Passive Device Types

In XJTAG, when categorising passive devices, there are several passive device types to choose from, with different properties:

Connections

Because JTAG is a digital technology, it is unaffected by precise resistance values. A low-impedance resistor or a 0 Ω resistor both have the effect that a signal passes through them, effectively joining the nets on each side of the resistor or link.

Pull resistors

A pull resistor is a resistor between two nets where one net is either Power or Ground, and the other net can be driven. XJTAG assumes that whenever the net is undriven, pins on it will read logic level 1 (if pulled to power) or logic level 0 (if pulled to ground).

If the resistor value is too small, it will not be possible to successfully drive the net to both values - it will cause excessive currents or will not achieve the value required. If the resistor is too large then the undriven net will take a long time to achieve the pulled value, and this may cause errors to be reported in testing.

Differential termination resistors

A differential termination resistor (in XJTAG's terminology) is a single resistor making an unreferenced termination between two differential signals.

This style of termination is often seen around devices supporting the 1149.6 version of the JTAG standard. It defines a resistor which will be detected as a short-circuit between two nets at DC if only one net is driven, but allows the nets to be driven differentially during testing.

Bias-termination resistors

A bias-termination resistor is a termination resistor placed between a signal and a termination reference voltage net. Differential signals will each have a bias-termination resistor between the signal net and the reference net. Often many nets will be terminated to the same reference, and this means that when many of them are driven simultaneously, the reference net voltage can change enough for this to be detected on undriven nets attached to the same reference.

This style of termination is very commonly found on nets attached to DDR memory devices.

Coupling capacitors

A capacitor is effectively open-circuit at DC and short-circuit at high frequencies. Most JTAG testing is carried out at low enough speeds to be regarded as DC, and so the capacitor (open-circuit) has no effect. However, if the circuit has devices which support the 1149.6 JTAG standard, higher-speed testing can be carried out, and in this situation the automatically-generated Connection Test expects that a coupling capacitor will join its two nets for high-speed signals to pass through.

XJEase testing is always carried out using the 1149.1 (DC) JTAG Instructions and so coupling capacitors only have an effect in the Connection Test, and only when there are 1149.6-compliant devices present. When this is the case, the Connection Test will expect to see the capacitor behaviour change from open-circuit to short-circuit when the high-speed 1149.6 tests are carried out.

Other passive devices

There will be other types of passive device in your circuit. Many, such as fuses, inductors, links, have the effect of linking their nets together, and can be treated as Connections. Others, such as capacitors attached to power supplies, cannot easily be tested using JTAG and in XJDeveloper such devices should be categorised as Ignored devices.

Although resistors, fuses, inductors, links etc all have the same effect of linking nets, XJTAG give the option to assign different (but identical) device files to them, to make the setup easier to understand when you come to review the circuit, and for this reason the library contains files with these names.