Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!brutus.cs.uiuc.edu!psuvax1!gatech!hubcap!rchampe
From: rchampe@hubcap.clemson.edu (Richard Champeaux)
Newsgroups: comp.sys.m68k
Subject: What's the difference between the MC68661 and the MC68681?
Message-ID: <6735@hubcap.clemson.edu>
Date: 11 Oct 89 03:46:06 GMT
Organization: Clemson University, Clemson, SC
Lines: 34

What's the difference between the MC68661 and the MC68681.  Both are
serial drivers (UARTS).  I have the data sheets for the 68661, but I
know nothing about the 68681.

The 68661, they say, is an enhanced Signetics 2651.  I need an RS232
port, and this does everything I need to do it.  It's got an internal
baud rate generater with 16 baud rates to choose from (as high as
38400 baud), and has all of the necessary RS232 control lines.  It
has some disadvantages, however.  They say it easily interfaces with
the 68000, but I don't think I agree with their definition of easy.

According to the example circuit they give, interfacing the 68661 to
the 68000 without using the 6800 synchronous memory cycle mode (VPA cycle)
takes 3 flip-flops, a 4 bit counter, a nor gate, 3 inverters, and an
open-collector buffer.  Half of the circuit is to insert a wait state,
and the other half is to hold off the next access for the 600ns
CE\ to CE\ delay time.

My definition of easy to interface to the 68000 is that it would accept
the 68000 control signals with a maximum of a few gates in between and
that it would either generate it's own DTACK or be fast enough to merely
use the chip select as the DTACK.

I guess the interface circuitry isn't all that bad, but if the 68681
does the same stuff (or more) and is easier to interface, I'd rather
use it.

So, can anyone tell anything about the 68681?  I don't need much info,
unless you want to tell me a lot about it.  I basically would like to 
know if it's easier to interface than the 68661, and if it's worth 
pursuing.

Thanks for any help.
Rich Champeaux  (rchampe@hubcap.clemson.edu)