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From: franke@rwthinf.UUCP (Christian Franke)
Newsgroups: comp.sys.ibm.pc
Subject: HELP! Interrupts on COM ports
Keywords: COM port, Interrupt, Device Driver
Message-ID: <1137@rwthinf.UUCP>
Date: 18 Jul 89 08:39:03 GMT
Organization: RBI - RWTH Aachen
Lines: 106


I've written a interrupt driver for the COM{12} ports of an AT.
If the receiver side uses interrupts and the transmitter side uses polling,
everything is OK.
If the transmitter uses interrupts also, the following problem occurs:

When the transmitter and the receiver interrupts occur simultaneously, then
sometimes the transmitter interrupt is lost: The IIR register of the
8250 does not report "Transmitter empty interrupt" after the
"Receiver data available interrupt" has been processed.
Both (8250 internal) interrupts should occur during the same hardware
interrupt cycle of the 8259.
After this situation, the LSR of the 8250 correctly reports 
"transmitter ready" (bit 5).

Is that a problem with the edge-sensitive mode of the 8259? 

Here is a fragment of the interrupt routine (MS-C 5.1):

--------------------------------------
/*	8250 Communikations Controller Port Offsets			*/
#define THR	0xf8	/* Transmitter holding register		w/o	*/
#define RBR	0xf8	/* Receiver buffer register		r/o	*/
#define IER	0xf9	/* Interrupt enable register			*/
#define IIR	0xfa	/* Interrupt identification register		*/
#define LCR	0xfb	/* Line control register			*/
#define MCR	0xfc	/* Modem control register			*/
#define LSR	0xfd	/* Line status register				*/
#define MSR	0xfe	/* Modem status register			*/

/*	8259 Interrupt Controller Port Numbers				*/
#define ICC	0x20	/* Control word register			*/
#define ICM	0x21	/* Interrupt mask register			*/

#define COMx	0x0300	/* Port base address				*/
#define IRQx	4	/* Hardware interrupt number			*/

/*
	COM interrupt handler routine
*/	

static void interrupt far ComIntHandler(void)
{
    register unsigned char ch;
	/* Determine interrupt source */
	for (;;) switch (inp(COMx+IIR)) {
	    case 0x6:
	    	/*
		 * Receiver line status interrupt
		 */
		ch = inp(COMx+LSR); /* get status, clear interrupt */
		/* .... */
	    	break;
		
	    case 0x4:
	    	/*
		 * Receiver data available interrupt
		 */
		ch = inp(COMx+RBR); /* get character, clear interrupt */
		/* ... queue data ... */
	    	break;
		
	    case 0x2:
	    	/*
		 * Transmitter empty interrupt
		 * (interrupt cleared by read of IIR)
		 */
		if (/* Data available in queue */)
		 	/* Send next character from queue */
		 	outp(COMx+THR,/* Next char in queue */);
	    	break;

	    case 0x0:
	    	/*
		 * Modem status interrupt
		 */
		ch = inp(COMx+MSR); /* get status, clear interrupt */
		/* ... */
	    	break;

	 /* case 0x1: */
	    default:
	    	/*
		 * No interrupt from COM port
		 */
		outp(ICC,0x20); /* Say 8259: End of Interrupt */
		return;
	}
	/*NOTREACHED*/
}
------------------------------

Perhaps someone can help me.

Thanks in advance

	Christian Franke

	Aachen University of Technology
	Lehrstuhl fuer Informatik I
	Ahornstrasse 55
	D-5100 Aachen
	Federal Republic of Germany
	Tel.: 0241 / 80-3586

	UUCP: franke@rwthinf.uucp ({...mcvax}!unido!rwthinf!franke)