Disabling J-Link-OB

Jumper JP19 disables the J-Link-OB-ATSAM3U4C JTAG functionality. When the jumper is installed, it grounds pin PA26 of the SAM3U4C that is normally pulled high. A quad analog switch (U31) is used to select the JTAG interface.

Jumper JP19 not installed: J-Link-OB-ATSAM3U4C is enabled and fully functional.
Jumper JP19 installed: J-Link-OB-ATSAM3U4C is disabled and an external JTAG controller can be used through the 10-pin JTAG port J18

Jumper JP19 disables only the J-Link functionality. The debug serial com port that is emulated through a Communication Device Class (CDC) of the same USB connector remains operational (if JP20 is open).

Table 1. J-Link-OB and CDC LED LD6 Status
LED LD6	State	USB Cable Connected
RED	Off	J-Link not programmed or JP19 and JP20 installed
GREEN	Off	J-Link not programmed or JP19 and JP20 installed
GREEN	Flashing	USB port not connected
GREEN	On	J-Link-OB connected and available