[time-nuts] Linux PPS clues?

Scott Stobbe scott.j.stobbe at gmail.com
Thu Oct 20 21:41:28 EDT 2016

I'm not sure if you were asking for scope adive or not, but to look for the
minimum seperation (also runt pulses) you would want to set your trigger
threshold as low as possible without false triggering on noise, and set
trigger hold off to a minimum.

National Instruments has lots of litteriture on this topic which may be
helpful, timestamping events with 10's of ns of resolution. NI
Timer/counters http://sine.ni.com/nips/cds/view/p/lang/en/nid/4529

On Thursday, 20 October 2016, Ilia Platone <info at iliaplatone.com> wrote:

> sorry, no attachment, this mail contains two images, one is the previous
> attempt, the second (IMG_003.JPG) was taken at 5us/div, 1v/div with a
> different oscilloscope setup.
> Best Regards,
> Ilia.
> On 10/20/16 18:12, Attila Kinali wrote:
>> On Thu, 20 Oct 2016 10:59:21 +0100
>> "David J Taylor" <david-taylor at blueyonder.co.uk> wrote:
>> Actually, of the 15 Raspberry Pi cards I have only one is used in a
>>> graphics
>>> application.
>> Yes, the rpi are used for all kind of stuff and there is a huge community
>> around them that helps with all kind of questions. Unfortunately, the
>> rpi is also used for all kind of stuff that it is a suboptimal choice
>> (to put it mildly), but people do not care or do not want to check
>> for alternatives. It kind of works, that's all they care about.
>> On the positive side they work very well with external devices for control
>>> and measurement,
>> And for most of these applications a 32bit uC that uses a fraction of
>> the power would be the right choice. Often a clock of 1MHz would be
>> enough.
>> and have a huge amount of software and hardware support for
>>> a vast range of devices which makes for fast and easy development.
>> That's the only plus side. But then, most of the code written in C
>> can be used on a uC just the same with little to no modification.
>> I will be interested to see what is recommended for a 100 kHz event rate.
>> This is actually a very tough question. 100kHz means that for each event
>> there is only 10µs available for detection, processing and output. Using
>> a uC that would be something in the order of 1000-2000 CPU cycles. On an
>> application processor (rpi and its cusins) that would be 2000 to 20'000
>> cycles.
>> While 1000 cycles on a uC is quite a lot, you cannot do any fancy
>> processing
>> with so few cycles.
>> On the application processor 20k cylces is plenty, but you have the
>> complex
>> OS that eats up a few thousand cycles itself. Addtionally there comes
>> the interrupt latency that the application processors suffer from, which
>> is in the order of 1-10µs... So they would need a kind of (hardware)
>> system
>> to queue up the events to process them in badges. Because of this, an rpi
>> wouldn't work at all (bitbanging takes several µs for each operation).
>> Going for an uC is easier in that regard as they have very little
>> interrupt
>> latency (usually just 5-10 cycles), but then you have problems with
>> getting the output out of the uC as their I/O subsystems are usually
>> optimized to work in a stand-alone fashion.
>> Maybe one way would be to use an arm9/cortex-a5 based uC (ie not an
>> application
>> processor) and use their high speed I/O.
>> For better answers, I would need to know what kind of events these are
>> and what exactly need to be done/measured.
>>                         Attila Kinali
> --
> Ilia Platone
> via Ferrara 54
> 47841
> Cattolica (RN), Italy
> Cell +39 349 1075999

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