•
Parity
: Can be odd, even, or none.
•
Hardware flow control
: Not supported.
•
Stop bits
: One or two bits are supported.
4.1.2.4
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. As this is a virtual control signal implemented
on the USB interface, it is not physically present on the board. Asserting the DTR signal from the host will indicate to
the on-board debugger that a CDC session is active. The debugger will then 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 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.
Remember:
Set up the terminal emulator to assert the DTR signal. Without the signal, the on-board
debugger will not send or receive any data through its UART.
Tip:
The on-board debugger’s CDC TX pin will not be driven until the CDC interface is enabled by the
host computer. Also, there are no external pull-up resistors on the CDC lines connecting the debugger and
the target, which means that during power-up, these lines are floating. To avoid any glitches resulting in
unpredictable behavior like framing errors, the target device should enable the internal pull-up resistor on
the pin connected to the debugger’s CDC TX pin.
4.1.2.5
Advanced Use
CDC Override Mode
In normal operation, the on-board debugger is a true UART bridge between the host and the device. However, in
certain use cases, the on-board debugger can override the basic operating mode and use the CDC TX and RX pins
for other purposes.
Dropping a text file into the on-board debugger’s mass storage drive can be used to send characters out of the
debugger’s CDC TX pin. The filename and extension are trivial, but the text file must start with the characters:
CMD:SEND_UART=
The maximum message length is 50 characters – all remaining data in the frame are ignored.
The default baud rate used in this mode is 9600 bps, but if the CDC is already active or has been configured, the
previously used baud rate still applies.
USB-Level Framing Considerations
Sending data from the host to the CDC can be done byte-wise or in blocks, which will be chunked into 64-byte USB
frames. Each such frame will be queued up for sending to the debugger’s CDC TX pin. Transferring a small amount
of data per frame can be inefficient, particularly at low baud rates, because the on-board debugger buffers frames
and not bytes. A maximum of four 64-byte frames can be active at any time. The on-board debugger will throttle the
incoming frames accordingly. Sending full 64-byte frames containing data is the most efficient method.
When receiving data on the debugger’s CDC RX pin, the on-board debugger will queue up the incoming bytes into
64-byte frames, which are sent to the USB queue for transmission to the host when they are full. Incomplete frames
are also pushed to the USB queue at approximately 100 ms intervals, triggered by USB start-of-frame tokens. Up to
eight 64-byte frames can be active at any time.
AVR-IoT Wx Hardware User Guide
Hardware User Guide
©
2020 Microchip Technology Inc.
User Guide
DS50002805B-page 11
