Difference between revisions of "WCH CH9325"
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== Protocol == | == Protocol == | ||
The chip | 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). | |||
Revision as of 21:43, 2 August 2013
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).