Classifying Nets

Not all nets on a PCB should be treated the same:

There are certain types of net that XJTAG needs to know about - this is partly used to help you set up tests correctly and partly to avoid damaging your PCB during testing.

Power nets

A power net is one which supplies power to some part of the PCB. Many devices have multiple power supplies at different voltages - all XJTAG wants to know is that they are power nets. XJTAG will not try to drive a signal on a power net, and if the net can be read through a JTAG device or via logic, we expect a power net to have a logic value of one.

Pull resistors may be connected to power nets, and it is expected that whenever a pulled-up net (the other side of the pull resistor to power) is undriven, it will read a logic value of 1.

Ground nets

A Ground net is the 0V return path for current on the PCB. Many PCBs have multiple ground nets which may or may not be linked, but devices on them will read them as a logic value of 0. XJTAG will not try to drive a signal on a ground net.

Pull resistors may be connected to ground nets, and it is expected that whenever a pulled-down net (the other side of the pull resistor to ground) is undriven, it will read a logic value of 0.

Termination reference voltage nets

Sometimes boards need a reference voltage supplied to certain ICs, and sometimes boards need a termination rail, often at half of the supply voltage for use with referenced terminations of signals. This is very commonly seen on later types of DDR memory. A reference voltage net is usually unable to source or sink significant current (unlike a power or ground net), whereas a termination rail may be able to source and sink significant current with relatively small voltage variation. The logic value of both reference and termination rail nets is undefined if read and may change during circuit operation as it is often near the threshold voltage for the attached components.

In XJDeveloper these types of net are both classified on the Power/Ground Nets screen as Termination Ref. Nets.

Because these nets are not defined as power supply nets, may not have current sourcing capabilities, and have undefined values, resistors connected to termination reference nets may not be defined as Pull resistors.

Bias-termination resistors may be connected to termination reference nets. They are automatically tested by the connection test but they do not restrict the operation of devices during XJEase testing.

Connect resistors may be applied to termination reference nets to extend them in the same way that they do for other nets.