Bus Disable Values

The Disable Value for a bus controls what the XJTAG system will do with the bus during the Connection Test. By default, the Disable Value will be set to None and in most cases this will be correct. A value of None means that the connection test is not restricted and can set the nets that make up the bus high and low as needed.

There are two reasons why you may need to set another value:

The bus will be outputting during the connection test.

In this case it is important that the connection test does not try to drive the nets connected to the bus. If the connection test does drive these nets then false errors are likely to be reported. In this case a Disable Value of Input should be defined.

This logic can seem contradictory as busses that cannot be prevented from outputting are assigned a Disable Value of Input. It is important to remember that XJTAG can only directly control the JTAG devices in the circuit and that the Disable Values in these files have to be applied to those JTAG devices, not the non-JTAG devices to which the files are assigned.

Another bus can be prevented from outputting by actively controlling this bus.

Typically that means you will specify a Disable Value on an output-enable pin designated nCE or nOE. Generally if you set an nOE pin to have a Disable Value of High then connection test will use this to prevent the non-JTAG device driving a data bus during the connection test.

Disable values have a similar effect as setting a Constant Pin in the circuit. The advantage of defining disable values in device files comes from the ability to reuse those files across circuits or, where there are multiple instances of a non-JTAG device in a circuit.

Bus Disable values are set on the Test Device Files screen inside the relevant device file.