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You could try new csp 0.1.2, have fix this bug, thank you!
Add ( source "$BSP_DIR/drivers/Kconfig" ) in the project folder's Kconfig to load driver folder's Kc
I have submit csp for samd21 serials, you could try it now.
You should open the setting, un-click & click the Enable SERCOM3 USART checkbox. Save setting, it wi