WCH CH9325

WCH, also known as Nanjing Qin Heng Electronics, makes a series of microcontrollers, PCI interface chips, and USB interface chips.

The CH9325 is a USB interface chip, which converts data received via UART to USB and vice versa. The chip presents a HID (Human Interface Device) USB profile, in an effort to not require USB driver installation: this is typically handled by the OS kernel. An example of the profile, in Linux 'lsusb' output format, is here.

The chip runs off of a 12MHz oscillator.

Pin layout

	1-	O	-16
UART TX	2-		-15	USB +5V
UART RX	3-		-14
	4-		-13
	5-		-12
USB D+	6-		-11
USB D-	7-		-10	OSC+
GND	8-		-9	OSC-

Protocol

The chip stores bytes received via the UART RX pin into a buffer. It transfers the contents of the buffer to a HID packet at regular intervals (maximum 12ms). The HID packets are always 8 bytes long, with the first byte containing the number of payload bytes. This is encoded as 0xf0 + length. The next 7 bytes contain the payload, padded with zeroes. Thus, even if no data is ever received on the UART, the chip always sends at least one packet every 12ms containing:

f0 00 00 00 00 00 00 00

Here's a packet with 2 bytes payload:

f2 35 41 00 00 00 00 00

Extracting the UART stream on the other side of the USB bus simply entails the reverse process: for every received packet, extract the payload according to the length byte (nibble).