4 io port-programming

Home Automation, Networking, and Entertainment Lab
Dept. of Computer Science and Information Engineering
National Cheng Kung University, TAIWAN
Chung-Ping Young
楊中平
I/O PORT
PROGRAMMING
The 8051 Microcontroller and Embedded
Systems: Using Assembly and C
Mazidi, Mazidi and McKinlay
I/O PORT PROGRAMMING

Department of Computer Science and Information Engineering
National Cheng Kung University, TAIWAN 2
HANEL
I/O
PROGRAMMING
8051
(8031)
(89420)
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
RST
(RXD) P3.0
(TXD) P3.1
(-INT0) P3.2
(-INT1) P3.3
(T0) P3.4
(T1) P3.5
(-WR) P3.6
(-RD )P3.7
XTAL2
XTAL1
GND
Vcc
P0.0 (AD0)
P0.1 (AD1)
P0.2 (AD2)
P0.3 (AD3)
P0.4 (AD4)
P0.5 (AD5)
P0.6 (AD6)
P0.7 (AD7)
-EA/VPP
ALE/PROG
-PSEN
P2.7 (A15)
P2.6 (A14)
P2.5 (A13)
P2.4 (A12)
P2.3 (A11)
P2.2 (A10)
P2.1 (A9)
P2.0 (A8)
1
2
3
4
5
6
7
8
9
10
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22
21
Provides
+5V supply
voltage to
the chip
Grond
P1
P3
8051 Pin Diagram
A total of 32
pins are set
aside for the
four ports P0,
P1, P2, P3,
where each
port takes 8
pins
P0
P2

HANEL
I/O
PROGRAMMING
I/O Port Pins
The four 8-bit I/O ports P0, P1, P2 and
P3 each uses 8 pins
All the ports upon RESET are
configured as input, ready to be used
as input ports
When the first 0 is written to a port, it
becomes an output
To reconfigure it as an input, a 1 must be
sent to the port
To use any of these ports as an input port, it
must be programmed
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
RST
(RXD)P3.0
(TXD)P3.1
(INT0)P3.2
(INT1)P3.3
(T0)P3.4
(T1)P3.5
(WR)P3.6
(RD)P3.7
XTAL2
XTAL1
GND
Vcc
P0.0(AD0)
P0.1(AD1)
P0.2(AD2)
P0.3(AD3)
P0.4(AD4)
P0.5(AD5)
P0.6(AD6)
P0.7(AD7)
-EA/VPP
ALE/PROG
-PSEN
P2.7(A15)
P2.6(A14)
P2.5(A13)
P2.4(A12)
P2.3(A11)
P2.2(A10)
P2.1(A9)
P2.0(A8)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
8051
(8031)

HANEL
I/O
PROGRAMMING
Port 0
It can be used for input or output,
each pin must be connected externally
to a 10K ohm pull-up resistor
This is due to the fact that P0 is an open
drain, unlike P1, P2, and P3
Open drain is a term used for MOS chips in the
same way that open collector is used for TTL
chips
P0.0
P0.1
P0.2
P0.3
P0.4
P0.5
P0.6
P0.7
Vcc
10 K
8051
Port0
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
RST
(RXD)P3.0
(TXD)P3.1
(INT0)P3.2
(INT1)P3.3
(T0)P3.4
(T1)P3.5
(WR)P3.6
(RD)P3.7
XTAL2
XTAL1
GND
Vcc
P0.0(AD0)
P0.1(AD1)
P0.2(AD2)
P0.3(AD3)
P0.4(AD4)
P0.5(AD5)
P0.6(AD6)
P0.7(AD7)
-EA/VPP
ALE/PROG
-PSEN
P2.7(A15)
P2.6(A14)
P2.5(A13)
P2.4(A12)
P2.3(A11)
P2.2(A10)
P2.1(A9)
P2.0(A8)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
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18
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20
40
39
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31
30
29
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8051
(8031)
P0.X

