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IRQ7 printer interrupt

I'm currently doing a project which needs to interface the PC's parallel port to a motor. The motor's optocoupler is connected to pin 10 (ACK pin) of the parallel port. This optocoupler outputs square waves, which is a measure of the motor's speed.

When there's a voltage transition, IRQ7 will be executed. Now, the problem is how to write a handler for IRQ7. When IRQ7 is asserted, how do I tell it to execute my own interrupt handler for IRQ7.

Please help.

Thanks :)


My rough program is as shown below, but it can't work!

/* Test IRQ (ACK pin 10) of the parallel port LPT1 */
/* The optocoupler's output is connected to pin 10 */
/* A high to low transition will assert IRQ7, print value of i, and increase i */

#include <conio.h>
#include <dos.h>

#define LPT_IRQ            0x0f      /* LPT1 interrupt number */
#define contr_reg      0x37a

void interrupt feedbk(void);

int i = 0;

void main()
unsigned char a;
void interrupt (*oldfunc)();

oldfunc = getvect(LPT_IRQ);  /* Save old interrupt vector */


setvect(LPT_IRQ, feedbk);  /* Install new interrupt */

/* Enable LPT1 IRQ7 */
/* Set bit 4 (IRQ) to high */
a = inportb(contr_reg);
a |= 0x10;
outportb(contr_reg, a);



setvect(LPT_IRQ, oldfunc); /* Restore original interrupt */


void interrupt feedbk()
      printf("%d\n", i++);
1 Solution
The primary problem I see with your code is that you are not enabling the interrupt through the PIC (Programmable Interrupt Controller).  You need to clear bit 7 of the interrupt mask register to enable the IRQ 7 to interrupt the processor.  Do this by the following:

        outportb(0x21, inportb(0x21)&0x7f);

I would recommend doing this after the setvect() call.  When you're done with the interrupt, be sure to reset the bit as follows:

        outportb(0x21, inportb(0x21)|0x80);

You also need to do the EOI (end of interrupt) inside your interrupt routine like this:

        outportb(0x20, 0x67);   /* I prefer specific EOI */

Lastly, I must let you know that you shouldn't do a "printf()" or any other functions in your interrupt handler that may use the operating system.  In general, the interrupt handler should do as little work as possible, and let the foregroung (your main program) do any real processing.

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