[ntp:questions] Fwd: Re: NetBSD GPS/PPS using 4.2.6p3
snews at lordynet.org
Thu Aug 25 10:38:41 UTC 2011
> On 2011-08-24, David Lord <snews at lordynet.org> wrote:
>> A C wrote:
>>> On 8/23/2011 15:27, unruh wrote:
>>>> On 2011-08-23, Uwe Klein<uwe at klein-habertwedt.de> wrote:
>>>>> unruh wrote:
>>>>>> But from his test, his system is labelling both edges.
>>>>> so he has bounces on the line?
>>>>> either that or rise/falltime is so low and noise so high
>>>>> that receiver hysteris is not sufficient to supress multiple
>>>>> HL/LH changes?
>>>> No, his test shows that the line changes and then 100ms later it changes
>>>> back and 900ms later it changes again (ie once per second it rises and
>>>> falls.) Ie, it is behaving exactly as it should if it were detecting a
>>>> pulse 100ms long.It is detecting both the rising and falling edge.
>>> I just checked and the pulse is almost exactly 100 ms low going and 900
>>> ms high (within about 1 ms) so it's 90% duty cycle high most of the time
>>> with a swing low. The signal itself is clean down to microvolt levels.
>>> The total voltage swing is about 12 volts (which would stand to reason
>>> since I'm feeding the TTL level PPS output of the GPS board through one
>>> channel of a MAX232 level shifter).
>>> Therefore the machine is receiving a nice, clean PPS signal on DCD (DCD
>>> pin was also verified yet again and is correct by hardware specifications).
>> You seem to be saying that the rs232 signal is low going,
>> ie low for 100ms then high for 900ms?
>> Previously I had the impression the pulse was high going
>> and you were using 'flag2 0'.
>> Ntpd requires the prefer peer to be within mindist before
>> even considering the PPS and then the system clock has to
>> be within a millisec of the PPS. Using one of your other
> Are you sure? That seems a very verysilly requirement to put on the time
> accuracy of the PPS. All that should be required is that the pulse be
> within .5 sec of the "true time" in order to establish the "seconds" for
From my reading of the docs the ntp time has to be within
about 1 millisec of the pps before pps will sync.
A pps pulse within .5 sec of the "true time" doesn't make
any sense to me. Perhaps you meant the NMEA sentence used
needed to be within .5 sec but again that doesn't make a
lot of sense in practice as all three different gps units
I've used each have large variations in NMEA time so
require both fudge factor and increase in the range of
values that will be accepted.
>> sources as prefer peer and having NMEA disabled might give
>> you a better chance of getting the pps working without
>> having to bother about the time2 value.
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