In addition to the J-Link-OB functionality, the SAM3U4C microcontroller provides a bridge to a debug serial port (DBGU) of the processor on a main 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 the microcontroller pin 25 is not grounded. The pin is normally pulled high and controlled by jumper JP20.
- Jumper JP20 not installed: J-Link-CDC is enabled and fully functional.
- Jumper JP20 installed: J-Link-CDC 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). Any PC software using a COM port works 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, giving access to UART0 on the SAMA5D27.
