I have two I2C messages 20 ms apart:


When I adjust the horizontal scale and look at them in normal trigger mode, I see both of them overlayed over each other:


(At least that is what I assume I'm seeing.)

After the second message there's silence for a second or so, so this image stays on screen until the next transmission.

If I press the RUN/STOP button, I see just one message:

for comparison

Display persistence is off. Is this a bug in my Rigol DS1052E or do I have some misconception about how normal trigger mode works? If the trigger holdoff time is short enough for the second message to trigger again, shouldn't the first message be cleared from the screen before recording the second one?

  • 3
    \$\begingroup\$ Not answering your question, but from those traces I can tell you that your I2C pull-up resistors are far too large for the clock frequency you're using. The rise-time is much too long and you can see on your blue clock trace that it doesn't even reach Vcc before being pulled low again. Whatever you're using, divide it by 4 at least. \$\endgroup\$ – brhans Jul 5 '17 at 1:28
  • \$\begingroup\$ @brhans We don't know how AndreKR is probing (could be a X1 rather than X10 attenuator probe). Might be OK after the probe is removed, but you do make a good point that should be checked out. \$\endgroup\$ – glen_geek Jul 5 '17 at 2:09
  • \$\begingroup\$ I recollect that early TEK digital 'scopes do normal trigger just as you've expected (each trigger event wipes the previous screenshot). Could trigger holdoff control work backwards to what you expect? \$\endgroup\$ – glen_geek Jul 5 '17 at 2:16
  • \$\begingroup\$ Yeah, probes on 1X and also quite long cables. Not the scope (no pun intended) of the question, though. :) \$\endgroup\$ – AndreKR Jul 5 '17 at 3:37
  • \$\begingroup\$ Are both messages identical - they don't look identical. \$\endgroup\$ – Andy aka Jul 5 '17 at 10:32

Your signals aren't necessarily far enough apart for the scope to only show one at a time while running.

When in "Run" mode you're potentially seeing the waveforms for multiple trigger events. But when you hit "stop" or "Single" you are only going to see one trigger event, which is why you only see one packet.

Normal trigger mode just means your oscilloscope won't automatically force a trigger after a specified timeout, I actually talk about this on our YouTube channel here: https://www.youtube.com/watch?v=AbjjF0vMPyo&index=5&list=PLzHyxysSubUkc5nurngzgkd2ZxJsHdJAb

So, just because you're in normal mode doesn't mean you'll only see one trigger at a time, it means you'll only see valid triggers.

  • \$\begingroup\$ "When in "Run" mode you're potentially seeing the waveforms for multiple trigger events." So that is by design? All DSOs work that way? Is there a number in the manual that tells me how long it takes for the screen to be cleared? \$\endgroup\$ – AndreKR Jul 7 '17 at 22:51
  • \$\begingroup\$ Yes, this is by design. We sometimes talk about oscilloscope waveform update rate, which hundreds of thousands or over a million waveform captures per second, but a screen only refreshes something like 120 times per second, so multiple captures are overlaid on top of each other in run mode. The spec to look for is "waveform update rate." But, you can also check the frequency of the oscilloscope's trigger out (if it has one) and that will let you know how fast your scope is triggering. \$\endgroup\$ – Daniel Bogdanoff - Keysight Jul 10 '17 at 19:55
  • \$\begingroup\$ That makes sense. I always wondered how can those InfiniiVision scopes refresh their screen a million times per second. Simple answer: They don't. Which is probably a good thing, otherwise you wouldn't be able to see anything. Anyway, so I can assume that if both triggers happen within about 1/120th of a second, they'll both be painted. \$\endgroup\$ – AndreKR Jul 10 '17 at 21:14
  • \$\begingroup\$ In general, yes, that's a fair assumption. \$\endgroup\$ – Daniel Bogdanoff - Keysight Jul 11 '17 at 16:13

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