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SAMD21: USB Device Descriptors
Try this it worked with little modification on a SAMD11  
Wednesday, 15 November 2017 - 23:38
SAM programming software for Linux
I use a Jlink edu which both 'JLinkExe -device AT91SAMD11D14A -if swd' and OpenOCD work.   To write I use QtCreator with JLinkGDBServer.    
Tuesday, 14 November 2017 - 13:52
SAMD21 Non-ASF/SAM-BA firmware starting place
I started on the SAMD11 by playing with a bare project from the github of Alex  Taradov.  If you get his simple example working, then with the data sheets and further...
Monday, 24 April 2017 - 21:57