JTAG port mainly includes 4 pins TCK/TDI/TDO/TMS. Those 4 pins can work in GPIO mode if the JTAG function is disabled, which is the normal case in application. To enable JTAG functionality, check the configuration of registers in the following, and reconfigure them if necessary. Set the IOMUX register to be 0 to enable JTAG, and make sure none of TCK/TDI/TDO/TMS pins works in GPIO mode. This can be done via the memory debugger in UCD3xxx Device GUI.

       MiscAnalogRegs.IOMUX.all = 0; //enable JTAG

                MiscAnalogRegs.GLBIOEN.bit.TCK_IO_EN = 0;
                MiscAnalogRegs.GLBIOEN.bit.TDI_IO_EN = 0;
                MiscAnalogRegs.GLBIOEN.bit.TDO_IO_EN = 0;
                MiscAnalogRegs.GLBIOEN.bit.TMS_IO_EN = 0;