parallel ports

Dr. Iyad Jafar

Parallel Ports

Outline

Why Do We Need Parallel Ports?Hardware Realization of Parallel PortsInterfacing to Parallel PortsThe PIC 16F84A Parallel Ports

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Why Do We Need Parallel Ports?

Almost any microcontroller needs to transfer digital data from/to external devices and for different purposesDirect user interface – switches, LEDs, keypads, displays

Input measurement - from sensors, possibly through ADC

Output control information – control motors and actuators

Bulk data transfer – to other systems/subsystems

Transfer could be serial or parallel !3

Hardware Realization of Parallel PortsOutput Parallel Port

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Hardware Realization of Parallel PortsInput Parallel Port

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Hardware Realization of Parallel PortsBidirectional Parallel Port

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The PIC 16F84 Parallel PortsPORT B

8-bit general-purpose bidirectional digital portPin RB0 is multiplexed with the external interrupt INT and has Schmitt trigger interface Data from/to this port is stored in PORTB register (0x06)Pins can be configured for input or output by setting or clearing corresponding bits in the TRISB register (0x86)Pins RB4 – RB7 have a useful ‘interrupt on change’ facility

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The PIC 16F84 Parallel PortsPORT B

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Configurable pull-up resistors using RBPU bit in the

OPTION register

Latches input data whenever the port is

read

Multiplexed input

The PIC 16F84 Parallel PortsPORT B

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Lathes data on port read

Holds previous latched data

Compares previous and present port

input values

Clearing the RBIF bit ?

The PIC 16F84 Parallel PortsPORT A

5-bit general-purpose bidirectional digital portPin RA4 is multiplexed and can be used as the clock for the TIMER 0 module Data from/to this port is stored in PORTA register (0x05)Pins can be configured for input or output by setting or clearing corresponding bits in the TRISA register (0x85)

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The PIC 16F84 Parallel PortsPORT A

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The PIC 16F84 Parallel PortsExample – configuring port B such that pins 0 to 2 are inputs, pins 3 to 4 outputs, and pins 5 to 7 are inputs

bcf STATUS, RP0; select bank0clrf PORTB ; clear port B latchesbsf STATUS , RP0 ; select bank1movlw 0xE7movwf TRISB ; PORTB<7:5> input, ; PORTB<4:3> output; PORTB<2:0> input

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Example on Using I/O PortsExample – on external interrupt (rising edge ON RB0), start flashing a LED connected to RB1 every 1 ms. If another interrupt occurs, stop flashing, and so on …

assume 4MHz clock.Requirements: 1)Configure RB0 as input and RB1 as output 2)Enable external interrupt (INTE) and global interrupts (GIE)3)Write a 1 ms delay routine 4)Keep track of the current status of flashing (on/off)

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Example on Using I/O Ports#include P16F84A.INCFLASH EQU 0X20 ; STORE THE STATE OF FLASHINGCOUNT EQU 0X21 ; COUNTER FOR DELAY LOOP

ORG 0X0000GOTO START ORG 0X0004GOTO ISR

; ---------------------------------- MAIN PROGRAM -----------------------------------------------START CLRF FLASH ; CLEAR FLASHING STATUS

BSF STATUS,RP0 ; SELECT BANK 1MOVLW B'00000001' ; CONFIGURE RB0 AS INPUT AND RB1 AS OUPUTMOVWF TRISBBSF OPTION_REG, INTEDG ; SELECT RISING EDGE FOR EXTERNAL INTERRUPT BSF INTCON , INTE ; ENABLE EXTERNAL INTERRUPT BSF INTCON , GIE ; ENABLE GLOBAL INTERRUPTBCF STATUS,RP0 ; SELECT BANK 0CLRF PORTB ; CLEAR PORTB

WAIT BTFSS FLASH , 0 ; IF BIT 0 OF FLASH IS CLEAR THEN NO FLASHING GOTO WAIT ; WAIT UNTIL BIT 0 IS SET MOVLW B'00000010' XORWF PORTB , 1 ; COMPLEMENT RB1 TO FLASHCALL DEL_1MSGOTO WAIT

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Example on Using I/O Ports; ---------------- DELAY ROUTINE ------------------------DEL_1MS NOP

MOVLW D'142'MOVWF COUNT

LOOP NOPNOPNOPNOPDECFSZ COUNT , 1 GOTO LOOPRETURN ; DELAY = (2+1+1+1+(4+1+2)*141+(4+2+2)*4/4MHz

= 1ms

; --------------- INTERRUPT SERVICE ROUTINE ---------------ISR XORWF FLASH , 1 ; COMPLEMENT THE STATUS

BCF INTCON , INTF ; CLEAR THE INTF FLAG RETFIE

END

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