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== Nexus debug cable ==
The nexus phones multiplexed the serial debug lines with the headphone audio lines. To switch it into serial mode you have to supply -3V to the mic port. The
uart lines themselves are 1.8V. If you use a 3.3V usb-to-uart adapter the signal from the phone will probably work, for the TX side you have to add a resistor divider to lover the voltage for the phone. Here's the schematic for a nexus debug cable:
Google open sourced their own [https://android.googlesource.com/device/google/debugcable/+/master debug cable design] in the AOSP repository.
== USB debug cable ==
The Suzy-Q debug cable isn't strictly a serial debug cable, it accesses the Closed Case Debugging system, it can access multiple busses on the device. This system was originally developed to debug chromebooks but has been used in the newer Pixel phones. Details for the Closed Case Debugging system can be found at https://chromium.googlesource.com/chromiumos/platform/ec/+/master/docs/case_closed_debugging.md
This cable should work with: