[ntp:questions] Second attempt at GPS-18 based NTP server

unruh unruh at invalid.ca
Thu Feb 2 20:12:24 UTC 2012

On 2012-02-02, Chris Adams <cmadams at hiwaay.net> wrote:
> Once upon a time, unruh  <unruh at invalid.ca> said:
>>In the latest linux kernels, the pps option is already included, if
>>selected by your distribution.
> I'm using parallel port PPS with Fedora 16.  I did the following:

As I mentioned, the problem is that parallel ports are becomeing rare on
motherboards, and the parallel ports on add in cards are almost always
ports with "shareable" interrupts, which means that they are level
triggered not edge triggered. This means that they keep pouring out
interrupts as long as the level is high, which means taht the system
gets 10s of thousands of interrupts for each time the interrupt comes in
from the GPS ( about 100ms long). At least this is what I found. 

The parallel port hardware does not seem to have anything which tells it
to stop sending interrupts on a high ACK after the first one has been

> - put this in /etc/sysconfig/modules/pps.modules (mode 755):
>   #!/bin/sh
>   exec /sbin/modprobe pps_parport > /dev/null 2>&1 || :
> - put this in /etc/udev/rules.d/99-local.rules
>   SUBSYSTEM=="pps", MODE:="0640", GROUP:="ntp"
> - put this in /etc/ntp.conf
>   server minpoll 4 maxpoll 4
> Serial port PPS would need to load pps-ldisc instead of pps_parport (who
> knows why one has an "-" and the other an "_") and an extra step to call
> ldattach to attach the PPS line discipline to the appropriate serial
> port (not sure where the best place to put that would be, maybe after
> loading the module?).

More information about the questions mailing list