CH340G Pinout

Per the WCH CH340 datasheet: the "#" suffix means active low. Pins 9-12 are inputs you can leave floating when you don't use hardware flow control — the driver handles the rest.

Pin Functions That Actually Matter

TXD (pin 2) and RXD (pin 3) — the serial pair. TXD transmits to the MCU's RX; RXD receives from the MCU's TX. Cross them — TX to RX, RX to TX. Wiring them straight-through is the most common "why is nothing coming through" mistake.

Note that RXD sits at 5V logic on a 5V-supplied CH340G; a 3.3V MCU's TX needs a divider or level shifter into it, and the CH340G's TXD at 5V needs protection before a 3.3V MCU's RX.

V3 (pin 4) — the mode switch. In 5V operation, decouple the internal 3.3V regulator with a 4.7-20nF capacitor to ground. In 3.3V operation, tie V3 directly to VCC. The 10nF value on most Arduino schematics sits inside the official window.

What you must not do: run 5V VCC and tie V3 to VCC — that shorts the internal regulator output to 5V.

XI (pin 7) and XO (pin 8) — the 12MHz clock. A 12MHz crystal with two ~22pF load caps across XI/XO is the standard circuit. Or feed an external oscillator into XI and leave XO unconnected.

Why does the clock matter so much? Because the CH340G has no internal oscillator — no 12MHz on XI, no enumeration, no COM port.

DTR# (pin 13) and RTS# (pin 14) — the auto-reset pair. On Arduino boards, DTR# goes through a 100nF capacitor to the MCU's RESET. When the host opens the serial port, DTR# pulses low and the capacitor couples the edge into RESET.

RTS# is the second flow-control output — some boards use it for the EN pin on ESP8266 modules. Why does the capacitor matter? Because a direct connection would hold the MCU in reset the whole time the port is open.

R232 (pin 15) — the RS232 mode latch. Asserting it (high) inverts the RXD signal, for use with external RS232 transceivers. It's latched during power-on reset, so it must be set at power-up, not mid-run. Floating it (internal pull-down) is the normal UART-TTL configuration.

Typical Application Circuit

Common Mistakes

1. TXD/RXD wired straight-through. The serial pair is crossed: CH340G TXD goes to the MCU's RX. Straight-through gives you a port that opens fine and transfers nothing.

2. V3 tied to VCC in 5V mode. That shorts the internal 3.3V regulator output to 5V. V3 gets a 4.7-20nF cap in 5V mode; it ties to VCC only in 3.3V mode.

3. No crystal on XI/XO. The CH340G has no internal oscillator. No 12MHz clock = the chip never enumerates, no COM port appears. (This is the main reason the CH340C with its built-in crystal exists.)

4. 5V TXD into a 3.3V MCU's RX. The CH340G runs its serial pins at VCC logic. A 5V TXD into a 3.3V STM32 or ESP32 RX can latch up or damage the pin. Divide it down or use a level shifter.

5. Forgetting the DTR auto-reset capacitor. DTR# must couple through a capacitor (typically 100nF) to RESET. A direct connection holds the MCU in reset while the port is open — "board won't upload, but serial monitor works" is the symptom.

6. Wrong V3 capacitor value. The official window is 4.7-20nF — the 10nF on Arduino schematics is correct. A 100nF there is out of spec, and leaving V3 empty in 5V mode leaves the internal reference noisy.

7. Assuming the 2023+ driver works with clone chips. WCH's newer Windows drivers dropped support for counterfeit CH340G parts. If a board stops enumerating after a driver update, that's the tell — genuine parts keep working. Buy from traceable stock.

Frequently Asked Questions

Q1: Is CH340G pin-compatible with CH340C?

A: Mostly, but the crystal pins differ in practice. The CH340C (SOP-16) has the same footprint but a built-in crystal — its XI/XO pins don't need external components. On a CH340G board, the crystal and caps sit on pins 7/8; a CH340C drops onto the same board and the empty crystal pads are harmless.

Q2: What is the maximum baud rate of the CH340G?

A: 2Mbps (the official spec). Standard rates up to 921600 are rock solid; above that, quality of the crystal and PCB layout start to matter. For production, stick to 115200 or 921600 unless the host and cable are known-good.

Q3: My CH340G stopped showing up after a Windows update — what happened?

A: The 2023 WCH drivers dropped clone-chip support. If your board uses a counterfeit CH340G, the newer driver refuses it. Fixes: install the 2019-era driver, or replace the chip with a genuine one. Genuine CH340G parts keep working with current drivers.

Q4: Why does my 3.3V MCU get hot when connected to a CH340G board?

A: The CH340G's serial pins run at VCC (5V) logic. A 5V TXD into a 3.3V MCU's RX pin can back-feed the MCU's ESD diodes and pull the rail up. Add a divider (2:1) or a level shifter between the CH340G TXD and the 3.3V MCU's RX.

Q5: What are RTS# and DTR# for on an ESP8266 board?

A: Auto-programming. RTS# drives the EN (enable) pin through a capacitor while DTR# drives GPIO0 — the esptool sequence toggles them to enter bootloader mode. Wire them through 100nF caps, and keep the polarity per the board's schematic; getting them swapped is a classic "can't flash the ESP" cause.

Q6: Can I leave the modem pins (CTS#, DSR#, RI#, DCD#) unconnected?

A: Yes. Without hardware flow control, all four float fine — the driver treats them as inactive. Only wire them if you're building an RS232-style modem interface with full flow control.

Q7: How do I tell a genuine CH340G from a clone?

A: Check the marking and the driver behavior. Genuine parts carry a clean WCH logo mark and keep working with the 2023+ drivers; clones typically fail after the driver update. On the bench, a genuine part also enumerates with a stable COM port across reboots — clones sometimes drop after sleep/wake cycles.