[time-nuts] 10MHz LTE-Lite - PPS accuracy?

Brian Martin brayniac at gmail.com
Wed Dec 10 22:28:41 UTC 2014


So just some comments based on personal experience with the raspi. Your experience may vary. 

I think it depends if they are phase aligned or offset. If the interrupts are occurring at the same time, I would anticipate increased jitter. If they are offset I would not expect any negative impact. 

A simple way to demonstrate the effect would be to feed the same pulse to two different PPS pins. The ISRs will compete when both interrupts fire. 

It's trivial to add more PPS inputs to the raspi.

I'd be curious if anybody has experience to share with multicore setups with interrupts pinned to different cores. 

- Brian


> On Dec 10, 2014, at 10:55 AM, Laszlo Hanyecz <laszlo at heliacal.net> wrote:
> 
> Time-nuts,
> 
> Is it a bad idea to have more than one PPS source on a single machine?  Would this cause additional jitter when trying to compare the timestamps on two sources?  I understand that USB can't deliver a real PPS, but what about using onboard serial ports or a PCI card?
> 
> Thanks,
> Laszlo
> 
> 
> -----Original Message-----
> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Paul
> Sent: Wednesday, December 10, 2014 18:43
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] 10MHz LTE-Lite - PPS accuracy?
> 
>> On Wed, Dec 10, 2014 at 12:17 PM, <gigneil at gmail.com> wrote:
>> 
>> Quite simply - if you either execute ldattach pps /dev/ttyUSBx, or are 
>> running GPSD (recommended) it will bind the USB appropriately to a 
>> ppsapi instance.
> 
> I can get a /dev/ppsN but ppstest says time-out.  I saw some hints that the USB DCD
> 
> might depend on the chipset .  I don't like gpsd so I try to avoid it but I might try it.
> 
> Thanks.
> 
>> Do not do it.  You will not approve of the results.
> 
> I don't intend to use it in production.
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