3.4.3 Hardware UART via J-Link-CDC

In addition to the J-Link-OB functionality, the ATSAM3U4C microcontroller provides a bridge to a debug serial port (UART DBGU) of the processor on a SOM board. The port is made accessible over the same USB connection used by JTAG by implementing Communication Device Class (CDC), which allows terminal communication with the target device.

This feature is enabled only if microcontroller pin 24 (PIO PA25) is not grounded. The pin is normally pulled high and controlled by jumper J9.

• Jumper J9 not installed: the J-Link-CDC is enabled and fully functional.
• Jumper J9 installed: the J-Link-CDC device is disabled.

The USB Communications Device Class (CDC) enables conversion of the USB device into a serial communication device. The target device running USB-Device CDC is recognized by the host as a serial interface (USB2COM, virtual COM port) without the need to install a special host driver (since the CDC is standard). All PC software using a COM port work without modifications with this virtual COM port. Under Windows®, the device shows up as a COM port; under Linux, as a /dev/ACMx device. This enables the user to use host software which was not designed to be used with USB, such as a terminal program.

The table below describes the pin assignment of USB connector J14.