JTAG Target Connection Issue

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1 Problem

No target connection with XDS220 ISO.

The VTRef pin 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 (document sprab21 and http://processors.wiki.ti.com/index.php/XDS_Target_Connection_Guide) recommend 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.


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