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I have an MCU controlling a LED sign. This sign speaks the HUB75 protocol, which is essentially a large shift register. Being a shift register, this requires a clock signal to be generated, which the MCU appears to do just fine however the sign appears to not interpret it correctly. Annoyingly, connecting a logic analyzer and probe to the offending line causes all glitches to go away, and therefore I ask: what effect does placing a probe onto the signal cause?

I'm using a Saleae Logic 8 with it's built-in probes. The datasheet cites an "input capacitance" of 10pf, however the effect is not replicated with a 10pf cap to ground.

The circuit is running at 3v, but the logic analyzer is powered off of USB, so I'd assume it has 5v -- could this mean there's a pull-up somewhere? I'm running out of ideas, and so I asked here.

EDIT: I've done some more testing and the "fixing" effect (for lack of a better term) occurs even if the analyzer has no power, just ground and the signal itself.

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  • \$\begingroup\$ (a) Is it a real Saleae Logic 8, or a "clone"? (b) Have you opened your specific analyser to view its actual input circuitry? If so, would you please edit your question and add some closeup, in-focus photos of the analyser PCB. \$\endgroup\$
    – SamGibson
    Jan 8, 2019 at 3:03
  • \$\begingroup\$ It is, in fact, a real Saleae bought straight off of their website and no I have not opened it up. \$\endgroup\$ Jan 8, 2019 at 3:09
  • \$\begingroup\$ Thanks for the reply. "It is, in fact, a real Saleae" OK, great, so in that case I'd suggest you ask Saleae, as it's a analyser-specific answer. It's always possible someone here knows (and will write) the answer for that specific analyser, but so will the manufacturer. \$\endgroup\$
    – SamGibson
    Jan 8, 2019 at 3:24
  • \$\begingroup\$ First just try a resistor of a few kilo ohms to ground. As for the analyzer, a series resistor and then diode clamps to 3v3 and ground would be likely. \$\endgroup\$ Jan 8, 2019 at 3:26
  • \$\begingroup\$ Exactly what logic is driving the clock, i.e. what MCU and what pin, and if it is configurable, what output type and current? How long is the wire from the MCU to the sign? What is the load presented by the sign, e.g. is it a single pin or multiple pins? A probe can subtly change the timing, especially if there are problems from transmission line effects. \$\endgroup\$
    – crj11
    Jan 8, 2019 at 6:07

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The noise is reduced not by the probe, but rather the earth gnd used by the probe.

The DC local circuit ground is probably floating with CM noise from SMPS.

I would connect 10nF from 0V to earth ground or inspect CM noise.

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  • \$\begingroup\$ I'm fairly new to a lot of this, so forgive me if I've butchered something here, but if the problem was only with the ground signal, wouldn't just attaching the GND of my circuit to the analyzer fix it? This isn't the case and I've updated the question to reflect this information. \$\endgroup\$ Jan 8, 2019 at 22:13
  • \$\begingroup\$ electronics.stackexchange.com/… \$\endgroup\$ Jan 8, 2019 at 22:14
  • \$\begingroup\$ So grounding does not solve it? \$\endgroup\$ Jan 8, 2019 at 22:16
  • \$\begingroup\$ I'm not quite sure how to test it, I don't have an earth ground coming out of my power supply (unless I'm being an idiot, which is quite possible, but I'm using a barrel jack which only has terminals for + and -), and therefore don't know where I'd place this 10nF cap. \$\endgroup\$ Jan 8, 2019 at 22:20
  • \$\begingroup\$ Does Logic analyzer use earth ground, then just connect logic probe ground . If that works then DO as I said in answer. get an earth ground and Cap couple to it or do the other things.. i.e. is USB coming from an earth grounded tower or a laptop? It may be your layout so shown pictures \$\endgroup\$ Jan 8, 2019 at 22:22

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