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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