UART Serial Console & Pinouts

When bringing up new Linux kernels or debugging early boot issues where USB networking has not yet initialized, having a physical UART serial console is invaluable.

1.8V Logic Level Warning!

The MSM8916 GPIO and UART lines operate strictly at 1.8V logic level. Connecting a 3.3V or 5V USB-to-UART adapter (like standard FTDI or CH340 modules configured for 3.3V/5V) directly to the board can permanently destroy the SoC’s internal I/O pins. Always use a 1.8V-capable USB-UART adapter or a bidirectional logic level shifter!

---

1. UART Pinouts on Common Boards

MSM8916 exposes hardware UART on GPIO 4 (UART_TX) and GPIO 5 (UART_RX).

   ┌──────────────────────────────────────────────┐
   │ [USB Plug]                                   │
   │                                              │
   │     ┌────────────┐        [O] TX (1.8V)      │
   │     │  MSM8916   │        [O] RX (1.8V)      │
   │     │    SoC     │        [O] GND            │
   │     └────────────┘                           │
   │                                              │
   └──────────────────────────────────────────────┘

• TX (Transmit): Connects to your USB adapter’s RX pin.
• RX (Receive): Connects to your USB adapter’s TX pin.
• GND (Ground): Connects to your USB adapter’s GND (you can solder to the metal casing/shield).

---

2. Recommended Hardware

1. 1.8V USB-to-UART Adapter:
   • FT232RL or FT2232H with 1.8V VCCIO jumper.
   • CH343G / CP2102N with 1.8V voltage regulator.
2. Fine-gauge enameled magnet wire (30–36 AWG) and a fine-tip soldering iron.

---

3. Terminal Connection Parameters

Connect your adapter to your PC and launch your preferred serial client:

# Using picocom
picocom -b 115200 /dev/ttyUSB0

# Using minicom
minicom -D /dev/ttyUSB0 -b 115200

# Using screen
screen /dev/ttyUSB0 115200

• Baud Rate: 115200
• Data Bits: 8
• Parity: None
• Stop Bits: 1
• Flow Control: None

---

4. What You Will See

Upon plugging the modem into USB power, you will observe the complete boot sequence:

1. Primary Boot Loader (PBL) initialization.
2. Secondary Boot Loader (sbl1) memory training.
3. TrustZone / KeyMaster (tz) validation.
4. lk2nd bootloader output and countdown timer.
5. Linux kernel dmesg stream (earlycon / console=ttyMSM0).
6. A standard Linux login prompt (openstick login:).