Digital Electronics: Principles, Devices and Applications

178 Digital Electronics74S00174S00274S002074S00Figure 5.61Output of one TTL subfamily driving another.4. The power supply rail must always be properly decoupled with appropriate capacitors so that thereis no drop in V CC rail as the inputs and outputs make logic transitions. Usually, two capacitorsare used at the V CC point of each IC. A 0.1 F ceramic disc should be used to take care ofhigh-frequency noise, while typically a 10–20 F electrolytic is good enough to eliminate any lowfrequencyvariations resulting from variations in I CC current drawn from V CC , depending upon logicstates of inputs and outputs. To be effective, the decoupling capacitors should be wired as close asfeasible to the V CC pin of the IC.5. The unused inputs should not be left floating. All unused inputs should be tied to logic HIGH inthe case of AND and NAND gates, and to ground in the case of OR and NOR gates. An alternativeis to connect the unused input to one of the used inputs.6. While using open collector devices, resistive pull-up should be used. The value of pull-up resistanceshould be determined from the following equations:R X = V CC max − V OL /I OL − N 2 LOW × 16 (5.11)R X max = V CC min − V OH /N 1 × I OH + N 2 HIGH × 40 (5.12)where R X is the external pull-up resistor; R X (max.) is the maximum value of the external pull-upresistor; N 1 is the number of WIRED-OR outputs; N 2 is the number of unit input loads beingdriven; I OH is the output HIGH leakage current (in mA); I OL is the LOW-level output current ofthe driving element (in mA); V OL is the LOW-level output voltage; and V OH is the HIGH-leveloutput voltage. One TTL unit load in the HIGH state = 40 mA, and one TTL unit load in theLOW-state = 1.6 mA.