SE PAI Unit 5_IO programming in 8051

Unit 5: 8051 I/O Port Programming
Subject: Processor Architecture & Interfacing
(SPPU,Pune2015CourseofInformation Technology)
Class: SEIT Semester: II
Prepared By,
Ms. K. D. Patil, Assistant Professor
Department of Information Technology
(NBA accredited)
Sanjivani College of Engineering, Kopargaon-423603.
Maharashtra, India.
(An Autonomous Institute, Affiliated with SPPU, Pune.)
NAAC ‘A’ Grade Accredited, ISO 9001:2015 certified

Learning Outcomes
• Students will be able to
• Learn different ports of 8051
• Apply knowledge of 8051 assembly language to write a
code for 8051 I/O port programming
Prepared By: Ms. K. D. Patil, Dept. of Information
Technology, Sanjivani COE, Kopargaon
2

Contents
• Introduction to 8051 Ports
• Port 0
• Port 1
• Port 2
• Port3
• Different Ways of Accessing Entire Byte
• I/O Ports and Bit addressability
• Single Bit Instructions
• Examples
3

Introduction to Ports of 8051
4

Introduction to Ports of 8051
● As shown in the figure, In 8051 there are a total of
four ports for I/O operations.
● Out of 40 pins, a total of 32 pins are set aside for
the four ports P0,P1,P2 and P3, where each port
takes 8 pins.
● All ports upon RESET are configured as inputs,
when first 0 (zero) is written to port, it becomes
output.
● To reconfigure it as an input, 1 must be sent to the
port.
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6
Port 0 (Pins 39 to 32 )
● A total of 8 pins.
● Can be used for Input or output.
● To use it as input or output,
each must be connected
externally to 10 k-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.

7
Port 0
● The following code will continuously send out to port 0 the
alternating values of 55H and AAH. By sending 55H (0101 0101)
and AAH (1010 1010) to given port continuously we can toggle
(switch from one state to another state) the bits.
BACK: MOV A, #55H
MOV P0, A
ACALL DELAY
MOV A, #0AAH
MOV P0, A
ACALL DELAY
SJMP BACK

8
Port 0 as Input
● With resistors connected to port 0, In order to make port 0 an input, the
port must be programmed by writing 1 to all the bits.
● In the following example, 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

9
Port 0 as Dual Role
● 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.
● The 8051 multiplexes address and data through port 0 to save the
pins.

10
Port 1 (Pins 1 to 8)
● Occupies a total of 8 pins.
● 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.
● The following code will continuously send out to port 1 the alternating
values of 55H and AAH. By sending 55H (0101 0101) and AAH (1010
1010) to given port continuously, we can toggle (switch from one state to
another state) the bits.
BACK: MOV A, #55H
MOV P1, A
ACALL DELAY
CPL A ; complement register A (A = 1010 1010 after executing instruction.)
SJMP BACK

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● To make port 1 an input port, it must be programmed as such by writing 1 to all its
bits.
● In the following example, 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 = FFH
MOV P1, A ;make p1 as input port by writing all 1s to it
MOV A, P1 ; get data from p1
MOV R7,A ; save data in R7 register
ACALL DELAY ; wait
MOV R6, A ; save data in R6 register
ACALL DELAY ;wait
MOV R6, A ; save data in R5 register
Port 1 as input

12
● Just like P1, this port 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.
● The following code will continuously send out to port 1 the alternating values of
55H and AAH.
BACK: MOV A, #55H
MOV P2, A
ACALL DELAY
CPL A ; complement reg. A (A = 1010 1010 after executing instruction.)
SJMP BACK

13
Port 2 as input
● To make port 1 an input port, it must be programmed as such by
writing 1 to all its bits.
● In the following example, Port 1 is configured first as an input port
by writing 1s to it, then data is received from that port and send it P1
continuously.
MOV A,#0FFH ;A=FF hex
MOV P2, A ;make P2 an i/p port by writing it all 1s
BACK: MOV A,P2 ;get data from P2
MOV P1,A ;send it to port 1
SJMP BACK ;keep doing it

14
Port 2 as Dual Role
● 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.
● Since 8031/51 is capable of accessing 64K bytes of memory, it needs
a path for the 16 bits of the address.
● When 8051/31 is connected to external memory, it is job of P2 to
provide bits A8-A15 of the address, and it cannot be used for I/O
operation.

● Just like P1 and P2, this port does not need any pull-up resistors.
● Although upon RESET, port 3 is configured as an input port, it is not the way it is
most commonly used.
● Port 3 has the additional function of providing some extremely important signals.
● P3.0 and P3.1 are used for the RxD and TxD serial communication signals.
● Bits P3.2 and P3.3 are set aside for external interrupts.
● Bits P3.4 and P3.5 are used for Timer 0 and Timer 1.
● Bits P3.6 and P3.7 are used to provide the Read and Write signals of external
memories connected in 8031-based systems.
● In systems based on 8751, 89V51, pin P3.6 and P3.7 are used for I/O while the
rest of the pins in port 3 are normally used in alternate function role.
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16
P3 Port Pin Pin No. Alternate Function
P3.0 10 RxD (Serial Input Port)
P3.1 11 TxD (Serial Output Port)
P3.2 12 /INT0 (External Hardware Interrupt 0)
P3.3 13 /INT1 (External Hardware Interrupt 1)
P3.4 14 T0 (Timer/ Counter 0 external Input)
P3.5 15 T1 (Timer/ Counter 1 external Input)
P3.6 16 /WR (External Data memory Write
Strobe)
P3.7 17 /RD (External Data memory Read Strobe)
Note : ‘/’ indicates active low

