USB-to-Serial adapters: Difference between revisions

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


After installation, adapters appear as:
After installation, adapters appear as:
* '''Windows''': COMx (e.g. `COM3`, `COM4`)   
* '''Windows''': COMx (e.g. <code>COM3<code>, <code>COM4<code>)   
* '''Linux/macOS''': `/dev/ttyUSBx`, `/dev/tty.SLAB_USBtoUART`, `/dev/cu.usbserial-XXXXX`
* '''Linux/macOS''': <code>/dev/ttyUSBx<code>, <code>/dev/tty.SLAB_USBtoUART<code>, <code>/dev/cu.usbserial-XXXXX</code>


=== Practical Usage ===
=== Practical Usage ===


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


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


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* '''Baud Rate Mismatch''': Check both sides match—common rates: 9600, 115200, 921600 bps.   
* '''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.   
* '''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).   
* '''Buffer Latency''': FT232 chips allow latency timer tuning via EEPROM (<code>ftdi_eeprom</code> on Linux).   
* '''Counterfeit Detection''': Genuine FTDI chips enumerate as “FT232R USB UART”; fakes may require VCP patch.   
* '''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.   
* '''Solder-Mod Kits''': Add solder-jumper pads to switch voltage level or enable 1.8 V rail.   

Revision as of 12:03, 15 May 2025

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

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 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
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