UART over USB for STM32 Micro-controller
I'm trying to implement UART over a USB interface on the STM324x9I-EVAL development board. The purpose is to send commands to a servo controller (or other hardware, for that matter) serially. I've successfully implemented the USB_Device_CDC example on the development board but am unsure exactly how this works without a PC with drivers on the other end. As far as other hardware is concerned, will the USB port now simply look like a serial port? Or is there still a need for a driver or some sort of interface on the other end?
I do want to point out that I'm aware of the following post:
but I don't believe my question is fully answered within the context of that answer.
Answer
A USB connection is not a peer-to-peer connection like a UART. It requires a host and a device in a master/slave relationship. The device cannot initiate data transfer; it must be continuously polled by the by the host.
A CDC/ACM class device presents a virtual COM port on a PC host, but that does not allow the device to communicate with a UART interface. It looks like a serial port at the software level, but does not implement a UART physical layer. There is an awful lot going on under the hood to make it look like a PC serial port, but none of it resembles UART communications at the physical level.
There are devices that act as UART/USB bridges (from FTDI and Prolific for example), and you could (somewhat expensively) build your own from a microcontroller that has a USB device controller and a UART, but the bridge is a USB device and must still connect to a USB host; these are normally used to connect a PC to a microcontroller that lacks a USB controller or where the software/CPU overhead of using a USB controller is too great.
In theory you could connect a microcontroller that has a USB host controller to one that has a USB device controller, but you need host and device software stacks on each respectively, and once you have the USB connection, implementing CDC/ACM is a somewhat inefficient use of the available bandwidth. The purpose of the CDC/ACM class is primarily to allow "legacy" software to work on a PC.
If you need to connect to a "real" serial port, you should use a real UART - which are far more ubiquitous than USB controllers on microcontrollers in any case.