[time-nuts] Linux PPS clues?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Oct 20 15:19:42 EDT 2016

On Thursday, October 20, 2016 08:12:12 PM 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 
> > 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 
> > 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 
> 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 
> 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 
> 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) 
> to queue up the events to process them in badges. Because of this, an 
> 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
The fly in the ointment so to speak is that the events in question (photon 
arrival times) are random.
The average rate is up to around 100KHz or so but they are not uniformly 
spaced in time.


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