[time-nuts] PC clock generator without 14.318MHz

Tom Van Baak tvb at LeapSecond.com
Tue Oct 18 17:30:35 EDT 2016

```Hi Vladimir,

1) Maybe you can use a DDS chip to create your 14.328 MHz signal, or

2) Do it yourself using a PLL at 45.454 kHz using 220 and 315 dividers:

10.000000 MHz / 220 = 45454.545 Hz
14.318181 MHz / 315 = 45454.545 Hz

This works because your 14.31818 MHz number is defined as 4x the 3.579545 MHz NTSC colorburst frequency,
which is defined precisely as 60 * 1000/1001 * 525/2 * 455/2,
which expands to (60 * 1000 * 3*5*5*7 * 5*7*13) / (7*11*13 * 2 * 2),
which reduces to 315 / 88 MHz.
So that's why 10.000000 MHz * 315 / 220 = 14.318181 MHz, exactly.

Note also that:
-  14.31818 MHz / 3 = 4.77273 MHz, the CPU clock rate for the original IBM PC
-  14.31818 MHz / 4 = 3.579545 MHz, the colorburst frequency (for PC video)
-  14.31818 MHz / 12 = 1.193182 MHz, the Intel 8253 timer clock (the historical root of all PC timekeeping)

Some of these numbers survive to the present. I'm typing this post on an XP laptop where QueryPerformanceCounter() has a Frequency.QuadPart of, you guessed it, 3579545 Hz, which is why my Win32 laptop's high-res clock has ~279 ns resolution.

For more fun with time, frequency, oscillators, and prime numbers, see:
http://www.poynton.com/PDFs/Magic_Numbers.pdf

/tvb

----- Original Message -----
From: "Vladimir Smotlacha" <vs at cesnet.cz>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Tuesday, October 18, 2016 7:53 AM
Subject: [time-nuts] PC clock generator without 14.318MHz

Hello,

I have operated own NTP servers with stable system clock for many years.
The principle is quite simple - I replaced 14.318 MHz quartz with OCXO
based circuit. Now I have to build few more servers with modern mini-ITX
motherboards, however on many of them (e.g. from ASUS) I canâ€™t find any
14.317 MHz quartz.  Such frequency is a relic of original PC design and
I wonder if it is used any other basic frequency in recent clock generators?

thanks,