[time-nuts] Linux PPS clues?

Ilia Platone info at iliaplatone.com
Fri Oct 21 06:08:04 EDT 2016

it's a BC546, 10k collector resistor... I thought it was fast enough, 
but also think the signal is inverted, since there's passive quenching 
in this circuit, so the breakdown voltage of the APD is lowering too 
slowly before next photon, causing this kind of positive edge.. if you 
see well there's the same slope in all the signals. I'm currently 
searching for an active quenching circuit that fits into my application.

Best Regards,

On 10/21/16 09:53, Adrian Godwin wrote:
> What is the circuit driving that signal ? It appears to have too little
> positive drive to overcome the capacitance. Perhaps it's an open collector
> with too large a pull-up ?
> On 21 Oct 2016 12:23 a.m., "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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Ilia Platone
via Ferrara 54
Cattolica (RN), Italy
Cell +39 349 1075999

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