HANEL
I/O
PROGRAMMING
Port 0
(cont’)
The following code will continuously send out to port 0 the
alternating value 55H and AAH
BACK: MOV A,#55H
MOV P0,A
ACALL DELAY
MOV A,#0AAH
MOV P0,A
ACALL DELAY
SJMP BACK
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
RST
(RXD)P3.0
(TXD)P3.1
(INT0)P3.2
(INT1)P3.3
(T0)P3.4
(T1)P3.5
(WR)P3.6
(RD)P3.7
XTAL2
XTAL1
GND
Vcc
P0.0(AD0)
P0.1(AD1)
P0.2(AD2)
P0.3(AD3)
P0.4(AD4)
P0.5(AD5)
P0.6(AD6)
P0.7(AD7)
-EA/VPP
ALE/PROG
-PSEN
P2.7(A15)
P2.6(A14)
P2.5(A13)
P2.4(A12)
P2.3(A11)
P2.2(A10)
P2.1(A9)
P2.0(A8)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
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40
39
38
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31
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26
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21
8051
(8031)

HANEL
I/O
PROGRAMMING
Port 0 as Input
In order to make port 0 an input, the
port must be programmed by writing 1
to all the bits
Port 0 is configured first as an input port by writing 1s to it, and then
data is received from that port and sent to P1
MOV A,#0FFH ;A=FF hex
MOV P0,A ;make P0 an i/p port
;by writing it all 1s
BACK: MOV A,P0 ;get data from P0
MOV P1,A ;send it to port 1
SJMP BACK ;keep doing it
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
RST
(RXD)P3.0
(TXD)P3.1
(INT0)P3.2
(INT1)P3.3
(T0)P3.4
(T1)P3.5
(WR)P3.6
(RD)P3.7
XTAL2
XTAL1
GND
Vcc
P0.0(AD0)
P0.1(AD1)
P0.2(AD2)
P0.3(AD3)
P0.4(AD4)
P0.5(AD5)
P0.6(AD6)
P0.7(AD7)
-EA/VPP
ALE/PROG
-PSEN
P2.7(A15)
P2.6(A14)
P2.5(A13)
P2.4(A12)
P2.3(A11)
P2.2(A10)
P2.1(A9)
P2.0(A8)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
40
39
38
37
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35
34
33
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31
30
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26
25
24
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22
21
8051
(8031)

HANEL
I/O
PROGRAMMING
Dual Role of
Port 0
Port 0 is also designated as AD0-AD7,
allowing it to be used for both address
and data
When connecting an 8051/31 to an
external memory, port 0 provides both
address and data
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
RST
(RXD)P3.0
(TXD)P3.1
(INT0)P3.2
(INT1)P3.3
(T0)P3.4
(T1)P3.5
(WR)P3.6
(RD)P3.7
XTAL2
XTAL1
GND
Vcc
P0.0(AD0)
P0.1(AD1)
P0.2(AD2)
P0.3(AD3)
P0.4(AD4)
P0.5(AD5)
P0.6(AD6)
P0.7(AD7)
-EA/VPP
ALE/PROG
-PSEN
P2.7(A15)
P2.6(A14)
P2.5(A13)
P2.4(A12)
P2.3(A11)
P2.2(A10)
P2.1(A9)
P2.0(A8)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
40
39
38
37
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35
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33
32
31
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26
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22
21
8051
(8031)

HANEL
I/O
PROGRAMMING
Port 1
Port 1 can be used as input or output
In contrast to port 0, this port does not
need any pull-up resistors since it already
has pull-up resistors internally
Upon reset, port 1 is configured as an
input port
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
RST
(RXD)P3.0
(TXD)P3.1
(INT0)P3.2
(INT1)P3.3
(T0)P3.4
(T1)P3.5
(WR)P3.6
(RD)P3.7
XTAL2
XTAL1
GND
Vcc
P0.0(AD0)
P0.1(AD1)
P0.2(AD2)
P0.3(AD3)
P0.4(AD4)
P0.5(AD5)
P0.6(AD6)
P0.7(AD7)
-EA/VPP
ALE/PROG
-PSEN
P2.7(A15)
P2.6(A14)
P2.5(A13)
P2.4(A12)
P2.3(A11)
P2.2(A10)
P2.1(A9)
P2.0(A8)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
40
39
38
37
36
35
34
33
32
31
30
29
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27
26
25
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23
22
21
8051
(8031)
The following code will continuously send out to port 0 the
alternating value 55H and AAH
MOV A,#55H
BACK: MOV P1,A
ACALL DELAY
CPL A
SJMP BACK

