What is the additional advantage of developing a full UMDF windows driver if a CDC device is detected as a virtual COM port?.
I have some experience with embedded examples using a microcontroller, both communicating with a terminal like teraterm or using a dedicated USB peripheral that allows for cdc or hid functionality.
Are these drivers the ones you download and install before using any USB device? It is not clear yet what features are avaialble through the host driver.
Often on Windows the "driver" for a USB serial device is no more than an inf file that maps the device VID/PID to the Microsoft usbser.sys CDC/ACM driver.
Recent releases of Windows 10 appear to have stopped insisting that each CDC/ACM have a VID/PID specific inf file, and will load the standard driver for any device that presents as a CDC/ACM device.
The advantage of having a VID/PID specific inf file is that your device can have a vendor specific "friendly name" which can be used in applications to more easily identify your device, rather than just appearing as a generic "USB Serial Device".
One problem with Microsoft's usbser.sys driver (and Linux and Mac OS are no different) until recently was that if you disconnect the USB device, the driver is unloaded even if an application has the COM port open, and the application must close and reopen the port to recover when reconnected. I have previously used a custom driver (provided by a third party), that does not unload if an application has the port open, so that when the USB cable is reconnected, the data connection continues as normal, just as it would if it were an RS-232 cable. However, again in recent versions of Windows 10 I have noticed that usbser.sys appears to exhibit this behaviour in any case.
Note that when you do provide your own driver file, or even just a custom inf file, you will be required to go through WHQL testing in order for your device to be allowed on Windows 10, or to load without warnings on earlier versions. To do that you will need a USB.org assigned VID, an Extended Validation code-signing certificate, and either the tools and facilities to perform the testing, or pay a test house to perform the testing. That all gets somewhat expensive, and may be prohibitive for low volume, low value or non commercial products.
At one time Microsoft too charged for WHQL processing submissions, but no longer do so. That is however little combination since at the same time they changed to requiring an EV certificate and stopped a deal the used to have for low-cost certificate.
The advantage of WHQL qualification is your driver will be provided by Windows Update.
If you are using a USB serial bridge chip rather than your own USB stack and USB controller, then there is a lower cost solution. These chips can be customised with your own VID/PID and descriptors, and the vendor's existing WHQL can be associated to your device so you get all the advantages of your own driver without the costs. Most vendors will even allow you to use their VID and will assign you a unique PID so you can avoid USB.org fees. I have used this route with both Prolific and FTDI devices; it is by far the most cost effective solution.
