[ntp:questions] Adjust serial device speed in ACTS refclock driver?

Mike S mikes at flatsurface.com
Wed Oct 22 14:26:54 UTC 2014


On 10/22/2014 6:41 AM, Uwe Klein wrote:> Am 06.10.2014 09:00, schrieb Rob:
 >> Rich Wales <richw at richw.org> wrote:
 >>> Unless at least one of the known modem time services supports or
 >>> requires
 >>> a very high baud rate, BTW, I'm unconvinced that supporting anything
 >>> higher
 >>> than 9600 is really necessary for this application.
 >>
 >> Using the highest possible communication speed removes one of the delays
 >> from the system that causes your time to be "so far off".
 >
 > which isn't all that interesting.
 >
 > defined constant offsets are easy to fix, aren#t they?

What makes you act like it's a constant offset? Assume that the 
timestamp is taken to be the end of the stop bit for the last character 
of the time message. Those bits are typically clocked out by a U(S)ART 
with an internal clock at 16x the bit rate. So, the jitter of that last 
bit is 1/16 the bit rate (or closer to 1/8 when you consider both ends). 
Upping the bit rate improves the jitter caused by the UART clocks, which 
make it NOT a constant offset.



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