[Thumbgps-devel] Project clarification

[Ron Frazier (NTP)]

On 3/13/2012 11:42 PM, Dave Taht wrote:
> On Tue, Mar 13, 2012 at 8:08 PM, Dave Taht<dave.taht at gmail.com>  wrote:
>    
>> On Tue, Mar 13, 2012 at 6:28 PM, Ron Frazier (NTP)
>> <timekeepingntplist at c3energy.com>  wrote:
>>      
>>> On 3/13/2012 9:10 PM, Dave Taht wrote:
>>>        
>>>> On Tue, Mar 13, 2012 at 4:06 PM, Eric S. Raymond<esr at thyrsus.com>    wrote:
>>>>
>>>>          
>>>>> Mike Hord<mike.hord at sparkfun.com>:
>>>>>            
>>      
>>> I'd like to get a further minor clarification on the project goal.  Do you
>>> want your time reading to be within plus or minus 1 ms of UTC, for a total
>>> range of 2 ms?  Or do you want it to be within plus or minus 500 us of UTC,
>>> for a total range of 1 ms?
>>>        
>> Heh. Good question.
>>
>> As good as it is possible to get without having to have rubidium
>> clocks embedded in the stick?
>>      
> Ooh! OOh! I found an americium clock for only 13.99!
>
> http://www.cafepress.com/randumbshirts.327160457
>
>    
>> Better than 10ms is desirable, and the 1ms is something of an
>> arbitrary goal. Nyquist theorem applies, and then there's slop/noise
>> elsewhere in the system that will start cropping up (interrupt
>> latency, etc)
>>      
> I kind of realized I wasn't being as clear as I might like.
>
> right now the jitter and delay is well over 10ms on all the gps
> devices we've got our hands on, so it's the most noisy source of
> 'stable' time there is, and the internal system clock, as corrected by
> ntp, does better.
>
> Take the blox gps chip that I'm half in love with. It can - with
> tweaking, do a 15ns pulse -
>
> but everything after that you attach to that signal - microcontroller,
> usb bus, basic interrupt latency in the os,  userspace response time,
> response to load, etc - will add additional jitter. And all that has
> to be measured.
>
> So starting with the least jitter possible and fixing it at every step
> of the way out of the chip is what would happen, which is why it's
> hard to specify (right now, without having a stable time source to
> play with at all - which I'm trying to aquire btw) whether .5 or 1ms
> jitter will affect things all that much.
>
> I did a lot of work with linux-rt back in the day, and with threaded
> interrupts it was possible to do really intense audio work with about
> 2.8ms sample period on high end (for the day) hardware.
>
> While much of the linux-rt code has made it into the mainline, that
> feature hasn't, and the box I'm mostly playing with runs at 680Mhz. I
> don't know what it's interrupt latency actually is...
>
>
>
>    

Hi Dave,

What you just said reminded me think of something.  A couple of years 
ago, I was doing research into microcontrollers and thought I might be 
designing an audiovisual product.  That never happened, but I ran across 
FreeRTOS, as in Free Real Time Operating System.  I have no idea if it 
would be relevant to what you all are doing, but I thought I'd pass the 
link along.

http://www.freertos.org/

To give an idea of what can be done with off the shelf non PPS USB 
technology, my GlobalSat BU-353 is capable of giving me time offsets 
from it's perception of true time of + / - 6 ms.  The problem is that 
its perception of true time, the NMEA sentences, wander 120 ms or so 
over a period of a few days.  I've been told that you can get + / - 400 
us with PPS emulation through a USB - serial converter.  I hope to test 
that theory soon.

Sincerely,

Ron

>    
>> Trying to clearly identify problem hops from multiple sides is
>> dependent on the 'distance' inherent in the underlying technology. If
>> we wanted to identify problems on a single 10GigE ethernet segment,
>> that's measured in *ns*. We don't, thankfully.
>>
>> First hop out of a router is generally 1ms, same for wireless. Cable
>> is in the 2ms-8ms range. DSL, up to 60ms, same for wimax...
>>
>> So<  5ms would be the outside goal, and anything less than that - so
>> long as it can be thoroughly catagorized - is a bonus.
>>
>> Continental US distances are 60-100ms...
>>
>>
>>
>>
>>      
>>> Sincerely,
>>>
>>> Ron
>>>        



-- 

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Ron Frazier
timekeepingdude AT c3energy.com