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When I monitor a SPI bus with only one probe, I see such output when data is transferred.

scope output

I know that I need to use a logic analyzer to monitor the SPI, but I wonder why I see such signal that looks like a glitch; why don't I see square signal waveform?

Or is it a sync function what I see on the output?

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You didn't give us the important information about what speed the SPI signals are or what exactly you expect, but it looks like you are sampling the signal way too slowly. That plots seems to be at about 20 µs per division, which is a very long time for lots of SPI busses. SPI could be operating at 10 MHz or more.

Crank up the time resolution and then show us what you get. Also tell us what you expect to see. A bug is a difference between observed and expected behavior. Without expected behavior there can obvioulsy be no bug, so trying to diagnose one like that is pointless.

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