CH340G USB to serial chip: pinout, the 12 MHz crystal, 5 V or 3.3 V wiring, and the CH340C

The CH340 is a family. The CH340G and CH340C are the two in SOP-16 that you find on Arduino Nano and Uno clones, ESP8266 and ESP32 boards and USB to TTL adapters. WCH also makes the CH340B (SOP-16, internal clock and EEPROM), the CH340N (SOP-8), the CH340K (ESSOP-10), the CH340E and CH340X (MSOP-10) and the CH340T (SSOP-20); their pin numbers differ, so check the marking before you follow a pinout.

On a board, a CH340G always has a 12 MHz crystal, usually a silver HC-49 can, right next to it. No crystal means a CH340C or one of the other clock-free versions. All of them use the same CH341 driver on the computer side.

The CH340C has a built-in clock. Its footprint and pinout match the CH340G except pins 7 and 8: pin 7 is an empty pin that must be left open, and pin 8 is OUT#, a modem output. On lots whose number starts with 4 and ends above B40, a 4.7 kΩ pull-down on pin 8 turns it into a second DTR# for boards that need both a reset and a boot line.

The internal clock is looser. WCH puts the CH340C's transmit baud error below 1.8 %, against 0.3 % for the crystal-driven CH340G, and rates it from −20 to 70 °C instead of −40 to 85 °C. The CH340C also draws less in USB suspend.

The supply may go to 6.0 V absolute, and no pin may sit more than 0.5 V above VCC. That second limit is the one that bites: a CH340 running from 3.3 V must not see 5 V on RXD or on any modem input, except on the newer lots of the CH340C, CH340N and CH340X that WCH calls 5 V tolerant.

In 5 V operation VCC runs from 4.0 to 5.3 V, which covers USB's 5 V bus. In 3.3 V operation the CH340G accepts 2.9 to 3.6 V and the CH340C 3.1 to 3.6 V. Design to the CH340C's floor if the board might be built with either.

V3 is the chip's internal 3.3 V node. From 5 V the chip makes that node itself and V3 only needs its 100 nF capacitor; from 3.3 V the supply already is 3.3 V, so V3 is joined to VCC. Either way VCC gets its own 100 nF at the pin.

It is a light load: 7 mA typical and 20 mA at most from 5 V (100 mW worst case), 4 mA and 12 mA from 3.3 V (39.6 mW). The outputs guarantee VCC − 0.6 V high and 0.5 V low, and an input reads high above 2.3 V on 5 V or 1.9 V on 3.3 V.

The clock decides the baud rate. A UART receiver samples each bit near its middle, and WCH says the CH340 receiver copes with about 2 % of rate error. The crystal-driven CH340G transmits within 0.3 %, leaving 1.7 points for the other end; the CH340C's 1.8 % leaves only 0.2.

• Programming and talking to a microcontroller over USB: Arduino clones, ESP8266 and ESP32 boards, STM32 with a serial bootloader.
• A USB to TTL serial adapter for a GPS module, a Bluetooth module or a router's debug header.
• USB to RS232 or RS485, with a MAX232-type level shifter or an RS485 transceiver after it (WCH's sheet shows both).
• Automatic reset and boot-mode entry: DTR# and RTS# are outputs the computer controls, so a download tool can reset a board and put it in bootloader mode with no buttons.

It is a poor choice above about 1 Mbps one way or 500 kbps both ways: WCH itself points those jobs to its CH343, which has hardware flow control. Its USB is full speed only.

A home-made programming board for an ESP32 module: USB in, a CH340G, and the ESP32's UART0. The ESP32's pins are 3.3 V only (3.6 V at most on an input), so the simplest plan is to run the CH340G from the same 3.3 V rail.

Many boards power the CH340G straight from USB's 5 V. Then its TXD high is at least 4.4 V and can reach 5.3 V, which is 0.8 V or more over the ESP32's 3.6 V limit. The other direction is fine: the ESP32's 2.64 V high clears the CH340G's 5 V threshold of 2.3 V by 0.34 V.

A 10 kΩ / 18 kΩ divider on TXD brings the high down to 2.83 V to 3.41 V, inside the ESP32's window. With 12 pF of pin and jumper capacitance the edge rises in 170 ns, against a half bit of 250 ns at the CH340's top rate of 2 Mbps.

The divider works, but running the CH340G on 3.3 V does away with it and its two resistors. The voltage divider lesson explains the arithmetic.

• Cross the data lines: CH340 TXD goes to the microcontroller's RX, and RXD to its TX.
• Put the 100 nF on VCC and, on 5 V, the one on V3 right at the pins.
• If USB keeps connecting and dropping, look at the crystal first: WCH suggests halving the capacitor on XI for a crystal that is slow to start, and the crystal maker's value (often 47 pF) for a ceramic resonator.
• When the board has its own supply, power the CH340 from it too. Two supplies let current flow through the I/O pins into whichever side is off; WCH's Section 7.7 shows the diode and MOSFET fix if you cannot avoid it.
• Unused modem pins can be left open. R232 has a pull-down, so leave it open for TTL levels.

The logic levels lesson explains VIH, VOH and margins, and crystals and resonators covers load capacitors.

The CH340C is the only drop-in, and only once the crystal is gone. The other CH340 versions are different packages with different pin numbers.

1. Plug it in and check the computer sees a new serial port (a COM port on Windows, /dev/ttyUSB0 on Linux). Nothing at all usually means a missing driver, a crystal problem or V3 wired wrong.
2. Measure VCC to GND: 4.0 to 5.3 V or 2.9 to 3.6 V. On 5 V, V3 should read about 3.3 V.
3. With nothing sending, TXD (pin 2) should sit high: at least VCC − 0.6 V. Only the discontinued CH340R idles low, so a low TXD on a CH340G or CH340C points to a short on that line or a damaged chip.
4. Loop back: join TXD to RXD, open a terminal at 9600 baud and type. Each character should come straight back. If it does, the chip and driver work, and any fault is in the wiring to the target.

For a 3.3 V microcontroller, run the CH340G from the same 3.3 V rail with V3 tied to VCC and wire TXD and RXD straight across: every level then has at least 0.23 V of margin and no divider is needed. Keep a 5 V CH340G only on 5 V boards, such as an ATmega328P. Choose the CH340C when board space or parts count matters more than baud accuracy, and stick to the CH340G with its crystal if the far end runs on an internal RC clock.

Indian sellers stock the bare CH340G and CH340C chips and, far more often, ready-made USB to TTL adapter boards built on them. On an adapter, look for a 3.3 V / 5 V jumper and check which pin it actually moves: some only switch the supply pin on the header, not the chip's VCC. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.