[ntp:questions] Thunderbolt at NTP ref clock.

unruh unruh at invalid.ca
Wed Jul 31 18:23:51 UTC 2013


On 2013-07-31, Thomas Laus <lausts at acm.org> wrote:
> On 2013-07-31, Thomas Laus <lausts at acm.org> wrote:
>> The pulse width wasn't adjustable.  It was was just on the ragged edge
>> of what the Soekris UART DCD was able to see for pps kernel
>> discipline.  It occasonally missed a pulse and that caused some
>> problems for ntp because it had the time solution from the serial
>> port data, but not the pps timestamp that matched.
>>
> From the Thunderbolt book:
>
> 1 PPS: BNC Connector TTL levels into 50 ohm 10 microseconds-wide
> pulse with the leading edge synchronized to GPS or UTC within
> 20 nanoseconds (one sigma) in static, time-only mode. The rising
> time is <20 nanoseconds and the pulse shape is affected by the
> distributed capacitance of the interface cable/circuit.
>
> My Soekris board could not handle something that narrow.

The problem was probably more with the impedence of the Soekris input
socket. If the time is only 0us and a 20ns the impedence of the end of
the line will mean that much of the pulse is reflected and is thus
rounded and much smaller than it should be. Ie, it might not have been
the time of the pulse, but the height of the pulse, which was causing
the trouble. (ie, 2Kohm termination impedence rathr than the line
termination of say 50 ohm meant that the signal was relected down the
line.) 

I will admit that 10us is pretty narrow. 

>
> Tom
>
>



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