USB-to-Serial adapters

USB-to-Serial Adapters

USB-to-Serial adapters' (often called “USB-UART” or “USB-TTL” adapters) provide a bridge between a host computer’s USB port and the UART (Universal Asynchronous Receiver/Transmitter) interface on an embedded hardware. They are indispensable tools for firmware flashing, console access, bootloader communication, and low-level hardware debugging.

Overview

USB-to-Serial adapters all share the core function of converting USB packets to asynchronous serial data, but differ by:

• **Signal Standard**: RS-232 vs TTL-level UART
• **Voltage Levels**: 3.3V, 5V, adjustable (1.8V, 2.5V)
• **Chipset**: FTDI (FT232R/FT231X), Prolific (PL2303), Silicon Labs (CP2102/CP2104), WCH (CH340G/CH341A), etc.
• **Form Factor & Connectors**: 6-pin 0.1″ header, micro-USB, USB-C, DB9 shell
• **Additional Lines**: RTS/CTS, DTR, DSR, RI, DCD for hardware flow control and reset signals

Key Components

USB-UART Bridge ICs

Chipset	Vendor	USB <> UART	Voltage(s)	Flow-Control	Notes
FT232R	FTDI	Full-Speed USB 2.0	1.8–5.0 V	RTS, CTS, DTR, DSR	Gold-standard, EEPROM-configurable
CP2102/CQ1	Silicon Labs	Full-Speed USB 2.0	3.3 V, 5 V	RTS, CTS	Low-cost, integrated EEPROM
PL2303HX	Prolific	Full-Speed USB 2.0	5 V	RTS, CTS	Widely cloned; watch for fake chips
CH340G	WCH	Full-Speed USB 2.0	3.3 V, 5 V	RTS, CTS	Very cheap; extremely reliable with good support
CP2104	Silicon Labs	Full-Speed USB 2.0	1.8–5.0 V	RTS, CTS, DTR	Newer silicon with lower power

TTL vs RS-232

• **TTL-Level UART**: Logic-level serial at 0 V–V_CC (3.3 V or 5 V). Used on microcontrollers and SoCs.
• **RS-232**: ±3 V to ±15 V swings with inverted logic. Requires level-shifter (MAX232) beyond TTL.

TTL Signal	Pin	Description
VCC	VCC	Adapter power output (3.3 V/5 V selectable)
GND	GND	Ground reference
TXD	TxD	Transmit data (adapter⇒target)
RXD	RxD	Receive data (target⇒adapter)
RTS	RTS	Request to Send (toggle for hardware flow)
CTS	CTS	Clear to Send

Connectors & Cabling

• **0.1″ 6-Pin Header**: Typical breakout boards; pin order varies—check silkscreen (“GND, CTS, VCC, TXD, RXD, RTS” common).
• **DB9 Male/Female**: Full-sized RS-232 adapters; DB9 pinout per TIA/EIA-232-F standard.
• **Inline Cables**: Must verify pin-color mapping (e.g. black=GND, red=VCC, orange=TX, yellow=RX).

Drivers & Host Configuration

Most USB-UART ICs require vendor drivers:

• **FTDI**: Native on Windows/macOS/Linux but may need VCP drivers from FTDI website.
• **CP210x**: Silicon Labs provides VCP drivers; Linux kernel has built-in support (`cp210x`).
• **CH340**: Linux requires `ch341` module; Windows installer from WCH.
• **PL2303**: Windows driver from Prolific; beware counterfeit devices with wrong PID.

After installation, adapters appear as:

• **Windows**: COMx (e.g. `COM3`, `COM4`)
• **Linux/macOS**: `/dev/ttyUSBx`, `/dev/tty.SLAB_USBtoUART`, `/dev/cu.usbserial-XXXXX`

Practical Usage

Console & Bootloader Access

1. **Connect GND first** to establish common reference. 2. **Wire TX↔RX** (cross): adapter TX ⇒ device RX; adapter RX ⇒ device TX. 3. **Power**: Some boards can be powered via adapter VCC, but prefer separate regulated supply. 4. **Open Terminal**: `screen /dev/ttyUSB0 115200,8N1` or `minicom -D /dev/ttyUSB0 -b 115200`. 5. **Reset Sequences**: Use DTR/RTS toggles for automatic reset into bootloader (e.g. Arduino auto-upload).

Firmware Flashing & Modding

• **UART Bootloaders**: Many MCUs (STM32, ESP32) include UART ROM bootloader—hold BOOT0, toggle reset.
• **Protocol Tools**: `stm32flash`, `esptool.py`, `flashrom` over serial protocols.
• **Voltage Translation**: When target uses 1.8 V logic, add a bidirectional level shifter (e.g. TXB0108).

Troubleshooting & Advanced Hacks

• **Baud Rate Mismatch**: Check both sides match—common rates: 9600, 115200, 921600 bps.
• **Flow-Control Issues**: Disable hardware flow in software if CTS/RTS not used or miswired.
• **Buffer Latency**: FT232 chips allow latency timer tuning via EEPROM (`ftdi_eeprom` on Linux).
• **Counterfeit Detection**: Genuine FTDI chips enumerate as “FT232R USB UART”; fakes may require VCP patch.
• **Solder-Mod Kits**: Add solder-jumper pads to switch voltage level or enable 1.8 V rail.
• **Signal Integrity**: Keep wiring <10 cm; add series 27 Ω resistors on TX lines to reduce overshoot.

Best Practices

• Always verify **GND continuity** before powering.
• Use **ESD protection** when probing exposed pads.
• Avoid back-feeding USB port power—disconnect VCC if powering target separately.
• Label and document pin mapping on custom adapters.

References & Further Reading

• FTDI Application Notes (AN_230, AN_232R-05)
• Silicon Labs CP210x Data Sheet
• WCH CH340 Datasheet
• “Serial Port Complete” by Jan Axelson