3.1.1.3 Signaling

During USB enumeration, the Host OS will start both communication and data pipes of the CDC interface. At this point, it is possible to set and read baud rate and other UART parameters of the CDC, but data sending and receiving will not be enabled.

On the Host PC when a terminal connects, it must assert the DTR signal. This is a virtual control signal that is implemented on the USB interface but not in hardware on the debugger.

Asserting DTR from the Host will indicate to the debugger that a CDC session is active, and it will enable its level shifters (if available) and start the CDC data send and receive mechanisms.

Disserting the DTR signal will not disable the level shifters, but it will disable the receiver. Without a DTR signal, no further data will be streamed to the Host. Data packets that are queued by the debug processor for the target MCU will continue to be sent, but no further data will be accepted.