I have a Cypress PSoC PL5 prototyping board. It has pins on the bottom which allow it to be mounted on a breadboard so that you can connect things to the GPIOs or ADCs. It gets its power from the host PC via USB. As well as providing power, it is also how the board is programmed and can be used for debugging code.

I have a standalone oscilloscope, and I'd like to probe the GPIO outputs with it (the tutorial I'm following shows this using an Nscope; this seems to provide power to the PSoC). However I don't know what to do with the ground clip on the oscilloscope probe. The PC the board is connected to is a desktop, so it is connected to the mains, as is the oscilloscope. Are there grounding issues to be aware of here?

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    \$\begingroup\$ Try it with and without; see the difference on fast waveforms! \$\endgroup\$ Sep 7, 2023 at 22:28
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    \$\begingroup\$ If I do connect it the ground clip to the board, it would be (could only be) on the board's GND pin, correct? \$\endgroup\$
    – fpd011
    Sep 7, 2023 at 22:43

1 Answer 1


Are there grounding issues to be aware of here?

Yup, you probably want to put it on the USB ground if probing on the Cypress Psoc

Let's think of the the Cypress Psoc as a single variable load. As switches turn on/off it will create current that needs to go down the USB cable back to the source. As this happens the current will create a voltage in R2 and L1, this voltage will be picked up by the scope if the ground is placed on the PC ground, but not if place on the USB ground.


simulate this circuit – Schematic created using CircuitLab

However this represents only one part of the equation as the oscopes ground is also tied to AC mains ground as well as the PC, so you might get more noise because both grounds will create a giant loop (you can make it smaller by placing the grounds of the scope and PC next to each other and keeping the loop size small. If you do have a big ground loop it may be better to use PC ground if it minimizes the loop size.

  • \$\begingroup\$ What is, then, the USB ground? I.e. how do I ground the probe to that USB ground? To reiterate, I'm following a tutorial, which is using a PC-based scope, but as I had access to a real one, I thought I would use that. I am just concerned that I might damage something and/or not be able to take reliable measurements. \$\endgroup\$
    – fpd011
    Sep 7, 2023 at 23:12
  • \$\begingroup\$ It's generally a good idea to use a DMM to see if there is any possibly problematic voltage between the grounds, and determine if it is capable of significant current. You could also look for noise spikes, and maybe beef up the connection, as well as perhaps a capacitor from USB GND to the 5V power on both ends. Remember that there are separate chassis (shield) grounds. \$\endgroup\$
    – PStechPaul
    Sep 7, 2023 at 23:24
  • \$\begingroup\$ If you take a meter to the Cypress board you could look at the grounding structure to see which points are ~0Ω and grounded to each other, I'm willing to bet that usb ground is GPIO ground \$\endgroup\$
    – Voltage Spike
    Sep 7, 2023 at 23:27
  • \$\begingroup\$ Either I don't know what I'm doing, or I'm doing it wrong (or quite frankly both). When I put one DMM probe on the USB GND (on the edge connector), and the other on either GND on the board, I get ~0 ohms. So far so good. When I do the same on the GPIO pins (per the schematic in the data), I get something in the mega-ohm range. Not sure what this means. \$\endgroup\$
    – fpd011
    Sep 10, 2023 at 2:03

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