CHAPTER 7. DIGITAL CIRCUITS138CoDAoBQDCBAFigure 7.2: The basic AND-OR-INVERT gate.7.4.3Exclusive-OR GateThe exclusive-OR gate (EOR or XOR) is a very useful two-input gate. The schematic symboland truth table are shown in ﬁgure 7.3.Q = A⊕ BXOR (EOR)ABQ000101011110EORQBAFigure 7.3: The schematic symbol for the exclusive-OR gate (EOR or XOR) and its truthtable.From the truth tableQ=A· B + A· B(7.28)Q=A· B + A· B(7.29)and we can draw the mechanization directly from the truth table (ﬁgure 7.4).7.4.4Timing DiagramsNormally signals ﬂip from one logic state to another. The time it takes the signal to movebetween states is the transition time tt, where the time is measured between 10% and 90%of the signal levels. Delays within the logic elements result in a propagation (pulse) delaytpd, where the time is measured between 50% of the input signal and 50% of the outputresponse. Deﬁnitions of the transition time and propagation delay are shown in ﬁgure 7.5.