[ntp:questions] NTP on embedded Linux with GPRS connection

David Lord snews at lordynet.org
Thu Nov 24 11:11:03 UTC 2011


mass85 at tlen.pl wrote:
>> I've used gprs with usb dongle and on either Ubuntu or NetBSD.
>> The main problem was the gprs being very slow to respond until
>> it had adjusted to any significant level of traffic. I'm not
>> sure if Ubuntu uses ntpdate at startup or if it's redundant
>> with NetBSD but either way it wasn't possible to get a reliable
>> timesetting at startup until I used 'burst'. It might be that
>> iburst would have been sufficient by then but I had setup a pool
>> server so using that option was ok for me and I had also set a
>> long default poll interval.
> Ntpdate works pretty fast, I can get time set in 1 or 2 seconds. The
> problem is that connection setup can last long or even fail.
> 
>> At startup rtt was sometimes around
>> 10 seconds and when connection was fully established the lowest
>> rtt was above 150ms. I would not rely on any timings being
>> useful for adjustment of hwclock.

In my case with rtt of 10s the difference between the paths
could result in the hw clock being wrongly set.

You seem to have a different problem.

> Why not? My NTP can synchronise in 15minutes and then keep control
> over system clock. I assume that when it synchronises time and adjusts
> system clock, it knows what it's doing and it is doing it quite
> accurately. RTC can be at least 1s off every 24h, so it is quite much.
> 
> Is there any other ready to use way than "hwclock --adjust" to
> calibrate RTC? Could NTP do it?

That is what the drift file is for after you have an initial
calibration. You should not need to continuously recalibrate.


David


> Marcin Adamski



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