[time-nuts] Using a frequency synthesizer replacement for motherboard oscillator

David davidwhess at gmail.com
Sun Dec 2 01:06:35 UTC 2012

On Sun, 02 Dec 2012 01:53:07 +0100, Magnus Danielson
<magnus at rubidium.dyndns.org> wrote:

>On 12/02/2012 01:29 AM, David wrote:
>> Originally the IBM PC design used an 8253 or 8254 PIT, programmable
>> interval timer, located at ports 40h to 43h with Timer 0 clocked at
>> 1.193182 MHz (1/3rd of 3.579545 MHz or 1/12th of 14.318 MHz) and set
>> to divide by 65536 which generated about an 18.2 Hz interrupt rate on
>> IRQ 0.  Timer 1 generated the since deprecated DRAM refresh clock and
>> Timer 2 goes to the PC speaker.
>In a modern world, look for things like HPET and TSC:
>You want to get a better reference clock to these timers.

I only mention and do not go into detail about the higher precision
timers later in my post because by the time they were available,
Intel's system management mode was adding so much jitter than I lost
interest in using PC hardware in applications where they otherwise
might have been useful.  To be fair, not all motherboards and BIOSes
had this problem but it was not worth weeding out the poorly
performing ones.

One of my favorite tricks back when the ISA bus was still available
was to use a custom expansion board I built and an oscilloscope to
measure the interrupt latency.

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