USB Mode Protocol #
To communicate between the USB controller and the UART on the FPGA we use a small protocol to indicate the USB endpoint on the client side. There are three different endpoints:
- QEMU (0x02).
- CH552 (0x04).
- CDC (0x08): USB CDC-ACM, a serial port on the client.
- FIDO (0x10): A USB HID security token device, useful for FIDO-type applications.
- CCID (0x20): CCID “smart card”.
- DEBUG (0x40): A USB HID special debug pipe. Useful for debug prints.
A small protocol header should always come first in every frame:
| Name | Size | Comment |
|---|---|---|
| Endpoint | 1B | Origin or destination USB endpoint |
| Length | 1B | Number of bytes following |
| Payload | See above | Actual data from or to app |
Note well: This protocol is not visible on the client.