# Minimum TTL 5.0V signal duration

I'm working on a little project in my physics lab.
We would like to create a photon coincidence counter. We were thinking to use the Pi GPIO pin to monitor a couple of photo diode which produce a 15ns 5V TTL signal.
Is this signal too short to be noticed by the Pi?
I was thinking that with a 700 MHz clock it could be possible but i'm not so sure that the CPU monitor with the same frequency the GPIO pins.
And if it can successfully read them, can it read multiple signal of this type in short delay one with another (or even better simultaneously)?
I'm pretty new with the Pi and I hope i explained clearly the problem. This is more a proof of concept to see if it can be done.
Thanks

• Hello and welcome. First important note: Thou shalt not apply 5V to the Pi's GPIO. (3V3 logic inputs!) As for the length of the signal. I have no idea how short pulses could be to be detected by edge-triggered interrupts.
– Ghanima
Dec 4, 2015 at 16:14
• OK, i'll take notice of that, thank you very much :D Dec 4, 2015 at 16:24

I haven't got a means of generating a 15 ns pulse so can't be sure.

I believe the Pi will be able to detect the signal although you might have to tailor the system slightly. You'd probably need to use something called "GPIO Asynchronous Edge Detect" which I'm fairly sure will catch a brief signal.

I don't know of a way to measure such a short duration with the Pi so you'd know a pulse happened but wouldn't know how long it was.

You could detect multiple simulatenous events on different GPIO. However I don't think you will be able to retrigger a GPIO for a new event without a microsecond or so delay.

You may have to go "bare metal" to achieve your aims.

• Thanks i don't need to measure the duration, i just need to know it arrived. The microsecond delay could be a problem, i'll look out for the bare metal solution thanks Dec 4, 2015 at 19:49
• @fox895 Have you considered asking the question on the electronics stack exchange? It seems fairly specialised as a software/Raspberry Pi problem but must be fairly common to an electronics engineer. For all I know an inexpensive Arduino may do the job.
– joan
Dec 4, 2015 at 20:27
• I will try tomorrow. We excluded the arduino uno (we alredy have one in lab) because it has a clock speed of 16 MHz (so we are not sure it can detect such short pulse) and it really can't handle simultaneous input, as it has to interrupt the listening mode on the pin if you want to perform some operation. We thought a Pi was more suitable and versatile. Also we looked around for similar project (a geiger count) and they used a Pi board Dec 4, 2015 at 21:12