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.

If the bus will be held at a constant known value (by something other than XJTAG, for example by the test device itself) during the connection test, then setting the disable value to be IsHigh or IsLow tells connection test the expected value of the bus, so that it can be checked if access permits.

If the value of the bus is not known, then a Disable Value of IsDriven or Excluded should be set. If the value is IsDriven the net will still be analysed for short-circuits to other nets by the connection test. If Excluded is chosen the net will be excluded from analysis.

In versions of XJTAG prior to 4.2.0, the Excluded value was called Input. This caused some confusion and so the value has been renamed in XJTAG 4.2.0, but behaviour has not changed and older projects will still work correctly.

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

In this case the values High or Low will be used. Typically that means you will specify a Disable Value on an output-enable pin. If the pin is designated *EN, nCE or nOE you will need to set an nOE pin to have a Disable Value of High; if the pin is in the positive sense (ie EN) then a value of Low is appropriate. XJTAG will drive the selected value to the net during the connection test in order 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.