CHAPTER 7. DIGITAL CIRCUITS147ND7=A0oA1oA2ND6=NA0oA1oA2ND5=A0oNA1oA2ND4=NA0oNA1oA2ND3=A0oA1oNA2ND2=NA0oA1oNA2ND1=A0oNA1oNA2ND0=NA0oNA1oNA2A2A1A0Figure 7.16: Octal decoder.7.6Schmitt TriggerA noisy input signal to a logic gate could cause unwanted state changes near the voltagethreshold. Schmitt trigger logic reduces this problem by using two voltage thresholds: a highthreshold to switch the circuit during low-to-high transitions and a lower threshold to switchthe circuit during high-to-low transitions. Such a trigger scheme is immune to noise as longas the peak-to-peak amplitude of the noise is less than the diﬀerence between the thresholdvoltages. A gate with the Schmitt trigger feature has a small hysteresis curve drawn insidethe gate symbol. Schmitt triggers are mostly used in inverters or simple gates to conditionslow or noisy signals before passing them to more critical parts of the logic circuit.7.7The Data BusA bus is a common wire connecting various points in a circuit; examples are the ground busand power bus. The data bus carries digital information. A data bus is usually a groupof parallel wires connecting diﬀerent parts of a circuit with each individual wire carrying adiﬀerent logic signal. The data bus is connected to the inputs of several gates and to theoutputs of several gates. You cannot connect directly the outputs of normal gates. For thispurpose three-state output logic is commonly used but will not be discussed here.A data bus line may be time multiplexed to serve diﬀerent functions at diﬀerent times.