CP316: Microprocessor Systems and Interfacing

Interrupts Prelab

Setup and test a base program that:

• flashes the left LED every 250 milliseconds (fairly rapid pulse) based on Timer0.  Use hardware timing that polls the interrupt flag. This timing routine will be used to keep the processor busy.   You will be interrupting this routine. You need a fairly rapid pulse so that it is immediately obvious if there are problems with your interrupts or interrupt handling routines.
• the signal that is driving the LED should be displayed on the oscilloscope (use the C2 pin).

Note: this program has no interrupts and should be a simple reconfiguration of work done in previous labs. Program must be working before you get to the lab.

Demonstration - demonstrate the base program

To be done in your lab notebook.

1. If working in the simulator, an interrupt can be "fired off" using Window | Simulator | Stimulus. What would you have to do in the Stimulus screen to configure and issue an INT1 interrupt?
   
   
2. The timer routine presented in class has been modified to protect the critical region from interrupts.
   • Explain how this code (shown below) works.
   • What is the advantage to using this instead of simply clearing/setting GIEL or GIEH?
   • Would time_adj have to be modified to use this code? If yes, what is the adjustment; if no, why not?
     

   	ten_ms:
   		btfss   INTCON,TMR0IF
   		bra 	ten_ms
         

   		movff	INTCON,INTCON_copy
   		bcf 	INTCON,GIEH
         

   		movff	TMR0L,tmr_l
   		movff	TMR0H,tmr_h
   		movlw	low time_adj
   		addwf	tmr_l,F
   		movlw	high time_adj
   		addwfc	tmr_h,F
   		movff	tmr_h,TMR0H
   		movff	tmr_l,TMR0L
         

   		movf	INTCON_copy,W
   		andlw	b'10000000'
   		iorwf	INTCON,F
   		bcf 	INTCON,TRM0IF
         

   		return
         

   
   Demonstration - Explain your results for the above questions.