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ch1hwsol - Embedded Controller Design W00 Problem Solutions...

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Embedded Controller Design, W00 Problem Solutions 1 Solutions to Chapter 1 Problems 1. If an open-drain N-channel FET transistor is used as a logic output, is it possible to connect more than one open-drain transistor output to the same signal? What would the effect of doing so be on the resulting combined signal? Yes, the outputs may be combined, as they provide parallel paths for sinking the load current to ground. If the transistors were working alone, there would not be a current source however, so the signal would never have any way of going high. A pull-up resistor is required to source current in the logic one output case. If any of the transistors are "ON," the output signal will be low. Only if ALL the transistors are in the "OFF," or open state, will the output be high, as it is pulled up by the resistor. This type of circuit is useful for bus signals that may be activated by any one of multiple sources. This is referred to as a "wired OR" circuit meaning that any one or more of the devices pulling the line low results in a logic zero. From a positive logic point of view however, it is an AND function, since the output is high only when all the inputs are high (off in this case).
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