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Categorising Power, Ground and Termination Nets

Once XJDeveloper has been given the circuit’s design data, the next step is to define which nets are power, ground, or termination voltages. It is important for XJDeveloper to identify those nets for several reasons:

  • Potential board damage is avoided by identifying power and ground nets because XJDeveloper can then ensure it does not attempt to drive any pins connected to them, either directly or via a low-value resistor.
  • Appropriate test methods can only be applied to the devices in the circuit if the power and ground nets are known.
    • Pull resistors are tested differently to other resistors during an interconnection test, and it is therefore important for XJDeveloper to correctly identify those devices. XJDeveloper will propose pull resistors during device categorisation by identifying resistors with appropriate values that are connected to power or ground; it is therefore important to have correctly identified those nets.
    • Because termination voltage supplies are often set to mid-rail, XJDeveloper cannot test resistors connected to those nets in the way it checks pull resistors; it therefore needs to differentiate between termination voltages and power supply rails.
    • A JTAG device’s pin that has input capability will automatically be read during the interconnection test to check it is at the expected logic level. Power and ground rails must therefore have been correctly categorised for XJDeveloper to know what level to expect on any pins connected to them.
  • Access to other nets can be affected if the power nets are not correctly categorised. Such problems can be hard to debug when trying to get the project setup working.
    • Failure to categorise a power net could cause the system to route the JTAG chain via its pull resistors and through the supply rail, resulting in XJDeveloper flagging a problem with the chain topology, or devices appearing in the wrong order.
    • Incorrect categorisation can cause parts of the circuit to appear untestable or undrivable. For example, if it leads to pull resistors not being correctly identified, an I2C bus could become inaccessible.
  • Test coverage figures will be misleading if the power and ground nets are not all correctly categorised.
  • Integration with ICT or external test equipment requires nets to be correctly categorised to ensure the tester does not apply a signal to a power or ground rail.

Nets that function as power, ground or termination voltages therefore need to be identified.

This guide covers the following topics: