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Design for Testability 71 2.6.1 Tristate Buses Bus contention occurs when two bus drivers force opposite logic values onto a tristate bus, which can damage the chip. Bus contention is designed not to happen during the normal operation and is typically avoided during the capture operation, as advanced ATPG programs can generate test patterns that guarantee only one bus driver controls a bus. However, during the shift operation, no such guarantees can be made; therefore, certain modifications must be made to each tristate bus in order to ensure that only one driver controls the bus. For example, for the tristate bus shown in Figure 2.21a, which has three bus drivers ( D 1 , D 2 , and D 3 ), circuit modification can be made as shown in Figure 2.21b, where EN 1 is forced to 1 to enable the D 1 bus driver, while EN 2 and EN 3 are set to 0 to disable both D 2 and D 3 bus drivers, when SE = 1. In addition to bus contention, a bus without a pull-up, pull-down, or bus keeper may result in fault coverage loss. The reason is that the value of a floating bus is
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