[ntp:questions] Atom PPS with parallel port

David Lord snews at lordynet.org
Sun Feb 23 15:17:33 UTC 2014

Rob wrote:
> I would like to use the Atom driver (22) on a Linux system with a
> parallel port.  It is not clear to me from the scattered info I have
> found on internet if this is going to work.
> Using a modern Linux kernel with the PPS module, is it possible to
> symlink /dev/pps0 to a parallel port device and then connect the PPS
> signal to the ACK input (pin 10)?
> If not, what else is required to get this working?
> Examples always refer to the use of a serial port DCD input, but for
> best accuracy (in the microsecond range) I think the parallel port
> is better.
> (no RS232 drivers/receivers, no funny UART that may delay interrupts)
> Any other suggestions for an accurate PPS input?

On NetBSD with stock ntpd, pre 2010, I did comparisons of
pps from Sure GPS with output from dcd at ttl level vs the
"serial" dcd but didn't really see any consistent difference.
 From loop_summary the rms offsets from either would be 4-6 us
due to temperature changes (ambient and from system load)
with peaks of 30-100us otherwise rms offset would have been
significantly lower. Weekly log on Saturdays gives an
additional and larger peak.

loopstats  range(us)   rms
20140219    9+/-38.6   4.5
20140220    7+/-37.9   4.5
20140221    9+/-39.1   4.6
20140222   20+/-49.2   6.2

I just symlinked from lpt to pps but not certain if I needed
to set a flag in ntp.conf. I also used this to compare two pps
signals, serial vs parallel one marked noselect.


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