114 VLSI Test Principles and Architectures That o i = Z indicates that G i has no effect on the bus wire y , leaving the control to other drivers. For the bus to function correctly, there should not be more than one active tristate control at a time. If multiple drivers are enabled and they intend to drive the bus to the same value, the bus wire is assigned the active drivers’ output value; however, if at least two drivers drive the bus wire to opposite binary values, a bus conflict occurs. Such situations may cause the circuit to be permanently damaged. Finally, if no driver is activated, the bus is in a floating state because it is not connected to V dd or ground. A pull-up or down network that connects the bus wire to V dd or ground via a resistor may be added to provide a default 1 or 0 logic value (Figure 3.6); otherwise, the bus wire will retain its previous value as a result of trapped charge in the parasitic wire capacitance. In addition to design errors, abnormal bus states could occur during testing when
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