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From: byron@cc.gatech.edu (Byron A Jeff)
Newsgroups: comp.sys.m68k
Subject: Re: 68000 vs. 68020
Message-ID: <27178@hydra.gatech.EDU>
Date: 24 Apr 91 16:42:50 GMT
References: <12964@pasteur.Berkeley.EDU>
Sender: gt8566a@prism.gatech.EDU
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Organization: Georgia Institute of Technology
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In article <12964@pasteur.Berkeley.EDU> spp@zabriskie.berkeley.edu.UUCP (Steve Pope) writes:
>
>Perhaps the obvious is worth stating in this discussion --
>not only is the 68000 a lower-cost part then the 68020,
>but it has a 16-bit bus vs. the 68020's 32 bit bus, and
>a somewhat simpler set of control signals.  Therefore, for a 
>low end system, the parts count and parts cost will be 
>considerably lower for a 68000 system.

I'd have to disagree with on on this point. I've built systems with each
of the 68008, 68000,68010,68020 and '020 is much easier to hook up because
of the dynamic bus sizing. The parts count is actually less than a 68000 or
'010 based system because everything can be done on a single 8 bit bus 
(Including EPROM, RAM, and I/O). 

Also I was able to pick up 2 20 Mhz '020s for $25 apiece (used).
Considering I get a more than %25 increase in speed using only half the bus 
width (due to the effects of double bus speed, 3 instead of 4 cycles, and the 
instruction cache) and reclaim more board space, I'd use a '020 any time. 

In my opinion a '020 system can be costly if you choose to exploit all of
the features available. However in minimal and small configured systems
a '020 can beat out 68000 anytime.

My '020 system consists of the following:

20 Mhz '020
1 32K battery backed static ram (70ns running at no wait states)
PC parallel port interface for program loading
PC keyboard interface
LCD driver interface
1 Mbyte of dynamic ram 
   (70ns running at no wait states, configured as as 1 32 bit x 256K bank)

All this fits on a 6 x 8 inch perfboard with enough space to fit another
Mbyte of DRAM on board. To exploit the 32 bit data bus all I need is a
single 16L8 pal to decode the SIZE and A1 and A0 address lines.

Also note that since the '020 does not force the use of the entire data
bus that all the program can fit in a single ram without having to be split.
This simplifies the programming of the board.

>
>Therefore, which part to use primarily depends on whether
>you're aiming for a 16-bit or 32-bit system.
>

The '020 (and '030) can give the best of both (and also 8 bit) because of
dynamic bus sizing.

>steve

BAJ
---
Another random extraction from the mental bit stream of...
Byron A. Jeff - PhD student operating in parallel!
Georgia Tech, Atlanta GA 30332   Internet: byron@cc.gatech.edu