[time-nuts] 60Hz mains clocking in computers

[Magnus Danielson]

Joe Gwinn wrote:
> At 1:44 AM +0000 12/13/09, time-nuts-request at febo.com wrote:
>> Date: Sat, 12 Dec 2009 17:29:17 -0800
>> From: Colby Gutierrez-Kraybill <colby at astro.berkeley.edu>
>> Subject: [time-nuts] 60Hz mains clocking in computers
>> To: time-nuts at febo.com
>> Message-ID: <3058527A-CC99-4174-BE75-21DD92334155 at astro.berkeley.edu>
>> Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes
>>
>>
>> I'm trying to get to the bottom of whether or not any computing 
>> equipment made around the advent of UNIX systems (or any time-slicing 
>> system) used the mains cycles of 60Hz as phase lock for the internal 
>> system clock.  My guess is that perhaps they did not as the computing 
>> logic is DC based, but, I have memories of using an 68000 based UNIX 
>> system that I thought had its internal clock based off of the 60Hz
>> mains...  Not sure the vendor anymore.
> 
> In the 1980s and 1990s, before networks capable of carrying NTP time to 
> the millions became common, the computer local clock was very often 
> derived from the local AC power mains, and the frequency was steered to 
> match atomic time once per day.  The POSIX standards reflect this common 
> approach by the tolerance on CLOCK_REALTIME, 20 milliseconds, this being 
> one cycle of 50 Hz power.

Which does not perfectly match the 60 Hz being used in some countries or 
for that matter traditional division for PC clocks (derived from 
14,31818 MHz, over 4,77 MHz and what the 8253 divider allows).

> The CPU logic clock was not generally phase-locked to the AC power 
> lines, instead being generated by a cheap crystal having a very large 
> tempco.  The exception to this was that video generators were (and still 
> are) often locked to the AC line so that hum bars would not drift across 
> the screen.

I have never seen this in any of the devices I've seen. It is certainly 
not what we do in the TV world either. Examples would be good.

Cheers,
Magnus