28.3.2.3 GNAK - Global Not Acknowledge

Writing a ‘1’ to the Global NAK (GNAK) bit in the Control B (USB.CTRLB) register will return a NAK response on all transactions for non-isochronous endpoints. Any transaction will be ignored for isochronous endpoints.

The application can write the GNAK bit, or it can be set by hardware when a SETUP packet is received, if the Automatic Global NAK (GNAUTO) bit in USB.CTRLB is ‘1’.

After GNAK is set to ‘1’, the GNAK Operation Done Interrupt Flag (GNDONE) in the Interrupt Flags B (USB.INTFLAGSB) register is set while effectuating the global NAK response.