Question 4 (35 marks)

A program written in assembly language targeted for PIC18F4550 microcontroller is listed as below:

ORG 0000H

;*****Set up the Constants****

STATUS equ 03h ;Address of the STATUS register

COUNT1 equ 08h ;First counter for our delay loops

COUNT2 equ 09h ;Second counter for our delay loops

;****Set up the port****

bsf STATUS,5 ;Switch to Bank 1

movlw 00h ;Set the Port B pins

movwf TRISB ;to output.

bcf STATUS,5 ;Switch back to Bank 0

;****Turn the LED on****

Start movlw 02h ;Turn the LED on by first putting it

movwf PORTB ;into the w register and then on the port

;****Start of the delay loop 1****

Loop1 decfsz COUNT1,1 ;Subtract 1 from 255

goto Loop1 ;If COUNT is zero, carry on.

decfsz COUNT2,1 ;Subtract 1 from 255

goto Loop1 ;Go back to the start of Loop 1.

;****Delay finished, now turn the LED off****

movlw 00h ;Turn the LED off by first putting

movwf PORTB ;it into the w register and then on the port

;****Add another delay****

Loop2 decfsz COUNT1,1

goto Loop2

decfsz COUNT2,1

goto Loop2

;****Now go back to the start of the program

goto Start ;go back to Start and turn LED on again

;****End of the program****

END ; End of program

Program 2

Given that the crystal frequency is 1MHz and each instruction takes 4 cycles to execute:

(a) Identify the output pin used for this program. [2 marks]

(b) Draw the circuit corresponding to this program. [5 marks]

(c) Determine the number of counting performed by Loop1 and Loop2. [5 marks]

(d) Explain why two delay loops, Loop1 and Loop2 are needed for the program above. [5 marks]

(e) Explain the drawback of using two delay loops for the program above. [3 marks]

(f) Rewrite the above program using only one delay loop (Loop1). [15 marks]