17
Different Ways of Accessing Entire
Byte (8 Bits)
● One way of accessing entire port
without using Accumulator (A)
BACK: MOV P0, #55H
ACALL DELAY
MOV P0, #0AAH
ACALL DELAY
SJMP BACK
● One more way to do same thing
MOV A, #55H
BACK: MOV P0, A
ACALL DELAY
CPLA
SJMP BACK
● The entire 8 bits of
Port 0 are accessed
BACK: MOV A, #55H
MOV P0, A
ACALL DELAY
MOV A, #0AAH
MOV P0, A
ACALL DELAY
SJMP BACK

18
I/O Ports and Bit Addressability
● Sometimes we need to access only 1
or 2 bits of the port instead of entire
8 bits.
● 8051 port have a capability to access
individual bits of the port without
altering the rest of the bits in that
port.
● When accessing a port in single bit
manner, we use the syntax, “SETB
X.Y”
– Where, X is the port number
(0,1,2,3)
– Y is the desired bit number from
0 to 7 for the data bits D0 to D7
● Example:
BACK : CPL P1.2
ACALL DELAY
SJMP BACK
Above code complements bit 2 of port 1
only. Other bits will be as it is
● The above example can be written
as
AGAIN: SETB P1.2
ACALL DALAY
CLR P1.2
ACALL DELAY
SJMP AGAIN

19
Single Bit Addressability of Ports

20
Single Bit Instructions
● Instructions that are used for single-bit operations
are as follows
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)

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Instructions for reading an Input Port
● The JNB and JB instructions are widely use 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

Example No. 1
Write ALP to 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.
Program:
HERE: SETB P1.0 ; set to high bit 0 of port 1
LCALL DELAY ; call delay subroutine
CLR P1.0 ; P1.0 = 0
LCALL DELAY
SJMP HERE ; keep doing it
• Another way to write
above program
HERE : CPL P1.0
LCALL DELAY
SJMP HERE
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Example No. 2
A switch is connected to pin P1.7. Write a program to get the
status of the switch and perform the following
a. if SW = 0 , send letter “N” to P2
b. if SW = 1 , send letter “Y” to P2
Solution:
SETB P1.7 ; set to high bit 7 of port 1 for input
AGAIN: JB P1.7, OVER ; jump IF P1.7 = 1
MOV P2, # ’N’ ; SW=0, send “N” to P2
SJMP AGAIN ; keep monitoring
OVER: MOV P2, # ’Y’ ; SW=1, send “Y” to P2

24
Example No. 3
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:
SETB P1.7 ; make P1.7 as input
AGAIN: MOV C, P1.0 ; read switch status into CF
MOV P2.7, C ; send switch status to LED
NOTE: mov P2.7, P1.0 is not valid instruction but
mov P2, P1 is valid instruction

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Example No. 4
Write a program to configure I/O ports of 8051 for the following
configurations using bit/byte addressable instructions:
(a) To generate square wave at P3.0
(b) alternate pins of port P1 in input and output mode
(c) P2.0 to P2.3 to read the data from keyboard and P2.4 to P2.7 to
write data to display
Solution:
(a) The 50% duty cycle means that the “on” and “off” state (or the high and low
portion of the pulse) have the same length.
ORG 00H
AGAIN: CPL P3.0
LCALL DELAY ; wait
END

26
Example No. 4
(b) alternate pins of port P1 in
input and output mode
Solution:
ORG 00H
AGAIN:
MOV P1,#10101010B
CPL P1
SJMPAGAIN
END
(c) P2.0 to P2.3 to read the data from
keyboard and P2.4 to P2.7 to write
data to display
Solution:
ORG 00H
SETB P2.0 ; P2.0 TO P2.3 AS INPUT
SETB P2.1
SETB P2.2
SETB P2.3
CLR P2.4 ;PIN 2.4 TO P2.7 AS OUTPUT PORT
CLR P2.5
CLR P2.6
CLR P2.7
END

Example No. 5
Write a program to perform the following:
(a) Keep monitoring the P0.1 bit until it becomes high
(b) When P0.1 becomes high, read in data from port 1
(c) Send a low-to-high (L-to-H) pulse on P0.2 to indicate that data
has been read
Solution:
ORG 00H
SETB P0.1 ; MAKE P0.1 HIGH
MOV P1, #0FFH ;MAKE P1 AS INPUT PORT
AGAIN: JNB P0.1, AGAIN ; CHECK IF P0.1 IS HIGH,IF NOT
; KEEP MONITORING
MOV A, P1 ; TAKE DATA FROM P1
CLR P0.2 ; LOW PULSE
SETB P0.2 ; HIGH PULSE
END
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Reference
• “The 8051 Microcontroller and Embedded Systems using
Assembly and C” , Muhammad Ali Mazidi, Janice Gillispie
Mazidi, Rolin D. McKinlay, Second Edition, Pearson
publication
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SE PAI Unit 5_IO programming in 8051

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