Ports
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Pins of I/O Port
The 8051 has four I/O ports – Port 0 （ pins 32-39 ）： P0 （ P0.0 ～ P0.7 ） – Port 1 （ pins 1-8 ） ： P1 （ P1.0 ～ P1.7 ） – Port 2 （ pins 21-28 ）： P2 （ P2.0 ～ P2.7 ） – Port 3 （ pins 10-17 ）： P3 （ P3.0 ～ P3.7 ） – Each port has 8 pins. Named P0.X （ X=0,1,...,7 ） , P1.X, P2.X, P3.X Ex ： P0.0 is the bit 0 （ LSB ） of P0 Ex ： P0.7 is the bit 7 （ MSB ） of P0 These 8 bits form a byte. Each port can be used as input or output (bi-direction).
A Pin of Port 1 Read latch
TB2
Vcc Load(L1)
Internal CPU bus
D
Write to latch
Clk
P1.X pin
Q
P1.X Q
M1
TB1 Read pin
8051 IC
P0.x
Writing “1” to Output Pin P1.X Read latch
Vcc
TB2
Load(L1) 2. output pin is
Vcc
1. write a 1 to the pin Internal CPU bus
D
Write to latch
Clk
1
Q
P1.X pin
P1.X Q
0
M1
TB1 Read pin
8051 IC
output 1
Writing “0” to Output Pin P1.X Read latch
Vcc
TB2
Load(L1) 2. output pin is
ground
1. write a 0 to the pin Internal CPU bus
D
Write to latch
Clk
0
Q
P1.X pin
P1.X Q
1
M1
TB1 Read pin
8051 IC
output 0
Reading “High” at Input Pin Read latch 1.
TB2
write a 1 to the pin MOV P1,#0FFH Internal CPU bus
2. MOV A,P1
Vcc
external pin=High Load(L1)
D
1
Q
1
P1.X Write to latch
Clk
0
Q
M1
TB1 Read pin 3. Read pin=1 Read latch=0 Write to latch=1
8051 IC
P1.X pin
Reading “Low” at Input Pin Read latch 1.
Vcc
2. MOV A,P1
TB2
write a 1 to the pin
Load(L1)
external pin=Low
MOV P1,#0FFH Internal CPU bus
D
1
Q
0
P1.X Write to latch
Clk
Q
0
M1
TB1 Read pin 3. Read pin=1 Read latch=0 Write to latch=1
8051 IC
P1.X pin
Other Pins P1, P2, and P3 have internal pull-up resisters. – P1, P2, and P3 are not open drain. P0 has no internal pull-up resistors and does not connects to Vcc inside the 8051. – P0 is open drain. – Compare the figures of P1.X and P0.X. However, for a programmer, it is the same to program P0, P1, P2 and P3. All the ports upon RESET are configured as output.
A Pin of Port 0 Read latch
TB2
Internal CPU bus
D
Write to latch
Clk
P0.X pin
Q
P1.X Q
M1
TB1
P1.x
Read pin
8051 IC
Port 0 with Pull-Up Resistors Vcc
Port 0
P0.0 DS5000 P0.1 P0.2 8751 P0.3 P0.4 8951 P0.5 P0.6 P0.7
10 K
RESET Value of Some 8051 Registers:
Register PC ACC B PSW SP DPTR RAM are all zero.
Reset Value 0000 0000 0000 0000 0007 0000
The 8051 has four I/O ports – Port 0 （ pins 32-39 ）： P0 （ P0.0 ～ P0.7 ） – Port 1 （ pins 1-8 ） ： P1 （ P1.0 ～ P1.7 ） – Port 2 （ pins 21-28 ）： P2 （ P2.0 ～ P2.7 ） – Port 3 （ pins 10-17 ）： P3 （ P3.0 ～ P3.7 ） – Each port has 8 pins. Named P0.X （ X=0,1,...,7 ） , P1.X, P2.X, P3.X Ex ： P0.0 is the bit 0 （ LSB ） of P0 Ex ： P0.7 is the bit 7 （ MSB ） of P0 These 8 bits form a byte. Each port can be used as input or output (bi-direction).
A Pin of Port 1 Read latch
TB2
Vcc Load(L1)
Internal CPU bus
D
Write to latch
Clk
P1.X pin
Q
P1.X Q
M1
TB1 Read pin
8051 IC
P0.x
Writing “1” to Output Pin P1.X Read latch
Vcc
TB2
Load(L1) 2. output pin is
Vcc
1. write a 1 to the pin Internal CPU bus
D
Write to latch
Clk
1
Q
P1.X pin
P1.X Q
0
M1
TB1 Read pin
8051 IC
output 1
Writing “0” to Output Pin P1.X Read latch
Vcc
TB2
Load(L1) 2. output pin is
ground
1. write a 0 to the pin Internal CPU bus
D
Write to latch
Clk
0
Q
P1.X pin
P1.X Q
1
M1
TB1 Read pin
8051 IC
output 0
Reading “High” at Input Pin Read latch 1.
TB2
write a 1 to the pin MOV P1,#0FFH Internal CPU bus
2. MOV A,P1
Vcc
external pin=High Load(L1)
D
1
Q
1
P1.X Write to latch
Clk
0
Q
M1
TB1 Read pin 3. Read pin=1 Read latch=0 Write to latch=1
8051 IC
P1.X pin
Reading “Low” at Input Pin Read latch 1.
Vcc
2. MOV A,P1
TB2
write a 1 to the pin
Load(L1)
external pin=Low
MOV P1,#0FFH Internal CPU bus
D
1
Q
0
P1.X Write to latch
Clk
Q
0
M1
TB1 Read pin 3. Read pin=1 Read latch=0 Write to latch=1
8051 IC
P1.X pin
Other Pins P1, P2, and P3 have internal pull-up resisters. – P1, P2, and P3 are not open drain. P0 has no internal pull-up resistors and does not connects to Vcc inside the 8051. – P0 is open drain. – Compare the figures of P1.X and P0.X. However, for a programmer, it is the same to program P0, P1, P2 and P3. All the ports upon RESET are configured as output.
A Pin of Port 0 Read latch
TB2
Internal CPU bus
D
Write to latch
Clk
P0.X pin
Q
P1.X Q
M1
TB1
P1.x
Read pin
8051 IC
Port 0 with Pull-Up Resistors Vcc
Port 0
P0.0 DS5000 P0.1 P0.2 8751 P0.3 P0.4 8951 P0.5 P0.6 P0.7
10 K
RESET Value of Some 8051 Registers:
Register PC ACC B PSW SP DPTR RAM are all zero.
Reset Value 0000 0000 0000 0000 0007 0000
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