3.1.4 Signaling

During the 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 back the baud rate and other UART parameters of the CDC, but data sending and receiving will not be enabled.

When a terminal connects on the host, it will assert the DTR signal. This is a virtual control signal that is implemented on the USB interface, but not in the hardware on the debugger. Asserting DTR from the host will indicate to the debugger that a CDC session is active, will enable its level shifters (if available), and will start the CDC data send and receive mechanisms.

Deasserting the DTR signal will not disable the level shifters, but it will disable the receiver, so no further data will be streamed to the host. Data packets that are already queued up for sending to the target will continue to be sent out, but no further data will be accepted.