We have an example firmware in FX3 SDK. USBFlashProg- after building and programming the device, you can send vendor command to read and write from SPI/I2C. The vendor command for each of these can be found in the firmware itself. Here, the data is coming to the FX3-CPU, which is then sending the data to the USB endpoint. I suppose this is exactly what you are looking for.
You have mentioned, that you are looking the device to come-up as USB-UART Bridge. Can you clarify, why is this required? Do you want a CDC interface, or do you want a Vendor interface?
Have you looked for CY7C65211A:
This is a configurable part, where you can configure the device to come up as CDC class, and the device can be configured for I2C/SPI. This works as a USB to serial (I2C/SPI) bridge.
The purpose of doing this is to implement audio IN using CDC class. CX3 will be registered as USB-UART bridge which is working. I have implemented a CPLD logic to convert the external I2S audio data to CX3 SPI. Now I want to send the audio data over USB serial/COM port. I have a application that will read serial audio data.
What would be the easiest way to do this? What are the parameters I need to tweak/consider in order to get this running?
For USB-SPI channel, you need to create DMA channel similar to UsbSpiDmaMode example in the FX3 SDK. Along-with this, you will have to use the descriptors similar to UAB-UART example. The USB-UART makes the device to enumerate as a COM device on the host. The UsbSpiDmaMode is responsible for the data transfer from SPI to the USB endpoint. Please try and let us know if you face any problem.