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

Deasserting the DTR signal will not disable the level shifters but 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.

Remember: Enable to set up your terminal emulator to assert the DTR signal. Without it, the on-board debugger will not send or receive any data through its UART.