Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!ubc-cs!alberta!calgary!!smit
From: smit@.ucalgary.ca (Theo Smit)
Newsgroups: comp.sys.m68k
Subject: Re: What's the difference between the MC68661 and the MC68681?
Summary: Interrupts work OK
Message-ID: <1924@cs-spool.calgary.UUCP>
Date: 12 Oct 89 15:31:39 GMT
References: <6735@hubcap.clemson.edu> <9306@pyr.gatech.EDU>
Sender: news@calgary.UUCP
Reply-To: smit@enel.UCalgary.CA (Theo Smit)
Organization: U. of Calgary, Calgary, Alberta, Canada
Lines: 31

In article <9306@pyr.gatech.EDU> byron@pyr.UUCP (Byron A Jeff) writes:
>....
>In addition to the 2 serial ports which can be driven by the internal
>baud rate generator or by an external clock the chip also provides
>an 8 bit output port and a 6 bit input port with edge sensitive
>inputs that can generate interrupts. The 68681 supposedly can interface
>with the 68000 interrupt vector fetch but to be honest I have never
>gotten it to work (so I use autovector interrupts).
>....
>
>BAJ

I built a 68000/68681/68440 based system for my thesis project. I used
a pair of interrupt-driven circular buffers to handle serial input. The thing
that got me the first time was the address decoding and the way the interrupt
acknowledge cycle works. I was using the upper 8 address lines for device
address decoding, and had a device decoded at 0xffxxxx. During an interrupt
acknowledge cycle, the upper 20 address lines are forced high, which selected
the device. This then clashed with the 68681 trying to put the vector on
the data bus. I then changed the decoding to select nothing during the
interrupt acknowledge cycle.
Some of my circuit was based on the Motorola ap note, "A terminal interface,
printer interface, and background printing for an mc68000 based system using
the mc68681 DUART", Motorola application note AN899, 1984. I also used
the one describing the 68008 minimum configuration system (AN897). Both of
these, however, connect the RS-232 line receiver gates to -12 V - they should
be connected to +5V.

Have fun.

Theo Smit (smit@enel.ucalgary.ca)