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From: lnewman@emdeng.Dayton.NCR.COM (Lee.A.Newman)
Newsgroups: comp.sys.m6809
Subject: Re: fast clock for 6809
Message-ID: <675@emdeng.Dayton.NCR.COM>
Date: 17 Apr 89 16:51:40 GMT
References: <1605@ccnysci.UUCP> <12160@paris.ics.uci.edu>
Reply-To: lnewman@emdeng.Dayton.NCR.COM (Lee.A.Newman)
Organization: E&M Dayton, NCR Corporation, Dayton, Ohio
Lines: 37

In article <12160@paris.ics.uci.edu> Ira Baxter <baxter@madeleine.ics.uci.edu> writes:
>My understanding is that Hitachi offers a 3Mhz 6809 part.  I never
>did understand why much faster parts are not made... surely the
>manufacturer's process technologies are up to it (look at the
>68K speeds!).

Two points: 1) Speeds of other processors usually are their 'clock speeds',
where the 6809's speed is its 'bus speed'.  What does this mean?  Since other
processors are microcoded, it takes several clock cycles  to accomplish the
same thing the 6809 does in one bus cycle.  An example: a 68000 takes four
of its 'clock cycles' to fetch or store one piece of data from memory.  Each
such fetch or store is a 'bus cycle'.  A 68020 uses three clock cycles per
bus cycle.  Intel processors have similar numbers.
A 6809, on the other hand, counts in 'bus cycles'.  So, a 2 MHz 6809 has as
many bus cycles per unit time as an 8 MHz 68000, or a 68020 running at 6 MHz.

2) Motorola processors (NOT Hitachi!) have been notorious for running at much
higher clock speeds than what their data sheets say.  Two cases in point:
The Radio Shack Color Computer (Coco) routinely ran at 1.78 MHz when the part
Tandy was buying was the 1 MHz part.  A few machines, though, did have trouble.
I personally  obtained one of the first 68020's available without a non-
disclosure agreement.  At that time, Motorola was only selling 12 MHz parts,
but their data sheets also listed 16 MHz specs.  The Motorola rep. who I talked
to said that I could run these parts at 16 MHz, as long as I kept them cool 
enough.  Obviously, Motorola would not guarantee proper operation, but,
you'se takes your's chances.  Also, just because the part would run with a 
higher crystal does not mean that all other propagation delays are better
than the data sheet for the part you bought.

So, in Conclusion, a two MHz 6809 is not as slow as it sounds, and you can
probably increase the clock speed of the processor, as long as you take
pains to insure that your processor does not get too hot.

         Enjoy!

Lee Newman
lnewman@emdeng.dayton.NCR.com