Glitch capture - Page 1
Forum section: electronics / beginners

author: desowin
author_rank: Contributor
timestamp: December 22, 2011, 12:54:26 pm
content: Assume there is software running on microcontroller which outputs logic clock signal with nominally X µs between rising/falling and falling/rising edges. In rare moments the delay between logic transitions can be longer - that's perfectly fine. Define problem as time between rising/falling (or falling/rising) edges is lower than X µs. If problem doesn't appear in trillion cycles, then it's assumed to be good.What is best suited tool to empirically check if such problem exists? Can a digital oscilloscope be used for that (if yes, what besides appropiate bandwidth does it need to have, and how to set the trigger)?

author: Psi
author_rank: Super Contributor
timestamp: December 22, 2011, 01:14:54 pm
content: You can set the scope to trigger on pulses shorter than X µs then check back every so often to see how long it takes before it captures something.But you can run into issues with some scopes not having the right trigger options.I think the rigol only has "pulse width greater than" and not "less than", but i could be wrong.There is probably some piece of test gear better suited to this though, something that automates the process.

author: Balaur
author_rank: Unknown
timestamp: December 22, 2011, 01:41:22 pm
content: I would guess that there is something like an universal counter or some data acquisition system that could be able to fulfill your task with some programming.However, the approach I would use in this case is to design a quick test rig with the following algorithm:I. Use a fast & stable clock (100 MHz let's say with a <1ps jitter and a few ppm stability) as a timebaseII. Record how many cycles of this signal you count between the each edge of your signalIII. Put that data into a spreadsheet, analyze the results, plot a histogram, etcBy preference, I would implement the counting/recording into a FPGA connected to a PC for data logging and analysis.Cheers,Dan

author: baljemmett
author_rank: Unknown
timestamp: December 22, 2011, 02:34:06 pm
content: Quote from: desowin on December 22, 2011, 12:54:26 pmI'm pretty sure my 80s-era HP logic analyser can do this sort of thing -- but it wouldn't be ideal for this situation because I think the time it takes to rearm after a trigger will leave a blind spot and slow proceedings right down. Although thinking about it, by defining the trigger as 'steady for less than X µs before edge' it should capture at full speed and only trigger/re-arm on the failing events, which might do the job. Then just leave it in continuous acquisition and see how many triggers it counts...However, regardless of whether it can do this precise job, knowing that it can do something similar suggests that maybe more modern LAs will have similar features that could be useful?

author: bfritz
author_rank: Regular Contributor
timestamp: December 22, 2011, 05:28:24 pm
content: Quote from: baljemmett on December 22, 2011, 02:34:06 pmThe above is absolutely correct. The 80's-era HP logic analyser can do this with no problem. It will capture at the full digitizing rate without having to re-arm and re-trigger. Some of the better Tek, Agilent, and LeCroy scopes have this option, but not sure exactly which models. You'd just have to wade through the manuals.I'd probably look for something like "Conditional Triggering" in the scope manual.

author: Rufus
author_rank: Super Contributor
timestamp: December 22, 2011, 09:48:17 pm
content: Quote from: desowin on December 22, 2011, 12:54:26 pmA digital scope with infinite persistence (a very common feature) can be set to trigger on one edge and collect and display the positions of multiple instances of the following edge. The attached trace shows the spread in delay between the edges on two signals for example.You didn't mention any clock frequency. Observing a trillion cycles of a 100kHz clock will take more than 3 years so you might want to reconsider your assumptions.

author: desowin
author_rank: Contributor
timestamp: December 23, 2011, 07:01:51 pm
content: I will have a check for these features on the lab equipment I can get access to - thanks guys.Quote from: Rufus on December 22, 2011, 09:48:17 pmFrequency basically depends on X, hence I didn't provide any.Speaking of time - well, there's where "empirical" comes in - just collect enough and hope it'll be the same later.This question was more about asking how to do such thing and what are the right tools, but well, I might have used the "infinite" instead of "trillion" to make it more generic.