[time-nuts] PC clock generator without 14.318MHz

Vladimir Smotlacha vs at cesnet.cz
Thu Oct 20 04:10:42 EDT 2016

Chris, I agree with you that additional HW to avoid interrupt latency is 
necessary. My NTP servers  with stable oscillator and HW card processing 
PPS (still in use but some mainboards failed after 10 years of reliable 
service) are described here:


On 10/20/2016 04:38 AM, Chris Albertson wrote:
> 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.

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