[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 
 > 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 

 > 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..

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