A Guide to JTAG Grounding

Because JTAG signals run at high speed—often tens of MHz—signal integrity is important, and many problems can be avoided by providing suitable grounding between the JTAG controller and the board under test. In particular, it is important that each TAP has at least one ground wire running down the cable with its signals to connect its ground to that of the XJLink.

An XJLink2 supports up to four TAPs simultaneously, but because its connector only has two hard ground pins (pins 10 & 20), the supplied Splitter / Signal Integrity Board (XJA-0030) or an equivalent should be used when interfacing to more than two TAPs. The example shown in Figure 2 distributes the grounds so that each group of signals can have at least one ground connection.

The XJLink-PF20 supports 4 TAPs simultaneously and has 20 GND pins, while the XJLink-PF40 supports 8 TAPs simultaneously and has 40 GND pins. It is receommended that you connect to GND near each TAP connection on the UUT.

Figure 2: Using XJA-0030 Splitter PCB improves grounding for connecting to XJLink2

• Failing to connect a ground pin on the XJLink to the ground used by the JTAG device on the UUT can cause communication problems.