USB-to-Serial adapters
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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 | Commonly used, 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 |
Signal Levels & Pinouts
TTL vs RS-232
- TTL-Level UART: Logic-level serial at 0 V–VCC (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
ch341module; 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
- Connect GND first to establish common reference.
- Wire TX↔RX (cross): adapter TX ⇒ device RX; adapter RX ⇒ device TX.
- Power: Some boards can be powered via adapter VCC, but prefer separate regulated supply.
- Open Terminal:
screen /dev/ttyUSB0 115200,8N1orminicom -D /dev/ttyUSB0 -b 115200. - 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,flashromover 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_eepromon 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