3.5.4 Debug Solution Selection

If the user does not require the on-board programming feature, this section can be left unpowered, with no impact on the rest of the system. A level shifter has been placed on the DEBUG UART line (U29) between the SAMA7G5 MPU and the on-board programmer (U26) to properly separate the two voltage domains.

Jumper JP5/J26 disables the J-Link-OB JTAG functionality. When installed (J26 shorted), a quad analog switch (U25/U27) routes the JTAG interface of the SAMA7G5 to the 20-pin header J27.

  • Jumper JP5/J26 not installed: J-Link-OB-ATSAM3U4C is enabled and fully functional.
  • Jumper JP5/J26 installed: J-Link-OB-ATSAM3U4C is disabled and an external JTAG controller can be used through the 20-pin JTAG port J27.
Figure 3-56. Debug Solution Selection Schematic

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

This feature is enabled/disabled by jumper J25.

  • Jumper J25 not shorted: the J-Link-OB CDC function is enabled and fully functional.
  • Jumper J25 shorted: the J-Link-OB CDC function is disabled.

The USB CDC converts the USB device into a serial communication device. The target device running the CDC is recognized by the host as a serial interface (USB2COM, virtual COM port) without the need to install a special host driver (the CDC is standard). All PC software using a COM port work without modifications with this virtual COM port. Under Microsoft® 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.