Bob kb8tq wrote:
>Without the ability to put out a “known good” time pulse there is no quick way 
>check NTP. GPS modules suffer a similar issue. They put out a pulse and a 
>“correction” (sawtooth error) to tell you what they just told you. Doing the 
>sort of thing with NTP may be possible. 
>Indeed the process of correcting this sort of data is open to a bit of debate. 
>It does 
>give you a way to get around the “hey, all I can do is 300 ns” issue. With 
>the correction is part of the standard firmware. It would be nice if one of 
>the NTP 
>guru’s popped up with an equivalent process. 
>One *could* monitor various bits and pieces of the OS’s timing generation 
>Somehow that does not seem quite as much fun as looking at the whole result all
>at once. Indeed it might be the only way to get it all worked out.

Linux has a pps output driver (pps_gen_parport), but  I've never used
it. A while back  I added an output mode to the Linux pps_parport
that I will eventually get around to using with the palisade(trimble)
NTP driver.

My modified driver's polled input mode has an input-to-echo delay of
1.16 to 1.93 microseconds (measured with an old Keithley 775 counter)
on my machine which has a parallel card attached directly to the pci-e
port on a sandybridge processor.  Interrupt mode echo delay is 3.8 to
4.3 microseconds when the machine is lightly loaded with occasional
spikes of 5 to 7 us. When the machine is idle delay falls to
Port read and write delays are equal at about 820ns each. I think that
pci-express always uses 'split transactions'  so reads can sometimes
seem to take only half the time depending on the input pulse time
relative to the start time of a read request.  Delays increase to
~1200ns when attached to the chipset pcie ports.
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