JTAG Target Connection Issue

1 Problem

No target connection with XDS220 ISO.

The VTRef pin (also called TVRef, TVD or VREF) on the JTAG header is used by most emulators to sense and detect the target's I/O voltage. Texas Instruments target board emulation design guidelines (http://processors.wiki.ti.com/index.php/XDS_Target_Connection_Guide#TVRef_2) recommends to connect VTRef via a 100 ohms current limiting series resistor to the target IO power supply.

The XDS220 ISO however draws more current from the VTRef pin to supply the target side isolation drivers. A 100 ohms series resistor causes too much voltage drop and the XDS220 ISO will fail to connect to the target.

2 Solution

Replace the 100 ohms resistor on your target board with a 0 ohms resistor or a ferrite bead with low DC resistance for better noise immunity.