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Sorry if I worded the question strangely, I'm just not exactly sure what's going on. I'm analyzing a PCM audio stream coming out of an ESP32. I've got the DOUT pin connected through a 10x probe to channel one of my scope, and CLK connected to channel 2. CLK looks normal to me, but DOUT is just 2 parallel lines. Normally I would assume the signal is just too fast, but even on the shortest time base, the signal looks like 2 continuous lines, and the signal shouldn't be that fast anyway. The pin can't be at zero and five volts at the same time, right? Am I doing something wrong, or interpreting the data wrong? 0.5us signal 20ns signal Connections PCM Standard

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  • \$\begingroup\$ Try triggering on a negative slope on channel 2. See that vertical line on channel 1 in your upper picture? \$\endgroup\$ – WhatRoughBeast Jul 22 '18 at 20:42
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The pin cannot be at zero and five volts at the same time, but it can quite easily have occurrences of both states in successive sweeps within the retention time of your analog scope's phosphor (or a digital scope's software model of phosphor behavior).

You appear to be set up to trigger on any chosen direction edge of the clock, so you're probably viewing many different bit positions in the word (or successive words), all folded to the same position of the screen.

Even if you managed to trigger on something more uniquely identifying in time, like a framing pulse or left/right word select, noise could still cause you to see both possibilities in the data at a given position on the screen - obviously so in the lower bits, but also even in the higher bits if the noise spans zero where all bits may change state, or some other power of two transition where many of them do.

To get a picture without this, you'd either need to view a consistent place in consistent serial data, or use a digital scope in a one-shot mode or some similar scheme so that you're only viewing a single sweep.

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  • \$\begingroup\$ I see now, I was thinking one clock pulse equals one bit, completely forgetting how the oscilloscope really works. I will get my hands on a digital scope to troubleshoot it further, thanks for the help. \$\endgroup\$ – nkizz Jul 22 '18 at 20:43
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    \$\begingroup\$ You can probably do this without a digital scope and with some cleverness. For example, trigger on the word sync and use your trigger holdoff so you're at least seeing the same position in each word. Or after using a scope to look at the analog aspects of the signal, you can use a simple CY7C68013A USB-fifo logic analyzer with sigrok and pulseview to look at the data. \$\endgroup\$ – Chris Stratton Jul 22 '18 at 20:50

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