[time-nuts] 60Hz mains clocking in computers

[Lux, Jim (337C)]

On 12/12/09 5:29 PM, "Colby Gutierrez-Kraybill" <colby at astro.berkeley.edu>
wrote:

> 
> 
> 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.
> 
>

There were a variety of computers synchronized not to the mains frequency
but to the horizontal retrace or vertical frame rate for video.  That way,
they could do things like DRAM refresh or video buffer updates in a clock
synchronous way.  To a certain extent, even the IBM PC was built like this,
running at 4.77 MHz, divided down by 3 from a 14.3 MHz crystal (which was
divided by 4 to get the 3.58 MHz color burst).  If I had to guess, at the
low end, boxes like the Atari 68K machines, at the high end, 3Rivers PERQ
(but that one sticks as using 2901 bitslice...)

Anything intended to generate video for integration with other video streams
would greatly benefit from being able to be synchronized to the NTSC 59.95
Hz frame rate, and if the video memory is the same as the system ram, then
running the CPU clock at an exact multiple makes designing the memory access
arbiters easier (they can be synchronous), so what you really want is the
pixel rate being a multiple of 59.95 and the CPU clock being a multiple of
the pixel rate, so that wait state generation is easy (or you can do
transparent/hidden access to RAM during a time when you KNOW the CPU won't
be looking at it).  More than one system used the video access to do DRAM
refresh, too.