Design for Testability712.6.1Tristate BusesBus contention occurs when two bus drivers force opposite logic values onto atristate bus, which can damage the chip. Bus contention is designed not to happenduring the normal operation and is typically avoided during the capture operation,as advanced ATPG programs can generate test patterns that guarantee only onebus driver controls a bus. However, during the shift operation, no such guaranteescan be made; therefore, certain modifications must be made to each tristate bus inorder to ensure that only one driver controls the bus. For example, for the tristatebus shown in Figure 2.21a, which has three bus drivers (D1,D2, andD3), circuitmodification can be made as shown in Figure 2.21b, whereEN1is forced to 1 toenable theD1bus driver, whileEN2andEN3are set to 0 to disable bothD2andD3bus drivers, whenSE=1.In addition to bus contention, a bus without a pull-up, pull-down, or bus keepermay result in fault coverage loss. The reason is that the value of a floating bus is

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