[ntp:questions] very confusing results though using pps and nmea
Nicola Berndt
nb at komeda-berlin.de
Wed Aug 13 11:53:32 UTC 2008
>> Now using the following ntp.conf I can see that things are working,
but not as expected. I would expect the pps clock to take over, no
matter what happens, since I understand it is either there or not and it
can't be wrong. Instead my system randomly marks either of these a false
ticker every now and then..
>
> Actually the pps CAN have noise on the line. I have found with my Garmin
> 18LVM that sometimes the interrupt fires of 6 or 7 hits in a few
> milliseconds,-- some sort of line noise coming in. Now, it is I admit
a bit
> of a kludge and I have not bothered to track down the problem-- bad
solder
> joint, TV interference,.....-- but it happens.
> What is best if you can record say a days worth of interrupt timings
> without haing the pps discipline your clock, and look at, or rather
plot (
> since looking at 86400 entries is liable to fry your brain) to see if you
> are getting glitches-- noise tends to occur randomly so plotting
> t_{i+1}-t_i over the day will show you if glitches have occured. If they
> differ from 1 second according to your computer clock ( and you have left
> your computer clock just rinning and not resetting it suddenly-- even ntp
> running on the outside sources is fine since it shifts the time slowly)
> then you have noise.
Ok, I will, I am just not sure, how to do so. Here are some assumptions,
please correct me, where I'm wrong:
cat /proc/interrupt shows me /counts/ of different interrupts, so I
assume,that is not what I am looking for. There is a IO-APIC-edge timer
interrupt (0) and a IO-APIC edge serial interrupt (4). The timer
interrupt grows constantly and the serial grows a couple of thousands
every start of a second (looks like) and If then stops until the next
second. I assume that's the one and that the interrupts are fired, als
long as the pps-signal is incoming.
Now I find cat /proc/irq/4/serial to reportI "Is a directory" and I find
nothing in there, so I am a bit stuck, since I simply don't know what to
record..
> Note that serial port nmea is liable to be out by up to 1/2 sec and
to have
> a lot ( 10s of msec) jitter on it, since the end of the string
received by
> a slow serial port ( eg at 4800Bd, it takes an nmea sentence of about 60
> characters 1/8 of a second to be delivered and will have a jitter of few
> characters in length-- or about 10ms jitter)
I don't understand why you are mentioning this, maybe you got me wrong.
I run the nmea gps over usb and the pps goes to the serial port. So
there is no data exept for the pps on my serial line..
More information about the questions
mailing list