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UART ground connection

+1 vote
Is the ground connection of the UART attached internally to the ground on the USB cable? If not, what are the allowable voltages relative to the USB ground?

I'm asking because I'm concerned about ground loops in my application. I have ideas for how to avoid it, but I'd like to understand what the JeVois has first. If you could provide a schematic and/or part number for the UART level shifting circuitry, that would also be appreciated.
asked Jan 6, 2019 in Hardware Questions by bsilver16384 (170 points)

1 Answer

+1 vote
Best answer

There is a single ground in this design, but we did use ferrite beads to absorb any high-frequency noise coming to the connectors. Please see the two relevant sections below. Note that we ended up using transistors with a lower gate threshold for the level shifters, BSS138 instead of 2N7002 in the schematics.

Here is the mini-USB connector. The metal shroud connects to BOX which goes to GND via a ferrite plus a TVS, 

and here are the level shifters. Note that BOX connects to two mounting tabs on the sides of the connector which are not pins. The black wire of the micro-serial cable (pin 2) goes to GND via a ferrite.

The CPU is direct connect to GND.

answered Jan 8, 2019 by JeVois (46,580 points)
selected Jan 9, 2019 by bsilver16384