[time-nuts] PC clock generator without 14.318MHz
albertson.chris at gmail.com
Wed Oct 19 22:38:52 EDT 2016
The last time I read about this it was on an ARM based board. They clocked
it with a GPSDO. I think the problem is MUCH easier if you can abandon
the PC platform.
The other story I read solved to problem by adding even more hardware and
some software changes. They moved the nanosecond counter out of the CPU
chip to a hardware counter and then the PPS signal connected to a latch.
This avoids the interrupt latency.
In most normal NTP servers the interrupt causes the CPU to snapshot its
internal nanosecond counter and store the snapshot in memory and set a flag
so the user space task can then read the value stated in RAM. This gets
you only microsecond resolution.
With special hardware the counter is latched with external hardware then
then on the interrupt handler only has read the latch and place that valuer
in RAM and set the same flag. The trouble is that EVERY routine that
reads the internal counters has to by modified to read the eternal counter.
As I remember these system ran BSD UNIX.
On Wed, Oct 19, 2016 at 9:02 AM, Vladimir Smotlacha <vs at cesnet.cz> wrote:
> On 10/18/2016 11:23 PM, Mike Cook wrote:
>> Le 18 oct. 2016 à 16:53, Vladimir Smotlacha<vs at cesnet.cz> a écrit :
>>> 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 can’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?
>> The 14.317MHz xtal was connected to the south bridge controller chip, but
>> for recent CPUs this has gone away as has northbridge and the system clock
>> has been integrated into the PCH (Platform Controller Hub) chip according
>> to Wikipedia, so I suspect that if you find the clock feeding that , then
>> you could stabilize it in that same way.
> Thank you Mike, PCH will be object of my experiments.
> I wonder than probably nobody solved stable clock source in "post 14.318"
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