PIC: EXAMPLE 3

EXAMPLE 3

Using nested loop

The connection scheme is again the same. To make this a bit more interesting, a different combination of port B bits change each other. And, that’s not all of course. As seen from the previous two examples, the microcontroller is very fast and often, it needs to be slowed down. The use of the built-in oscillator LF, as in example 2, is the last measure that should be applied. The problem is more often solved by using nested loops in a program. In this example, the variable "counter1" is decremented 255 times by 1 in the shorter loop1. Prior to leaving this loop, the program will countdown 255 times from 255 to 0. It means that between only two LED diode’s blink on the port, there are 255x255 pulses coming from the quartz oscillator. Precisely speaking, the number of pulses amounts to approximately 196 000 since it also takes some time to execute jump instructions and decrement instructions. Yes, it’s true, the microcontroller mostly waits and does nothing...

Source Code

;******************* Header ***********************************************
;************* DEFINING VARIABLES *****************************************

       cblock      0x20          ; Block of variables starts at address 20h
       counter1                  ; Variable "counter1" at address 20h
       counter2                  ; Variable "counter2" at address 21h
       endc
;**************************************************************************
       org         0x0000        ; Address of the first program instruction
       
       banksel     TRISB         ; Selects bank containing register TRISB
       clrf        TRISB         ; Clears TRISB
       
       banksel     PORTB         ; Selects bank containing register PORTB
loop
       movlw       B'11110000'   ; Binary number 11110000 is moved to W
       movwf       PORTB         ; Number is moved to PORTB
       movlw       h'FF'         ; Number hFF is moved to W
       movwf       counter2      ; Number is moved to variable "counter2"
       
loop2
       movlw       h'FF'         ; Number hFF is moved to W
       movwf       counter1      ; Number is moved to "counter1"       
loop1
       decfsz      counter1      ; Decrements "counter1" by 1. If result is 0
       goto        loop1         ; skip next instruction
       
       decfsz      counter2      ; Decrements "counter2" by 1. If result is 0
       goto        loop2         ; skip next instruction
       
       movlw       B'00001111'   ; Binary number 00001111 is moved to W
       movwf       PORTB         ; Number is moved to PORTB
       movlw       h'FF'         ; Number hFF is moved to W
       movwf       counter2      ; Number is moved to variable "counter2"
       
loop4
       movlw       h'FF'         ; Number hFF is moved to W
       movwf       counter1      ; Number is moved to variable "counter1"
loop3
       decfsz      counter1      ; Decrements "counter1" by 1. If result is 0
                                 ; skip next instruction
       goto        loop3
       decfsz      counter2      ; Decrements "counter2" by 1. If result is 0
       goto        loop4         ; skip next instruction
       goto        loop          ; Jump to label loop
       end                       ; End of program