Explaining PIC16F84A Flasher Program in Detail

You may have felt that the Flasher program is difficult. But this is not true.

Now, we 'll explain it step-by-step. Just remember, when I started learning Microcontroller programming, I started by understanding this program in detail and put on it my own comments to make me remember what each piece of code did.

list p=16f84
include "p16f84.inc"

; This part is necessary for the compiler to
; know the type of the PIC you are using.

org 0x00
goto start

; The word ORG tells the compiler to put the following code ( goto
; start label ) in the address 0x00 which is the reset vector of the PIC


org 0x20
start

; Again. Puts the label start at the address 0x20

bcf INTCON,7

; Clear the R7 bit in the register INTCON which disables the interrupts

movlw 0x00

; Put 0x00 in the W register

bsf STATUS,5

; Set the Bit 5 in the status register which selects Bank 1 in the RAM

movwf TRISB
; Copy the W register content into TRISB register
; [ makes PORTB output]

bcf STATUS,5
; Clear the Bit 5 in the status register which selects Bank 0 in the RAM

again

; This is a label for the repeating part of the program

movlw 0x80
movwf PORTB

; Copies 0x80 [binary 1000 0000 ] in the W register
; Copy the W register content into PORTB register

call delay

; This command calls the Delay routine.

movlw 0x00
movwf PORTB

; Copies 0x00 [binary 0000 0000 ] in the W register
; Copy the W register content into PORTB register


goto again

; This label is important to keep the program running forever

delay

; This is the label for the delay routine
; The delay consists of 3 nested loops

movlw 0x01
movwf 0x0e

; Put 0x01 in W register
; And copy it to the memory address 0x0e in RAM

loop3

; Label

movlw 0xfa
movwf 0x0d

; Put 0xfa in W register
; And copy it to the memory address 0x0d in RAM

loop2

; Label

movlw 0xfa
movwf 0x0c

; Put 0xfa in W register
; And copy it to the memory address 0x0c in RAM

loop1

; Label

decfsz 0x0c,1
; Decrease contents of memory address 0x0c by 1 ,
; then skip the next command if the result is zero


goto loop1

; goto the outer loop

decfsz 0x0d,1

; Decrease contents of memory address 0x0d by 1 ,
; then skip the next command if the result is zero

goto loop2

; goto the outer loop

decfsz 0x0e,1

; Decrease contents of memory address 0x0e by 1 ,
; then skip the next command if the result is zero

goto loop3

; goto the outer loop

return
; Retrun from the Delay routine

end

; must be put at the end of the program

;*********************************************************************

This is the end of the program. Now that you understand this basic program of LED flasher, you can understand more complex programs and tricks.






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