I often get asked about why the USB Serial doesn’t seem to work on Maple mini or generic STM32F103 boards loaded with the STMduino-bootloader.
If you buy a brand new Maple mini, and plug it into your PC or Mac or Linux box. Do not be suprised if you only see a DFU device. This is normal.
Just compile and upload a blank sketch. Even a blank sketch will contain the Serial USB code, and your board should then appear as a “Maple Serial” device.
There is a common missconception that the STM32duino-bootloader, and the older Maple bootloader, contain both DFU (upload) USB support and also “Maple Serial” (USB Serial) functionality. However this is not the case.
Originally when Leaflabs wrote the Maple bootloader (back in around 2012) they tried to include both DFU and Serial functionality, however they found issues with this under Windows. This is because the DFU and Serial functionality requires the use of 2 completely different (built in) drivers in Windows; when the Maple board is only one USB device.
Normally manufacturers create composite USB devices in this case, i.e one USB driver that performs both the DFU and Serial functions. But creation and certification of custom drivers for Windows is non-trivial and expensive. Hence LeafLabs chose to separate the DFU and Serial USB functionality, so that only the DFU driver is used when the bootloader is running and only the Serial driver is used when the Arduino sketch is running.
This method allows the use of 2 separate Windows drivers – both of which come as standard on all Windows installations.
Although the functionality of both DFU and USB Serial could have been included in the bootloader, it would make the bootloader and sketch core code (libmaple) more complicated, as the sketch would need to communicate with the bootloader in order to send and receive serial data.
So Leaflabs chose to go with the simpler option of the bootloader only containing the DFU functionality, and for the USB Serial to be compiled as part of the sketch. Although this is the simpler option, its probably the best one, because if the Serial USB support was contained in the bootloader, the amount of additional code to allow the sketch to send and receive Serial via the bootloader would make the overall size of the bootloader and sketch slightly larger.
Sketches do take slightly longer to upload than if they did not contain the USB Serial functionality, but uploads via DFU are generally faster than AVR Arduinos, so this isn’t a serious concern.
Another consequence of splitting the functionality, is that if the sketch crashes, then the USB Serial will also crash, and often the USB Serial device will not appear at all.
And… any board that has been freshly installed with a bootloader, will only appear as a DFU device, because the bootloader checks to see if a valid sketch is in the program (flash) memory, and if not the bootloader continues to stay in DFU mode until a valid sketch has been upload.