On 2013-07-31, Thomas Laus <[email protected]> wrote: > On 2013-07-31, Thomas Laus <[email protected]> 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 > > _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
