William Unruh wrote:
On 2014-02-23, David Lord <[email protected]> wrote:
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.
The problem with trying to compare interrupts from two different sources
on a single machine is that there will be something like a 10us
difference always simply because the first interrupt processed has to
read the clock to timestamp the interrupt before the interrupts are
released and then the interrupt handling routine has to send the second
interrupt to the relevant program for processing before the second can process.
As I said when I did this I found about a 10us delay between the two
interrupt processings.
I was testing some homebrew xtal oscillators that were divided
down to give around 1 Hz. They were sometimes at a stable offset
and varied within a range of 10 ms but other days the would have
gained or lost a second or more.
For comparing different PPS outputs at least one of my receivers
has a presettable offset that can then be fudged out by ntpd so
avoiding interrupts being too close together.
David
_______________________________________________
questions mailing list
[email protected]
http://lists.ntp.org/listinfo/questions
