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i seem to have forgotten to take the microUSB portion of my microUSB carkit UART cable with me. i could order a new microUSB plug and put it back together... but if i'm already ordering stuff, why not make the debug cable of my dreams?
the general idea:
- the cable can switch between two modes: USB passthrough and UART
- in USB passthrough, the microUSB connection goes directly from the host to the device (in this case, it also acts as an USB-C to microUSB converter)
- in UART mode, the host connects to an onboard UART chip, which in turn connects TX/RX to D+/D- on the cable, as per the carkit schematics (https://wiki.postmarketos.org/wiki/Serial_debugging/Cable_schematics#microUSB/Carkit_debug_cable). we also need to place a resistor between ID and GND; this is configurable between two common values - 150kOhm and 619kOhm - with a second onboard switch.
attached is my first naive attempt at creating this, using two TI TS3USB30E USB 2.0 multiplexers (one on the device side, one on the host side) and an onboard FTSI FT232R.
i have a few questions about this design:
- can this all be connected to one ground, or are there going to be problems? in passthrough mode microUSB GND needs to connect to host GND, in UART mode we need a gnd connection between the UART chip and the microUSB plug at least
- likewise, can VCC from the host/USB-C receptacle act as VCC for the whole board? (it has to...)
- the USB ICs do the D+/D- switching, but how do i cut the connection between GND and ID in passthrough (not UART) mode, and connect it when the UART mode switch is set to high? quick web search shows a bunch of different transistors or MOSFETS or other things and i'm not sure what any of that is about or which one applies in this case (ID and GND carry no voltage, the MUIC on the target device checks if ID pin is floating or pulled down with a specific resistance)
- is there a problem with using two multiplexers for this? i.e., is it possible that if one mux switches faster than the other, something is going to break?
- ...and, uh, is any of this even correctany thoughts? not really sure where to ask for help with this stuff, figured maybe fedi would know...