Emulating UART over USB
Does anybody know if it's possible to emulate UART (simple serial transmit and receive) over USB? How would this be accomplished?
I found this link on the Microchip website, but it's not very forthcoming.
http://www.microchip.com/forums/m522571-print.aspx
Any ideas? Thanks.
Answer
You need to implement the device stack as a CDC ACM device (also known as Virtual COM port or VCP). Most vendors of microcontrollers with USB support have example code or app notes.
Given that, your device will look like a COM port as far as Windows is concerned. At the device end, you will get raw blocks of data transferred. An appropriate abstraction layer can be implemented for both UART and USB interfaces to give then the same interface if necessary.
One gotcha is that USB devices require a Vendor ID allocated by the USB Implementer's Forum, at a $5000 fee(correct 23 JUly 2016). If you are going to release your device in the wild, you really will need one if your device is to be recognised and behave correctly with other devices. Some microcontroller vendors will allow you to use their vendor ID for a subset of product IDs for free or a smaller fee, but they might only do that if you were purchasing significant quantities of devices from them.
Another issue is that while on OSX or Linux a CDC/ACM is recognised without any additional drivers, Windows is more fussy and required an INF file to associate the specific USB Vendor and Product ID to the usbser.sys driver. Then you get into the whole world of driver signing, which is essential if using Windows Vista 64, or any version of Windows 7. A code-signing signature will also cost you money. If your vendor has provided example VCP code, they will also probably provide a signed driver. STMicroelectronios's STM32 VCP example is even WHQL certified so can be acquired automatically via Windows Update.
So the upshot is that for experimentation you can do it if your vendor already provides code and a signed driver (or you are not using Windows), but to deploy a product you will need an Vendor ID and a code-signing certificate. It is a bit of a minefield to be honest.
A simpler approach is to use an FTDI USB<->Serial chip. This is especially useful for a microcontroller without a USB controller of its own, but the data transfer rate will be limited by the micro's and/or the FTDI's UART interface rather than USB speed. An FTDI chip can be used as-is using FTDI's VID/PID or you can customise it with your own VID/PID. Customising puts you back into needing to acquire a VID and a signing certificate, but allows your device to be identified uniquely rather than as a generic serial port.