Digital Electronics: Principles, Devices and Applications

398 Digital ElectronicsDQ –QEnableTo-otherD-LatchesFigure 10.42 Example 10.6.Example 10.6Figure 10.42 shows the internal logic circuit diagram of one of the four D latches of a four-bit Dlatch in IC 7475. (a) Give an argument to prove that the Q output will track the D input only whenthe ENABLE input is HIGH. (b) Also, prove that the Q output holds the value it had just before theENABLE input went LOW during the time the ENABLE input is LOW.Solution(a) When the ENABLE input is HIGH, the upper AND gate is enabled while the lower AND gate isdisabled. The outputs of the upper and lower AND gates are D and logic ‘0’ respectively. Theyconstitute inputs of the NOR gate whose output is D . The Q output is therefore D.(b) When the ENABLE input goes LOW, the upper AND gate is disabled (with its output going tologic ‘0’) and the lower AND gate is enabled (with its output becoming the same as the Q outputowing to the feedback). The NOR gate output in this case is Q , which means that the Q outputholds its state as long as the ENABLE input is LOW.10.8 Synchronous and Asynchronous InputsMost flip-flops have both synchronous and asynchronous inputs. Synchronous inputs are those whoseeffect on the flip-flop output is synchronized with the clock input. R, S, J, K and D inputs are allsynchronous inputs. Asynchronous inputs are those that operate independently of the synchronousinputs and the input clock signal. These are in fact override inputs as their status overrides the statusof all synchronous inputs and also the clock input. They force the flip-flop output to go to a predefinedstate irrespective of the logic status of the synchronous inputs. PRESET and CLEAR inputs areexamples of asynchronous inputs. When active, the PRESET and CLEAR inputs place the flip-flop Qoutput in the ‘1’ and ‘0’ state respectively. Usually, these are active LOW inputs. When it is desiredthat the flip-flop functions as per the status of its synchronous inputs, the asynchronous inputs are keptin their inactive state. Also, both asynchronous inputs, if available on a given flip-flop, are not madeactive simultaneously.