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From: jkg@gitpyr.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: Why 4.77MHz on clones?
Message-ID: <3017@gitpyr.gatech.EDU>
Date: Mon, 2-Feb-87 22:09:47 EST
Article-I.D.: gitpyr.3017
Posted: Mon Feb  2 22:09:47 1987
Date-Received: Wed, 4-Feb-87 04:22:16 EST
References: <1292@ho95e.ATT.COM> <5426@amdahl.UUCP> <1326@whuts.UUCP> <1696@hoptoad.uucp>
Reply-To: jkg@gitpyr.UUCP (Jim Greenlee)
Distribution: na
Organization: Georgia Institute of Technology
Lines: 25

In article <1696@hoptoad.uucp> pozar@hoptoad.UUCP (Tim Pozar) writes:
>Side_note: Ever wonder why 4.77 was picked?   The 8088 bus was designed 
>around the Multi-bus specs.  As I remember 4.77 or some mutilple was use
>on the Multi-bus for timeing.

I always thought it was because the crystal input to the 8284 (8088 clock
driver) is 3X the processor speed - IBM PCs and compatibles use a 14.31818
Mhz crystal to provide basic timing. It just so happens that 14.31818/4 (a
couple of D flip-flops !) is 3.58 - very close to the colorburst crystal 
frequency. I can just see the conversation at the design meeting now...

IBM Design Engineer 1 - "Oh boy! - we saved ourselves one whole chip!"

IBM Design Engineer 2 - "Yeh - and now we can drive a color TV directly 
			for a monitor!"

In Unison -		"Won't the bean counters be proud of us!"


						Jim Greenlee
-- 
The Shadow...!{akgua,allegra,amd,hplabs,ihnp4,seismo,ut-ngp}!gatech!gitpyr!jkg

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