Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!cs.utexas.edu!uunet!mcvax!unido!rwthinf!franke
From: franke@rwthinf.UUCP (Christian Franke)
Newsgroups: comp.sys.ibm.pc
Subject: Re: HELP! Interrupts on COM ports
Summary: Final solution
Keywords: COM port, Interrupt, Device Driver
Message-ID: <1239@rwthinf.UUCP>
Date: 31 Jul 89 14:07:23 GMT
References: <1137@rwthinf.UUCP>
Organization: RBI - RWTH Aachen
Lines: 68

In article <1137@rwthinf.UUCP>, I [franke@rwthinf.UUCP (Christian Franke)]
wrote:
> 
> 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).
> 
> [remaining stuff deleted]
>

Thanks to all who answered. I've found the solution last week.

The behaviour seems to be a problem in some COM12/LPT12 cards from 
taiwan. These cards don't have a real 8250 for serial i/o 
but a custom chip named CTC83747 for serial and parallel i/o.
I could not reproduce the error with our V24/V11 card, which has
a fast 8250 (up to 115200 baud) on it.

With a special test routine, I was able to trace the error:
The following situation occurs sometimes when transmitter & receiver
interrupt occur nearly simultaneously: 


	Action		Data		Comment
=========================================================================
ComInterrupt:				Start of interrupt routine
	Read LSR	**0****1	Transmitter busy, receiver ready
	Read IIR	****0100	Receiver Interrupt (ok)
	Read RBR	********	Received character read (ok)
	Read LSR	**1****0	Transmitter changed to ready
	Read IIR	****0000	Modem status interrupt (disabled!)
					should be ****0010 !
	Read MSR	****0000	No modem status changes !
	Read IIR	****0001	No interrupt pending now,
					transmitter interrupt lost.
	Write ICC	00100000	Say 8259: EOI 
	RTI				End of interrupt routine

The LSR reads are not necessary to reproduce this error.
The IER is set to 00000011 (only transmitter & receiver interrupt enabled).
The error does not depend on the baud rate (checked with 300-19200 Baud).

In order to overcome this problem, it is necessary to loop on the
Transmitter Empty bit in the LSR when the IIR says "No interrupt".


Regards,

	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)