Info:
For all operating systems: Be sure to use a terminal emulator that supports DTR signaling. See
.
3.1.2.3
Limitations
Not all UART features are implemented in the on-board debugger CDC. The constraints are outlined here:
•
Baud rate
: Must be in the range of 1200 bps to 500 kbps. Any baud rate outside this range will be set to the
closest limit, without warning. Baud rate can be changed on-the-fly.
•
Character format
: Only 8-bit characters are supported.
•
Parity
: Can be odd, even, or none.
•
Hardware flow control
: Not supported.
•
Stop bits
: One or two bits are supported.
3.1.2.4
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 DTR in debugger firmware version 1.20 or earlier has the following behaviour:
• Debugger UART receiver is disabled, so no further data will be transferred to the host computer
• Debugger UART transmitter will continue to send data that is queued for sending, but no new data is accepted
from the host computer
• Level shifters (if available) are not disabled, so the debugger CDC TX line remains driven.
Deasserting DTR in debugger firmware version 1.21 or later has the following behaviour:
• Debugger UART receiver is disabled, so no further data will be transferred to the host computer
• Debugger UART transmitter will continue to send data that is queued for sending, but no new data is accepted
from the host computer
• Once the ongoing transmission is complete, level shifters (if available) are disabled so the debugger CDC TX
line will become high-impedence.
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.
ATtiny1627
Curiosity Nano
©
2020 Microchip Technology Inc.
User Guide
DS40002199A-page 9
