JTAG Chain Bring-up Problems

Common initial problems encountered when trying to connect to a JTAG chain are:

• The pin mapping for your project does not match the hardware you are connecting to.
• Compliance patterns are not being correctly applied to JTAG devices in the circuit.
• One of the JTAG devices is being held in reset (this often means that too few devices are detected in the JTAG chain).
• One of the JTAG devices requires a specific Test Reset sequence to be applied (using the nTRST pin and perhaps others) before it will operate in JTAG mode.
• One of the links between devices is broken.
• One of the external signals (TDI, TMS, TCK or TDO) is either open-circuit or shorted.
• One of the global signals (TMS or TCK) is open-circuit to a JTAG device.
• The JTAG chain TCK frequency is too high.
• The JTAG TAP signals are not correctly terminated, or other major signal integrity issues are causing problems reading data reliably.

See the JTAG chain errors page for more information on the symptoms seen with various types of connection problems in the JTAG chain.

Debugging JTAG Chain problems

The JTAG Chain Debugger utility is a very useful tool for troubleshooting problems with the JTAG chain.

It is installed as a standalone application although XJDeveloper users can also access this functionality through the Chain Operations panel in the Pin Mapping screen of XJDeveloper. Its use is described on the Using the Chain Debugger page.

In addition to the JTAG chain errors page, which describes the symptoms of certain types of error, there is information about debugging JTAG chain setup in the XJTAG User Guide section on Setting up a Physical JTAG Chain and in the help for the JTAG Chain Debugger application.

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For more information about improving the testability of circuits, download the Design For Test (DFT) guidelines from the XJTAG website.