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What the title says. I looked into flow control, as that seems like the obvious answer to "what are these extra lines in my UART communication?" But what I'm seeing doesn't really look like flow control, even if we assume that one line is tied to gnd. Also, I didn't see any other data moving across other lines. As far as I can tell, what I'm seeing is a standard UART TX/RX arrangement, except that there's this third line which is controlled (it seems like) by the master.

It looks like it "ticks" (pulses) every time the master finishes sending a byte, and then it also pulses to indicate that the slave is clear to communicate. I've attached some pictures below of traces that I've taken with a logic analyzer. This is this mighty gecko development board driving communication with the smaller mighty gecko module (EFR32MG12).

third line pulses when tx finishes a byte

third line clears slave to talk

EDIT: for detail, this is a mighty gecko dev board talking to the mighty gecko chip/module (EFR32MG12). The lines are unlabeled because they're not listed in the schematic of the device that I'm talking to (the gecko), so I basically went around tapping lines while it talked until I found something. I'm assuming that host TX is the top/black line, as it talks first, and that host RX is the second/brown line, as it's clearly another serial comm line. That leaves the red line as the mystery line.

EDIT 2: changed beaglebone black to mighty gecko dev board. Apologies for the confusion.

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    \$\begingroup\$ Some context would be nice \$\endgroup\$ – Voltage Spike Sep 4 at 18:14
  • \$\begingroup\$ You should investigate this from the software perspective of the beaglebone, in particular figure out if the extra line is being driven in software as a GPIO or if it is coming from a serial engine, perhaps in a pseudo mode. Also identify the peripheral - questions missing critical detail tend to be quickly closed. \$\endgroup\$ – Chris Stratton Sep 4 at 18:16
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    \$\begingroup\$ Third line (if you're talking about the red label 02), could just be a clock. It's very hard to interpret since you don't have your channels labeled and you're not telling us what object you're probing. \$\endgroup\$ – KingDuken Sep 4 at 18:19
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    \$\begingroup\$ That's a good edit. Thank you. I don't know anything about this software that you're using so I don't know if you're simulating this if you actually have wires hooked up to the oscilloscope. If it's the ladder, then could you tell us what pin on the board you're probing? If it's simulation, is there some way you can view your configuration? \$\endgroup\$ – KingDuken Sep 4 at 18:47
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    \$\begingroup\$ Looks to me like it's identifying the final bit of each serial byte. \$\endgroup\$ – TonyM Sep 4 at 19:44
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It looks like UART with hardware flow control.

Page 15 in the data sheet states that it supports hardware flow control on the USART https://www.silabs.com/documents/public/data-sheets/efr32mg12-datasheet.pdf

By searching for “uart hardware flow control” images, several plots with similar data and “ticks” appear.

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