[time-nuts] Linux PPS clues?

Hal Murray hmurray at megapathdsl.net
Thu Oct 20 18:08:05 EDT 2016

info at iliaplatone.com said:
> These events are random photon arrivals (converted to 5vTTL pulses),  their
> rate was measured using the pulse width of the smaller detected,  which was
> 5~10 uS during an observation in low-light environment. The photon arrival
> and pulse width were random with a minimum pulse  width of 10uS. What I want
> to do is measuring the photon arrival  precisely (low to high transition -
> interrupt I guess), consider that  the maximum rate would be 100Kcps because
> the photon events would  overlap if higher. If the 3130 controller can
> handle such rate it would  be great :) 

I think your 100K samples per second is going to be exciting.

You can collect some data by writing some code and connecting up a signal 
generator.  Start with a slow frequency to debug the software than turn it up 
and see how fast you can go.

I assume it's OK if some (but not many) samples are lost.

You (or I) may be confusing the data rate.  There are two issues.  One is the 
peak rate.  That sounds like your 100K above.  The other is the average.  How 
many samples do you expect in a typical second?  If that is low enough, you 
can solve the peak problem with enough buffering.

My straw man would be a FPGA.  The idea is to collect a batch of pulse 
arrival times and DMA them into memory.  When the buffer fills up or a timer 
expires, it would finish that block and switch to the next one.  You could 
feed a PPS to the FPGA if you want timing linked to the outside world rather 
than just relative times.

These are my opinions.  I hate spam.

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