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Interrupts

Interrupts are important features of modern microprocessors, as they facilitate an efficient mechanism for servicing requests from a variety of peripherals.

The example assembly program below uses the external interrupt pin irq1 (ACTIVE LOW), which should be connected to one of the debounced pushbuttons (active low). When the button is pressed, the interrupt handler routine prints the character i and delays for about 2 seconds before another button press will be handled (software pulse catch). The delay is implemented using the pdelay routine used above, and is available in a separate file: pdelayc.c. Download this file and the assembly program irq1as.s:

/*      irq1as.s
        test of external irq 1 (autovectored)
        connect IRQ1 to a PB (active L)
        prints 'i' each time an interrupt occurs
        pulse catching delay of 2 sec
*/
	
        .text
        .even
        .global   main
main:    movea.l   #irq1_handler,%a5          /* set up irq 1 vector */
	 move.l    %a5,IRQ1_VECADDR

         move.l    #0x00003F7C,%d0            /* enable irq1 */ 
	 move.w    %d0,IMR

        
	
loop:                           /* infinite loop */
         bra  loop              /* terminate with ABORT button */
         rts

irq1_handler:                   /* irq 1 service routine */
         move.l   #0x00003F7E,%d0      /* disable irq1 to catch 1 irq pulse */
         move.w	  %d0,IMR

         move.l   #'i',%d1            /* print i */ 
         move.l  #0x0013,%d0
         trap    #15
	 
         move.l  #2000,-(%sp)         /* delay for ~2 sec */
         jsr     pdelay               /* a C routine */
         addq.l  #4,%sp

         move.l  #0x00003F7C,%d0      /* enable irq1 */
         move.w  %d0,IMR
         rte
	
MBARx   =   0x10000000     /* Module Base Address value */
IMR     =   MBARx+0x0036   /* interrupt mask register */
IPR  	=   MBARx+0x003A   /* interrupt pending register */

IRQ1_VEC      = 25          /* irq1 vector number */
IRQ1_VECADDR  = 4*IRQ1_VEC  /* irq1 vector address */

Exercise.

1.

Make the necessary hardware connections, including ground. Refer to the Appendix 18.5 describing the SBC cable pins.

2.

Examine the code and determine what the program does and how.

3.

Compile, assemble and link the program and delay routine. Here is a makefile that may be of use: irq1as.make.

4.

Load the program irq1as.x into the SBC.

5.

Connect a debounced pushbutton to the irq1 pin on the SBC cable connector. Use active low.

6.

Run and test the code:

go 10000

7.

Terminate the program with the black ABORT button.

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