[ntp:questions] Atom PPS with parallel port

Charles Elliott elliott.ch at verizon.net
Tue Feb 25 13:05:25 UTC 2014

Several years ago, I was asked to evaluate the Real Time O/S that Intel was
for sale as a project in an electrical engineering course.  Interrupt
latencies and
servicing times are very important in real time control, so I evaluated the
parallel port.  As I recall, I output a signal to one pin on the parallel
port and then
sent out a digital output to a point where I could measure it with an
oscilloscope after
receipt of the interrupt in my test program.  I used the oscilloscope to
the time from the first signal to its recognition by my program.  I don't
the exact results, but I do remember that the latencies were highly
Intel subsequently sold the real time O/S, and I have not seen it for sale

Charles Elliott

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From: questions-bounces+elliott.ch=verizon.net at lists.ntp.org
[mailto:questions-bounces+elliott.ch=verizon.net at lists.ntp.org] On Behalf Of
Sent: Sunday, February 23, 2014 12:36 PM
To: questions at lists.ntp.org
Subject: Re: [ntp:questions] Atom PPS with parallel port

David Lord <snews at lordynet.org> 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.

Did you try the parallel port?
I am interested not only in jitter but also in any constant offset between
the PPS pulses and system time on different systems (possibly using
different makes of serial card).  4-6 us would be good enough for my
purpose, but it would not be good when one system had a 10us offset because
of a propagation delay in a linedriver/receiver.

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