HANEL
I/O
PROGRAMMING
Port 1 as Input
To make port 1 an input port, it must
be programmed as such by writing 1
to all its bits
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
RST
(RXD)P3.0
(TXD)P3.1
(INT0)P3.2
(INT1)P3.3
(T0)P3.4
(T1)P3.5
(WR)P3.6
(RD)P3.7
XTAL2
XTAL1
GND
Vcc
P0.0(AD0)
P0.1(AD1)
P0.2(AD2)
P0.3(AD3)
P0.4(AD4)
P0.5(AD5)
P0.6(AD6)
P0.7(AD7)
-EA/VPP
ALE/PROG
-PSEN
P2.7(A15)
P2.6(A14)
P2.5(A13)
P2.4(A12)
P2.3(A11)
P2.2(A10)
P2.1(A9)
P2.0(A8)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
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8051
(8031)
Port 1 is configured first as an input port by writing 1s to it, then data
is received from that port and saved in R7 and R5
MOV A,#0FFH ;A=FF hex
MOV P1,A ;make P1 an input port
;by writing it all 1s
MOV A,P1 ;get data from P1
MOV R7,A ;save it to in reg R7
ACALL DELAY ;wait
MOV A,P1 ;another data from P1
MOV R5,A ;save it to in reg R5

HANEL
I/O
PROGRAMMING
Port 2
Just like P1, port 2 does not need any pull-
up resistors since it already has pull-up
resistors internally
Upon reset, port 2 is configured as an input
port
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
RST
(RXD)P3.0
(TXD)P3.1
(INT0)P3.2
(INT1)P3.3
(T0)P3.4
(T1)P3.5
(WR)P3.6
(RD)P3.7
XTAL2
XTAL1
GND
Vcc
P0.0(AD0)
P0.1(AD1)
P0.2(AD2)
P0.3(AD3)
P0.4(AD4)
P0.5(AD5)
P0.6(AD6)
P0.7(AD7)
-EA/VPP
ALE/PROG
-PSEN
P2.7(A15)
P2.6(A14)
P2.5(A13)
P2.4(A12)
P2.3(A11)
P2.2(A10)
P2.1(A9)
P2.0(A8)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
8051
(8031)

HANEL
I/O
PROGRAMMING
Port 2 as Input
or Dual Role
To make port 2 an input port, it must
be programmed as such by writing 1 to
all its bits
In many 8051-based system, P2 is used
as simple I/O
In 8031-based systems, port 2 must be
used along with P0 to provide the 16-
bit address for the external memory
Port 2 is also designated as A8 – A15,
indicating its dual function
Port 0 provides the lower 8 bits via A0 – A7
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
RST
(RXD)P3.0
(TXD)P3.1
(INT0)P3.2
(INT1)P3.3
(T0)P3.4
(T1)P3.5
(WR)P3.6
(RD)P3.7
XTAL2
XTAL1
GND
Vcc
P0.0(AD0)
P0.1(AD1)
P0.2(AD2)
P0.3(AD3)
P0.4(AD4)
P0.5(AD5)
P0.6(AD6)
P0.7(AD7)
-EA/VPP
ALE/PROG
-PSEN
P2.7(A15)
P2.6(A14)
P2.5(A13)
P2.4(A12)
P2.3(A11)
P2.2(A10)
P2.1(A9)
P2.0(A8)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
8051
(8031)

HANEL
I/O
PROGRAMMING
Port 3
Port 3 does not need any pull-up resistors
Port 3 is configured as an input port upon
reset, this is not the way it is most
commonly used
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
RST
(RXD)P3.0
(TXD)P3.1
(INT0)P3.2
(INT1)P3.3
(T0)P3.4
(T1)P3.5
(WR)P3.6
(RD)P3.7
XTAL2
XTAL1
GND
Vcc
P0.0(AD0)
P0.1(AD1)
P0.2(AD2)
P0.3(AD3)
P0.4(AD4)
P0.5(AD5)
P0.6(AD6)
P0.7(AD7)
-EA/VPP
ALE/PROG
-PSEN
P2.7(A15)
P2.6(A14)
P2.5(A13)
P2.4(A12)
P2.3(A11)
P2.2(A10)
P2.1(A9)
P2.0(A8)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
8051
(8031)

