Message-ID: <3ED7BDE1.614E51F4@yahoo.com>
Date: Fri, 30 May 2003 16:24:01 -0400
From: CBFalconer <cbfalconer AT yahoo DOT com>
Organization: Ched Research
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To: djgpp-workers AT delorie DOT com
Subject: Re: uclock() still out by 1 in 65536
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Reply-To: djgpp-workers AT delorie DOT com

Eric Rudd wrote:
> 
... snip ...
> 
> If you are concerned about small frequency errors, you may be interested
> in the origin of the 1.19-MHz rate that is used in the PC timer chip.
> In the early days of personal computing, it was considered important to
> maintain compatibility with NTSC, since people used NTSC monitors on
> their computers.  According to the EIA RS 170 A spec, the chroma
> subcarrier is at 3.579545 MHz +/- 10 Hz, and the PC clock was chosen to
> be 1/3 of that, or 1.19 MHz approximately.  The specified 3.579545 MHz
> rate itself is an approximation; it is exactly 455/2 times the
> horizontal sweep rate, which was originally 15750 Hz in the early
> black-and-white days, but was changed to 15750/1.001 when NTSC was
> devised.  Thus, the chroma subcarrier works out to 39.375/11 MHz, and
> the PC timer chip runs at 13.125/11 MHz.  As I recall, the system clock
> on the original IBM PC was 4 times that, or 4.77 MHz.

The fundamental reason for the selection was that 3.579545 MHz
crystals were cheap and plentiful- they existed in every color TV
receiver.  About 6 years earlier I selected a frequency for my
systems based on exact generation of baud rates from 110 baud
through 19200, and the resultant 8080 clock was slightly below 2
Mhz.  Don't remember the exact result any more.

-- 
Chuck F (cbfalconer AT yahoo DOT com) (cbfalconer AT worldnet DOT att DOT net)
   Available for consulting/temporary embedded and systems.
   <http://cbfalconer.home.att.net>  USE worldnet address!