Constant Pins and Bus Disable Values

Two closely related concepts in XJTAG are Constant Pins and Bus Disable Values.

Both are used to specify values to the automatically-generated parts of XJTAG such as the Safe Bitstream and Connection Test that have to be observed.

Both Constant Pins and Bus Disable Values are used to either:

• prevent XJTAG driving a net which is being driven by some other device, or which, if driven would cause undesirable consequences (e.g. reset pin).
• drive a net to a particular value - perhaps to disable a non-JTAG device, to enable a power supply or to keep a JTAG device in JTAG mode.

In general, Bus Disable Values should be used where the purpose is related to a device type, where you would want to do the same thing wherever the device is used. Constant pins on the other hand are useful where the setting is specific to a PCB design.

Neither Constant Pins nor Bus Disable Values truly set a pin or bus as "constant".

Both Constant Pins and Bus Disable Values can be overridden during XJEase testing by explicitly setting a bus to a different value, but performing a SAFE statement or running a connection test will reset them back to their original value.

However, both of these types of value can be changed permanently during a test run using XJTAG's Default Overrides feature, which can be useful if a value needs to depend on which chains are currently in use when the safe bitstream is applied.

Constant Pin Values and Effects

The possible values of a constant pin are:

High

XJTAG will try to drive the pin to the logic 1 state. If it cannot drive it, it will assume that something else is driving the state. Connection Test will monitor the state if it can read it. If the pin is already connected to Power, XJTAG will not drive it.

Low

XJTAG will try to drive the pin to the logic 0 state. If it cannot drive it, it will assume that something else is driving the state. Connection Test will monitor the state if it can read it. If the pin is already connected to Ground, XJTAG will not drive it.

Input

XJTAG will attempt to set all outputs on the net to a High-Z state. Connection Test will not drive the net, and will exclude any values read from the net from its analysis.

Related concepts: Compliance Values and TRST Constants

Compliance Values for JTAG devices behave in much the same way as Constant Pins or Bus Disable Values. However, Constant Pins and Bus Disable Values are easily edited by the user, whereas Compliance Values are set by the JTAG device manufacturer and are required in order to make sure a device is kept in JTAG mode.

Finally, there is another type of constant pin - the TRST Constant. This is a much stricter type of constant pin which applies to pins that are used in the graphical TRST sequence waveform or Pulse nTRST steps.