[time-nuts] Time stamping with a PICPET

Chuck Forsberg WA7KGX caf at omen.com
Mon Oct 28 03:07:23 EDT 2013


NTP itself runs over a network.  Even on a gigabit local area net
there will be temporal uncertainties in packet assembly and
transmission.  These may be comparable to interrupt latency
responding to 1 PPS.

On 10/27/2013 10:58 PM, Tom Van Baak wrote:
>> On most CPU architectures, the low level hardware has a register that counts on the CPU clock.
> Merely counting the CPU clock is only half the requirement. Reading the clock precisely when an external event occurs is the other half.
>
> In my mind, there are three levels of NTP precision:
>
> 1) The traditional scheme, in use for decades, where a PC uses interrupts (e.g., DCD) and OS s/w acts as a crude time interval counter. This is quite limiting, but Dave Mill's worked with what he had, and it's far better than nothing.
>
> 2) The external h/w capture/counter scheme, which is what any external time interval or time-stamping counter can provide. By using external h/w instead of internal s/w you get a precise, low-jitter time comparison at every pulse. The OP was talking about the picPET, a sub-microsecond $1 time-stamping counter. But you can use a $1000 HP sub-nanosecond time interval counter if you prefer.
>
> 3) An internal h/w capture/counter scheme, which is what the latest crop of SBC offer. Here, not only is the counter tied to the CPU clock, but timestamping is pure h/w, so all problems with s/w and latency and jitter are removed from the equation. This is the ideal solution, not unlike how any GPSDO is designed.
>
> In my mind, the modern SBC architectures with single CPU clock source (could be OCXO or atomic), and real GPIO pins (instead of serial or USB ports) and a native h/w capture/compare register should allow NTP to work at the 10 to 100 ns level. If any of you could verify this, it would be time-nut worthy.
>
> /tvb
>
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-- 
      Chuck Forsberg WA7KGX   caf at omen.com   www.omen.com
Developer of Industrial ZMODEM(Tm) for Embedded Applications
   Omen Technology Inc      "The High Reliability Software"
10255 NW Old Cornelius Pass Portland OR 97231   503-614-0430



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