HANEL
I/O
PROGRAMMING
Port 3
(cont’)
Port 3 has the additional function of
providing some extremely important
signals
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
RST
(RXD)P3.0
(TXD)P3.1
(INT0)P3.2
(INT1)P3.3
(T0)P3.4
(T1)P3.5
(WR)P3.6
(RD)P3.7
XTAL2
XTAL1
GND
Vcc
P0.0(AD0)
P0.1(AD1)
P0.2(AD2)
P0.3(AD3)
P0.4(AD4)
P0.5(AD5)
P0.6(AD6)
P0.7(AD7)
-EA/VPP
ALE/PROG
-PSEN
P2.7(A15)
P2.6(A14)
P2.5(A13)
P2.4(A12)
P2.3(A11)
P2.2(A10)
P2.1(A9)
P2.0(A8)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
8051
(8031)
P3 Bit Function
P3.0 RxD
TxD
INT0
INT1
T0
T1
WR
RD
P3.1
P3.2
P3.3
P3.4
P3.5
P3.6
P3.7
Pin
10
11
12
13
14
15
16
17
Serial
communications
External
interrupts
Timers
Read/Write signals
of external memories
In systems based on 8751, 89C51 or
DS89C4x0, pins 3.6 and 3.7 are used for I/O
while the rest of the pins in port 3 are
normally used in the alternate function role

HANEL
I/O
PROGRAMMING
Port 3
(cont’)
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
RST
(RXD)P3.0
(TXD)P3.1
(INT0)P3.2
(INT1)P3.3
(T0)P3.4
(T1)P3.5
(WR)P3.6
(RD)P3.7
XTAL2
XTAL1
GND
Vcc
P0.0(AD0)
P0.1(AD1)
P0.2(AD2)
P0.3(AD3)
P0.4(AD4)
P0.5(AD5)
P0.6(AD6)
P0.7(AD7)
-EA/VPP
ALE/PROG
-PSEN
P2.7(A15)
P2.6(A14)
P2.5(A13)
P2.4(A12)
P2.3(A11)
P2.2(A10)
P2.1(A9)
P2.0(A8)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
8051
(8031)
Write a program for the DS89C420 to toggle all the bits of P0, P1,
and P2 every 1/4 of a second
ORG 0
BACK: MOV A,#55H
MOV P0,A
MOV P1,A
MOV P2,A
ACALL QSDELAY ;Quarter of a second
MOV A,#0AAH
MOV P0,A
MOV P1,A
MOV P2,A
ACALL QSDELAY
SJMP BACK
QSDELAY:
MOV R5,#11
H3: MOV R4,#248
H2: MOV R3,#255
H1: DJNZ R3,H1 ;4 MC for DS89C4x0
DJNZ R4,H2
DJNZ R5,H3
RET
END
Delay
= 11 × 248 × 255 × 4 MC × 90 ns
= 250,430 µs

HANEL
I/O
PROGRAMMING
Different ways
of Accessing
Entire 8 Bits
The entire 8 bits of Port 1 are accessed
BACK: MOV A,#55H
MOV P1,A
ACALL DELAY
MOV A,#0AAH
MOV P1,A
ACALL DELAY
SJMP BACK
Rewrite the code in a more efficient manner by accessing the port
directly without going through the accumulator
BACK: MOV P1,#55H
ACALL DELAY
MOV P1,#0AAH
ACALL DELAY
SJMP BACK
Another way of doing the same thing
MOV A,#55H
BACK: MOV P1,A
ACALL DELAY
CPL A
SJMP BACK

