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I am connecting the SPI bus pins of two development boards:

They are each connected to their own wall-wart power supplies. I have confirmed that the connected pins are all operating at 3.3v. However, I know that "3.3v" is only in reference to their own grounds.

Are there any potential risks to connecting the VDD (3.3v only) and ground pins between the boards to make sure the voltages match up correctly?

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  • \$\begingroup\$ Don't connect the Vdd's. If you do, a current may flow if the voltages differ between the boards. As the impedance of the supply-paths is low, this current may be substantial. \$\endgroup\$ – peter Mar 2 '17 at 7:08
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    \$\begingroup\$ It would be better to use one supply for both. Weird things can happen if one board powers up before the other one. There are ways to deal with it, but that would be a whole topic on its own. \$\endgroup\$ – mkeith Mar 2 '17 at 7:22
  • \$\begingroup\$ @mkeith I hadn't thought about doing that... The Cypress board requires 12VDC, while the NVIDIA board can support 5.5VDC-19.6VDC, so they should both work from a 12VDC supply, assuming it meets the current requirements as well. Thanks for the suggestion. \$\endgroup\$ – skrrgwasme Mar 2 '17 at 8:06
  • \$\begingroup\$ @peter Yeah, in retrospect, that was kind of a stupid thought. Connected grounds should be good enough. \$\endgroup\$ – skrrgwasme Mar 2 '17 at 8:08
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If you are connecting two boards (powered from two different wall adapter), make sure you have made ground common. Short the Ground Pins of two boards together. This way, both boards will have common reference.

Unless you are using the 3.3 V from first board to power the second board, you don't have to connect the power lines. Once, common ground is established, the voltage levels definition remain same for both boards. 3.3 V for one board now means 3.3 V for the other board too.

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