups
Port 3 also serves the functions of various special
features of the MCS-51 Family as listed below
Pin Name
Alternate Function
P3 0
P3 1
P3 2
P3 3
P3 4
P3 5
P3 6
P3 7
RXD
TXD
INT0
INT1
T0
T1
WR
RD
Serial input line
Serial output line
External Interrupt 0
External Interrupt 1
Timer 0 external input
Timer 1 external input
External Data Memory Write strobe
External Data Memory Read strobe
Port 3 also receives some control signals for
EPROM programming and program verification
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-
tion when a minimum VIH1 voltage is applied wheth-
er the oscillator is running or not An internal pull-
down resistor permits a power-on reset with only a
capacitor connected to VCC
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
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
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