How to trigger on the first pulse after a long stable time?

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PhilV
Newbie
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Joined: Thu May 11, 2023 11:09 am

How to trigger on the first pulse after a long stable time?

Post by PhilV »

I have an SPI signal chain that looks like this:
picoSig1.png
This one has triggered in the correct place (I am using a simple falling edge trigger on the blue trace), but sometimes the scope will trigger like this:
picoSig2.png
Is there any setup I can do on the trigger to say "trigger on falling edge, but only if signal has been high for x uS previously" ?
Last edited by PhilV on Mon Apr 15, 2024 6:53 am, edited 1 time in total.
PS: 7.1.21.18179
Scope: 3405B

Willem43
Newbie
Posts: 1
Joined: Wed Jul 21, 2021 11:56 am

Re: How to trigger on the first pulse after a long stable time?

Post by Willem43 »

Try using an "Interval Trigger." Setting an interval trigger of “falling, greater than Time” will set the trigger point to the first falling edge after the long Time interval. Select Time to suit - longer than the times in your required pulse train.

PhilV
Newbie
Posts: 0
Joined: Thu May 11, 2023 11:09 am

Re: How to trigger on the first pulse after a long stable time?

Post by PhilV »

Thanks I tried this, but for this to work it seems that the whole transition has to be available on the scope screen, so much like the trigger icon I'd need the low section of the blue trace, then the LONG high section, then my PSI pulses.
However the gap between my SPI pulse chains is significantly longer than the chain itself; increasing the interval per division will allow me to see multiple "chains" but then I lose the resolution to be able to actually inspect them.
EVEN then, the scope does not reliably trigger on the first pulse in the chain:

At a 10uS interval on the trigger the trigger fires, but doesn't reliably lock to the first pulse, (other pulses probably also qualify):
pico_10us_trigInt.png
At 20uS interval the trigger fails to fire at all (on "normal" it never fires, on "auto" the trace just scrolls as captured below):
pico_20us_trigInt.png
PS: 7.1.21.18179
Scope: 3405B

Willem43
Newbie
Posts: 1
Joined: Wed Jul 21, 2021 11:56 am

Re: How to trigger on the first pulse after a long stable time?

Post by Willem43 »

I do not understand why there is a problem with your 20us case. You have about 1ms between chains, I would make the time, to trigger on a chain start, 300 - 400us. Do you have continuous chains or bursts of chains?

Here is an example for a case of mine.
Scope is 2207B MSO so I used digital pulses, amounts to the same thing in the end.
I control the bursts of SPI communication with instructions so can capture "sets" of pulses. I simply used a simple trigger on the first negative SS pulse edge which starts the sequence (I do have cases where I used Trigger Interval - cannot find them right now).
I used 1ms/div, that is 10ms per screen. The sequence occupies about a third of that. First capture shows zoom to full sequence (overall zoom shows the full screen).
Capture A.png
Zooming in on the last two chains still shows plenty of detail, as you can see in the next capture.
Capture B.png
The big difference is my sample rate. I have 625kS per buffer at 62.5MS/s (could probably get away with less - but if you have it, use it), this equates to 16ns Sample interval - sampling was set to 1MS buffer target. (I still do not understand how the scope decides how to round off the values).
I suggest you start by upping the sample rate.

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