USB stack confusion/serial emulator

Question

I am looking at implementing USB communication on a MCU which has a USB engine built into it. Basically you have access to the pipes/endpoints.

I am a little bit confused on the USB stack now. It appears that drivers operate on another level above the pipe/endpoint setup, so the pipe/endpoint is like a middle level layer that drivers are built on. Is this correct?

Secondly, I am interested in simulating serial communication over USB. It appears windows has a premade driver so on the computer side I do not need to program the pipe level.

How do I find out what I need to implement on the MCU to make it behave correctly with the generic serial driver?

Answer 1

This is an answer to your second question regarding the serial communication.

The USB standard defines a communication device class (CDC) for serial communication. The required drivers on the host side are implemented by Windows, macOS, Linux and many more operation systems.

The relevant CDC subclass is PSTN. The relevant documents are found in Class definition for Communication Devices 1.2.

The device basically implements four endpoints:

• Control endpoint for configuration requests (baud rate, DTR state etc.). Have a look at SetLineCodeing, GetLineCoding and SetControlLineState.
• Bulk endpoint for USB to serial transmission
• Bulk endpoint for serial to USB transmission
• Interrupt endpoint for notifications (DCD state, errors). See SerialState.

And of course you need to get the device descriptor right.

On top of that, you need to implement all the standard USB requests.

Chances are high that this has already been written for your MCU, both the standard requests and the serial communication. So why not use the existing code?