Making an UART cable for the Nexus 5

Prototyping

I had to prototype using only the components I already had, since I would already buy some components for the final board, and didn't want to do two purchases.

I started following the schematic on the postmarketOS wiki, since it was based on a schematic from Google.

For the audio jack I used an old earphone I had around, and cut it so that I ended up with only the cable with the wires exposed and the connector. I followed the same resistor values on the schematic, using a 1kΩ and two 100Ω resistors in series to get the 1.2kΩ for the voltage divider on the TX pin.

To bridge UART to USB I used the Adafruit FT232H breakout I had. Since it didn't have a 3.3V output, I tried some combination of resistors to divide the 5V. That got me to 3.26V but it still didn't work.

I was almost giving up, but remembered that Brian Masney wrote in that page that it really needs to be at 3.3V, and I also remembered I had an SPI to USB converter board that had a 3.3V regulated output. I decided to try it out.

And after only a minor hiccup, with having to change the baud rate to 115200, what do you know, it worked! 🥳

This is how the prototype looked:

I know, it looks horrible, but hey, it works!

Having got it to work I ordered some more parts to make the circuit neater.

Final board

The components I used to build the board were:

• wires
• 1 universal PCB (at least 9x7 holes)
• 1 1kΩ resistor
• 1 1.2kΩ resistor
• 1 1x7 male pin header
• 1 TRRS audio jack cable (from an old earphone) Note: it really needs to be a TRRS, that is, to have 4 separate conductors.
• 1 UART <-> USB converter (from a store in Brazil, but any with 3.3V is fine)

For reference, this is how the pins should be connected (but you really should refer to the schematic at postmarketOS):

I invested a little time thinking how was the most neat way I could solder the connections and resistors to the board, and I ended up with this:

And with the connections on the back side and corresponding pins shown:

My PCB was bigger than the converter board so I sawed it to have approximately the same size (17x7), but the connections only take up a 9x7.

I also changed the jumper position on the UART to USB converter board so that VCC was 3.3V.

Finally I soldered the male pin headers to the converter board, and that on top of my PCB, ending up with a pretty cool 2-story board 😎.

This is how the final board looks:

Testing

After connecting the board's USB to my computer and the audio jack to the Nexus 5, I opened a serial console using picocom with a 115200 baud rate:

picocom /dev/ttyUSB0 -b 115200

And booted the Nexus 5 into fastboot mode by holding the power and volume down buttons, to be greeted with this heartwarming message:

welcome to hammerhead bootloader
[10] Power on reason 80
[10] DDR: hynix
[110] Loaded IMGDATA at 0x11000000
[110] Display Init: Start
[190] MDP GDSC already enabled
[190] bpp 24
[230] Config MIPI_CMD_PANEL.
[230] display panel: ORISE
[230] display panel: Default setting
[360] Turn on MIPI_CMD_PANEL.
[410] Display Init: Done
[410] cable type from shared memory: 8
[410] vibe
[610] USB init ept @ 0xf96b000
[630] secured device: 1
[630] fastboot_init()
[680] splash: fastboot_op
 FASTBOOT MODE
 PRODUCT_NAME - hammerhead
 VARIANT - hammerhead D821(H) 16GB
 HW VERSION - rev_11
 BOOTLOADER VERSION - HHZ20h
 BASEBAND VERSION - M8974A-2.0.50.2.30
 CARRIER INFO - None
 SERIAL NUMBER - ***
 SIGNING - production
 SECURE BOOT - enabled
 LOCK STATE - unlocked
[790] splash: start
[1820] Fastboot mode started
[1820] udc_start()

Since I verified that the pinout on nexus-5-upstream was wrong, I sent a pull request fixing it, so there's no such confusion anymore 😉.

Great! Now that I can read all boot logs I'm ready to dive into the kernel code. Although I'm sure this was the easiest part of the project 😅...