HANEL
I/O BIT
MANIPULATION
PROGRAMMING
I/O Ports
and Bit
Addressability
Sometimes we need to access only 1
or 2 bits of the port
BACK: CPL P1.2 ;complement P1.2
ACALL DELAY
SJMP BACK
;another variation of the above program
AGAIN: SETB P1.2 ;set only P1.2
ACALL DELAY
CLR P1.2 ;clear only P1.2
ACALL DELAY
SJMP AGAIN P0 P1 P2 P3
P1.0 P2.0
P2.1
P2.2
P2.3
P2.4
P2.5
P2.6
P2.7
P1.1
P3.0
P3.1
P3.2
P3.3
P3.4
P3.5
P3.6
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7 P3.7
P0.0
P0.1
P0.2
P0.3
P0.4
P0.5
P0.6
P0.7
Port Bit
D0
D1
D2
D3
D4
D5
D6
D7

HANEL
I/O BIT
MANIPULATION
PROGRAMMING
I/O Ports
and Bit
Addressability
(cont’)
Example 4-2
Write the following programs.
Create a square wave of 50% duty cycle on bit 0 of port 1.
Solution:
The 50% duty cycle means that the “on” and “off” state (or the high
and low portion of the pulse) have the same length. Therefore,
we toggle P1.0 with a time delay in between each state.
HERE: SETB P1.0 ;set to high bit 0 of port 1
LCALL DELAY ;call the delay subroutine
CLR P1.0 ;P1.0=0
LCALL DELAY
SJMP HERE ;keep doing it
Another way to write the above program is:
HERE: CPL P1.0 ;set to high bit 0 of port 1
LCALL DELAY ;call the delay subroutine
SJMP HERE ;keep doing it
8051
P1.0

HANEL
I/O BIT
MANIPULATION
PROGRAMMING
I/O Ports
and Bit
Addressability
(cont’)
Instructions that are used for signal-bit
operations are as following
Instruction Function
SETB bit Set the bit (bit = 1)
CLR bit Clear the bit (bit = 0)
CPL bit Complement the bit (bit = NOT bit)
JB bit, target Jump to target if bit = 1 (jump if bit)
JNB bit, target Jump to target if bit = 0 (jump if no bit)
JBC bit, target Jump to target if bit = 1, clear bit
(jump if bit, then clear)
Single-Bit Instructions

HANEL
I/O BIT
MANIPULATION
PROGRAMMING
Checking an
Input Bit
The JNB and JB instructions are widely
used single-bit operations
They allow you to monitor a bit and make
a decision depending on whether it’s 0 or 1
These two instructions can be used for any
bits of I/O ports 0, 1, 2, and 3
Port 3 is typically not used for any I/O, either
single-bit or byte-wise
Mnemonic Examples Description
MOV A,PX MOV A,P2 Bring into A the data at P2 pins
JNB PX.Y, .. JNB P2.1,TARGET Jump if pin P2.1 is low
JB PX.Y, .. JB P1.3,TARGET Jump if pin P1.3 is high
MOV C,PX.Y MOV C,P2.4 Copy status of pin P2.4 to CY
Instructions for Reading an Input Port

HANEL
I/O BIT
MANIPULATION
PROGRAMMING
Checking an
Input Bit
(cont’)
Example 4-3
Write a program to perform the following:
(a) Keep monitoring the P1.2 bit until it becomes high
(b) When P1.2 becomes high, write value 45H to port 0
(c) Send a high-to-low (H-to-L) pulse to P2.3
Solution:
SETB P1.2 ;make P1.2 an input
MOV A,#45H ;A=45H
AGAIN: JNB P1.2,AGAIN ; get out when P1.2=1
MOV P0,A ;issue A to P0
SETB P2.3 ;make P2.3 high
CLR P2.3 ;make P2.3 low for H-to-L

HANEL
I/O BIT
MANIPULATION
PROGRAMMING
Checking an
Input Bit
(cont’)
Example 4-4
Assume that bit P2.3 is an input and represents the condition of an
oven. If it goes high, it means that the oven is hot. Monitor the bit
continuously. Whenever it goes high, send a high-to-low pulse to port
P1.5 to turn on a buzzer.
Solution:
HERE: JNB P2.3,HERE ;keep monitoring for high
SETB P1.5 ;set bit P1.5=1
CLR P1.5 ;make high-to-low
SJMP HERE ;keep repeating

HANEL
I/O BIT
MANIPULATION
PROGRAMMING
Checking an
Input Bit
(cont’)
Example 4-5
A switch is connected to pin P1.7. Write a program to check the status
of SW and perform the following:
(a) If SW=0, send letter ‘N’ to P2
(b) If SW=1, send letter ‘Y’ to P2
Solution:
AGAIN: JB P1.2,OVER ;jump if P1.7=1
MOV P2,#’N’ ;SW=0, issue ‘N’ to P2
SJMP AGAIN ;keep monitoring
OVER: MOV P2,#’Y’ ;SW=1, issue ‘Y’ to P2

HANEL
I/O BIT
MANIPULATION
PROGRAMMING
Reading Single
Bit into Carry
Flag
Example 4-6
A switch is connected to pin P1.7. Write a program to check the status
of SW and perform the following:
(a) If SW=0, send letter ‘N’ to P2
(b) If SW=1, send letter ‘Y’ to P2
Use the carry flag to check the switch status.
Solution:
AGAIN: MOV C,P1.2 ;read SW status into CF
JC OVER ;jump if SW=1
MOV P2,#’N’ ;SW=0, issue ‘N’ to P2
OVER: MOV P2,#’Y’ ;SW=1, issue ‘Y’ to P2

HANEL
I/O BIT
MANIPULATION
PROGRAMMING
Reading Single
Bit into Carry
Flag
(cont’)
Example 4-7
A switch is connected to pin P1.0 and an LED to pin P2.7. Write a
program to get the status of the switch and send it to the LED
Solution:
AGAIN: MOV C,P1.0 ;read SW status into CF
MOV P2.7,C ;send SW status to LED
SJMP AGAIN ;keep repeating
The instruction
‘MOV
P2.7,P1.0’ is
wrong , since such
an instruction does
not exist
However ‘MOV
P2,P1’ is a valid
instruction

HANEL
I/O BIT
MANIPULATION
PROGRAMMING
Reading Input
Pins vs. Port
Latch
In reading a port
Some instructions read the status of port
pins
Others read the status of an internal port
latch
Therefore, when reading ports there
are two possibilities:
Read the status of the input pin
Read the internal latch of the output port
Confusion between them is a major
source of errors in 8051 programming
Especially where external hardware is
concerned

HANEL
READING
INPUT PINS VS.
PORT LATCH
Reading Latch
for Output Port
Some instructions read the contents of
an internal port latch instead of
reading the status of an external pin
For example, look at the ANL P1,A
instruction and the sequence of actions is
executed as follow
1. It reads the internal latch of the port and
brings that data into the CPU
2. This data is ANDed with the contents of
register A
3. The result is rewritten back to the port latch
4. The port pin data is changed and now has the
same value as port latch

HANEL
READING
INPUT PINS VS.
PORT LATCH
Reading Latch
for Output Port
(cont’)
Read-Modify-Write
The instructions read the port latch
normally read a value, perform an
operation then rewrite it back to the port
latch
Mnemonics Example
ANL PX ANL P1,A
ORL PX ORL P2,A
XRL PX XRL P0,A
JBC PX.Y,TARGET JBC P1.1,TARGET
CPL PX.Y CPL P1.2
INC PX INC P1
DEC PX DEC P2
DJNZ PX.Y,TARGET DJNZ P1,TARGET
MOV PX.Y,C MOV P1.2,C
CLR PX.Y CLR P2.3
SETB PX.Y SETB P2.3
Instructions Reading a latch (Read-Modify-Write)
Note: x is 0, 1, 2,
or 3 for P0 – P3

HANEL
I/O BIT
MANIPULATION
PROGRAMMING
Read-modify-
write Feature
The ports in 8051 can be accessed by
the Read-modify-write technique
This feature saves many lines of code by
combining in a single instruction all three
actions
1. Reading the port
2. Modifying it
3. Writing to the port
MOV P1,#55H ;P1=01010101
AGAIN: XRL P1,#0FFH ;EX-OR P1 with 1111 1111
ACALL DELAY
SJMP BACK

4 io port-programming

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4 io port-programming