8051 IO Port Programming

In the 8051 there are a total of four ports for I/O operations. A total of 32 pins are set aside for the four ports PO, P1 P2, and P3, where each port takes 8 pins.

The four ports PO, P1, P2, and P3 each use 8 pins, making them 8-bit ports. All the ports upon RESET are configured as inputs, 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.

Toggle all the bits:

	ORG 0H
	MOV P0, #0H	; make P0 an output port
	MOV P1, #0H	; make P1 an output port
	MOV P2, #0H	; make P2 an output port
	MOV P3, #0H	; make P3 an output port
BACK :	MOV P0, #55H
	MOV P1, #55H
	MOV P2, #55H
	MOV P3, #55H
	ACALL DELAY
	MOV P0, #0AAH
	MOV P1, #0AAH
	MOV P2, #0AAH
	MOV P3, #0AAH
	ACALL DELAY
	SJMP BACK

; Delay Subroutine

	ORG 500H	; put time delay subroutine at program memory location 500H
DELAY :	MOV R3, #0FFH	; initialize the counter
LOOP :	DJNZ R3, LOOP	; stay her until r5 = 0
	RET	; return to caller
	END

Get a byte from port 0 and send it to port 1:

	ORG 0H
	MOV P0, #0FFH	; make P0 an input port
	MOV P1, #0H	; make P1 an output port
BACK :	MOV A, P0
	MOV P1, A
	SJMP BACK
	END

Bit Addressability:
Sometimes we need to access only 1 or 2 bits of the port instead of the entire 8 bits. When accessing a port in single-bit manner, we use the syntax “SETB X. Y” where X is the port number 0, 1,2. or 3, and Y is the desired bit number from 0 to 7.

Toggle P0.1:

	ORG 0H
	MOV P0, #0H	; make P0 an output port
AGAIN:	SETB P0.1
	ACALL DELAY
	CLR P0.1
	ACALL DELAY
	SJMP AGAIN

; Delay Subroutine

	ORG 500H	; put time delay subroutine at program memory location 500H
DELAY :	MOV R3, #0FFH	; initialize the counter
LOOP :	DJNZ R3, LOOP	; stay her until r5 = 0
	RET	; return to caller
	END

Single Bit Instruction:

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
JNB bit, target	Jump to target if bit = 0
JBC bit, target	Jump to target if bit = 1, Clear bit

Monitoring an IO pin:

	ORG 0H
	SETB P0.1	; make P0.1 an input pin
AGAIN :	JNB P0.1, AGAIN
	MOV P0, #55H
	END

Instruction for Reading an Input Port:

Mnemonic	Example	Description
MOV A, PX	MOV A, P0	Bring in to A the data at P0 pins
JNB PX.Y, target	JNB P0.1, LABEL	Jump if P0.1 = 0
JB PX.Y, target	JB P0.1, LABEL	Jump if p0.1 = 1
MOV C, PX.Y	MOV C, P0.1	Copy the status of P0.1 to Carry flag

Instructions reading a Latch:
In reading a port, some instructions read the status of port pins while others read the status of an internal port latch.

Mnemonic	Example
ANL PX, A	ANL P0, A
ORL PX, A	ORL P0, A
XRL PX, A	XRL P0, A
JBC bit, target	JBC P0.1, LABEL
CPL PX.Y	CPL P0.1
INC PX	INC P0
DEC PX	DEC P0
DJNZ PX.Y, target	DJNZ P0.1. LABEL
MOV PX.Y, C	MOV P0.1, C
SETB PX.Y	SETB P0.1
CLR PX.Y	CLR P0.1

The instructions that read the port latch normally read a value, perform an operation (and possibly change it), then rewrite it back to the port latch. This is often called “Read-Modify-Write”. A single instruction can be used to read the port, modify its value and write the result back to port.

	ORG 0H
	MOV P0, #55H
AGAIN:	XRL P0, #0FFH	; read P0, XOR P0 with FFH and write the result back to P0
	ACALL DELAY
	SJMP AGAIN

; Delay Subroutine

	ORG 500H	; put time delay subroutine at program memory location 500H
DELAY :	MOV R3, #0FFH	; initialize the counter
LOOP :	DJNZ R3, LOOP	; stay her until r5 = 0
	RET	; return to caller
	END

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Related topics:
8051 Loop   |   8051 Delay   |   8051 Timer Programming   |   8051 Counter   |   8051 Serial Port Programming   |   8051 Programming Timer Interrupt   |   8051 Programming Serial Interrupt   |   8051 Programming External Interrupt

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List of topics: 8051

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