Answer: The CH340B is the SOP-16 CH340 with a built-in clock and a built-in EEPROM. The EEPROM holds the VID (default 1A86h), PID (default 7523h), an 8-character serial number and a product string, so several boards on one PC can be told apart. Its pinout differs from CH340G on three pins: pin 7 is RST#, pin 8 is NC and pin 15 is TNOW. (WCH CH340DS1 V3D, pp.2–9.)
Key specifications
| Parameter | Value | Condition | Source |
|---|---|---|---|
| Package | SOP-16, 3.9 mm body, 1.27 mm pitch | — | §3, p.2 |
| Pin 7 / 8 / 15 | RST# (active low, internal pull-up) / NC (leave open) / TNOW | — | Pin table, pp.3–4 |
| EEPROM defaults | VID 1A86h, PID 7523h, MaxPower 31h, SN “12345678” | Config area 00h–3Fh | §5.2, p.6 |
| Serial number | 8 ASCII characters (bytes 10h–17h) | Disabled if first byte is not 21h–7Fh | §5.2, p.6 |
| Supply, 5 V mode | 4.0 / 5 / 5.3 V; 6 typ / 15 max mA | V3 to 0.1 µF | §6.2, p.8 |
| Supply, 3.3 V mode | 3.1 / 3.3 / 3.6 V; 3 typ / 9 max mA | V3 tied to VCC | §6.3, p.9 |
| USB suspend current | 0.09 typ / 0.2 max mA (5 V) | — | §6.2, p.8 |
| TX baud-rate error | < 1.8 % | — | §5.4, p.7 |
| Internal clock error | ±1.2 % (−5 to 55 °C); ±1.8 % (−25 to 75 °C) | Max | §6.4, p.9 |
| Ambient temperature | −20 to 70 °C | Operating | §6.1, p.8 |
Pins that differ from CH340G
| Pin | CH340G | CH340B |
|---|---|---|
| 7 | XI | RST#: external reset input, active low, built-in pull-up |
| 8 | XO | NC: leave unconnected |
| 15 | R232 | TNOW: high while transmitting |
All other pins (GND, TXD, RXD, V3, UD+/UD−, CTS#, DSR#, RI#, DCD#, DTR#, RTS#, VCC) are in the same places.
EEPROM configuration area (§5.2)
| Bytes | Field | Default | Notes |
|---|---|---|---|
| 00h | SIG | 00h | Must be 5Bh for the internal configuration to be used |
| 01h | MODE | 23h | Must be 23h |
| 02h | CFG | FEh | Bit 5 = 0 enables the serial-number string |
| 03h | WP | 00h | 57h makes the area read-only |
| 04h–05h | VID | 1A86h | 0000h or FFFFh = use the vendor defaults |
| 06h–07h | PID | 7523h | — |
| 0Ah | PWR | 31h | Max power in 2 mA steps (31h = 49 × 2 = 98 mA) |
| 10h–17h | SN | 12345678 | 8 ASCII characters |
| 1Ah–3Fh | PROD | — | Unicode product string, length byte ≤ 26h, second byte 03h |
WCH writes these “through special computer tool software”; the datasheet does not name the tool.
Why choose CH340B
- Telling boards apart: with unique serial numbers, a host can map each adapter to a stable port name instead of whichever COM number it gets.
- Lowest current in the family: 6 mA typical and 15 mA maximum at 5 V, against 7 and 20 mA for the others (§6.2).
- Board-level reset: RST# lets a supervisor or button reset the bridge without unplugging it.
Pitfalls specific to CH340B
- Swapping CH340B for CH340G pin for pin. Pin 7 is RST#, so remove the crystal and caps from it. Pin 15 is now the TNOW output, so do not tie it to a fixed level as an R232 strap might.
- Setting WP to 57h before the data is final. The area becomes read-only.
- Forgetting SIG = 5Bh. Without it the chip ignores the configuration and uses its defaults.
- Assuming the default serial number is unique. Every unprogrammed part reports “12345678”.
Part numbers (information only)
CH340B (SOP-16). WCH lists no packing suffix.
FAQ
- What is pin 7 on CH340B? RST#, an active-low external reset input with a built-in pull-up.
- Can I change the USB VID and PID on CH340B? Yes. They are in the EEPROM area at bytes 04h–07h (defaults 1A86h / 7523h).
- Does CH340B need a crystal? No, it has a built-in clock.
- How do I make the CH340B configuration read-only? Write 57h to the WP byte at 03h.
- Is CH340B pin-compatible with CH340G? Same SOP-16 footprint, but pins 7, 8 and 15 differ.
Related (internal links)
Sources (retrieved 2026-10-26)
- WCH, USB to Serial Port chip CH340 Datasheet (CH340DS1), Version 3D: PDF https://www.wch-ic.com/download/file?id=79 ; download page https://www.wch-ic.com/downloads/CH340DS1_PDF.html (page numbers are the printed page footers).
Disclaimer
Values are copied from WCH CH340DS1 Version 3D, read on 26 Oct 2026. We have not programmed a CH340B EEPROM ourselves; follow WCH’s tool and instructions.
Reviewed by: Jasper · Updated: 2026-10-26