PPI time stamp accuracy problem

RE: PPI time stamp accuracy problem

Cedric Honnet — Wed, 06 Jun 2018 15:49:01 GMT

Alright, hanks for the link anyway ;)

RE: PPI time stamp accuracy problem

Susheel Nuguru — Wed, 06 Jun 2018 10:28:35 GMT

I normally should have taken some snapshots, Not in a habit of saving all my desk test setups in a repository.

But the setup looks more like the code in this thread. I instead used TIMER->CC to be 1 instead of 2.

In any case, the difference should not be as big as you see in your tests. That is more than 300 PPI sample clock ticks, which cannot be true for any of our systems that depend on PPI to work.

RE: PPI time stamp accuracy problem

Cedric Honnet — Tue, 05 Jun 2018 13:51:52 GMT

Hi,

Thanks a lot for your help!

Could you share the link of your repository?

It looks like our configurations are different: GPIOTE -> PPI -> TIMER

(The idea is to trigger timer captures to measure GPIO pulse widths)

Thanks again !!

RE: PPI time stamp accuracy problem

Susheel Nuguru — Tue, 05 Jun 2018 13:43:26 GMT

sorry for the late reply, I have done some basic tests to check time delays between events from PPI -> TIMER->GPIOTE and still see a maximum of 1 tick error from the event source to the toggle. I believe in your setup you are exercising more peripherals? I cannot test your code as I do not have that photodiode to test with. 21 us see way off from what it needs to be.

RE: PPI time stamp accuracy problem

Cedric Honnet — Thu, 24 May 2018 11:16:47 GMT

Hi Aryan!

Thanks a lot for your answer!

You'll find below another example with a 54 us error (= 3294 us - 3240 us), and the code is attached.

Hardware: this test was done with a TS4231 photodiode, and signals generated by HTC vive base stations.

Software: the tool chain used is on github, but it should not be very different to what you use.

If needed, there's also a signal_generator folder to simulate fairly similar pulses.

Thanks again!!

Cedric :)

devzone.nordicsemi.com/.../6355.PPI.zip

RE: PPI time stamp accuracy problem

Susheel Nuguru — Wed, 23 May 2018 06:34:06 GMT

Very interesting. Can you please attach a minimalistic project to reproduce this. I cannot access the github page.

Maybe you are doing some casting conversions to be able to print that value onto serial port and loosing some accuracy in the conversion?? This seems like a long shot, I would like to reproduce this and see.

My other theory is that it is possible that the chain of peripherals this event goes through by the time it gets latched in the timer capture register might be long and takes more than one PPI cycle to reach there.