I/O PROGRAMMING

I/O Programming

• 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

Why P0 Need Pull Up Resistor?

P0 is open drain, so each pin of P0 must be connected externally to a 10K ohm pull up resistor.

• Open drain is a term used ofr MOC chips in the same way that open collector is use for TTL chips.

Example:

• 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

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

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=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

Additional Functions of Port 3

Port 3 has the additional function of providing some extremely important signals

Complete Program Example

• Write a program 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 Calculation

= 11 × 248 × 255 × 4 MC × 90 ns

= 250,430 μs

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

Bit Addressing of Port:

• Sometimes we need to access only 1 or 2 bits of the port

Example:

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

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

Example:

Write a program to perform the following:

1. Keep monitoring the P1.2 bit until it becomes high
2. When P1.2 becomes high, write value 45H to port 0
3. 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

Example:

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

Example:

A switch is connected to pin P1.7. Write a program to check the status of SW and perform the following:

1. If SW=0, send letter ‘N’ to P2
2. If SW=1, send letter ‘Y’ to P2

Solution:

SETB P1.7                             ;make P1.7 an input

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

SJMP AGAIN                      ;keep monitoring

Example:

A switch is connected to pin P1.7. Write a program to check the status of SW and perform the following:

1. If SW=0, send letter ‘N’ to P2
2. If SW=1, send letter ‘Y’ to P2

Use the carry flag to check the switch status.

Solution:

SETB P1.7                             ;make P1.7 an input

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

SJMP AGAIN                      ;keep monitoring

OVER:

MOV P2,#’Y’                      ;SW=1, issue ‘Y’ to P2

SJMP AGAIN                      ;keep monitoring