Nexus 5 UART Cable
A serial console can be obtained from the phone through the headphone jack and requires building a custom cable that converts between the different voltage levels found on the PC and the phone.
Parts list
- USB to TTL Serial Cable
- SparkFun Solder-able Breadboard - Mini
- SparkFun TRRS 3.5mm Jack Breakout
- Audio Cable TRRS - 1ft
- 3 x 1K resistors to make a voltage divider to convert the 3.3V TX output from the USB serial cable to 1.8V for the RX pin on the phone.
- 3.3V 250mA Linear Voltage Regulator - L4931-3.3 TO-92 to convert the 5-5.2V from the PC to a fixed 3.3V.
- 2 x 10 uF capacitors for the input and output on the voltage regulator.
Pin mapping and voltages
| USB | Headphone | |
|---|---|---|
| RX | TIP (TX 1.8V) | |
| TX (3.3V) | Voltage Divider | Ring 1 (RX 1.8V) |
| GND | Ring 2 | |
| 5V | Voltage Regulator | Sleeve (3.3V) |
Version 1 of my cable used a voltage divider for the 3.3V on the sleeve of the headphone jack, however I moved to the 3.3V voltage regulator because the USB-C on my laptop puts out 5.2V, and the USB-A puts out 5.0V. The phone expects close to 3.3V on the sleeve pin in order to enable the serial console on startup and this slight voltage difference was enough to cause the serial console to not work.
Booting the phone
With the phone off, plug the cable in, and start the phone with the volume-down and power buttons pressed simultaneously to boot into the phone’s bootloader. The following messages will come through over the serial cable:
welcome to hammerhead bootloader
[10] Power on reason 10
[10] DDR: elpida
[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 D820(E) 16GB
HW VERSION - rev_11
BOOTLOADER VERSION - HHZ20h
BASEBAND VERSION - M8974A-2.0.50.2.29
CARRIER INFO - None
SERIAL NUMBER - XXX
SIGNING - production
SECURE BOOT - enabled
LOCK STATE - unlocked
[790] splash: start
[1820] Fastboot mode started
[1820] udc_start()
[2190] -- reset --
[2190] -- portchange --
[2270] -- reset --
[2270] -- portchange --
[2360] fastboot: processing commands
You can use the minicom package to access the serial console. The baud rate should be 115200. Here’s my ~/.minirc.dfl file:
pu port /dev/ttyUSB0
pu rtscts No