1.1.6.A Component Identification: Digital



Activity 1.1.6 Component Identification: DigitalIntroductionIn this activity you will investigate both combinational and sequential logic gates. You will be asked to simulate simple circuits using basic logic gates. You will then complete a truth table for each logic gate based on the outputs generated from your simulation. The name of many of the fundamental logic gates in digital electronics are based on the logic output from the gate. From analysis of a truth table, could you determine the name and understand function of the gate?You will examine the basic building block of sequential logic: the flip-flop. The investigation will conclude with a look at the 555 IC and how it is used to trigger events in a circuit.Equipment Circuit Design Software (CDS)ProcedureUsing the Circuit Design Software (CDS), enter the circuit shown for each binational LogicUse a switch for the input X and a probe for the output Z. Toggle the switch to complete the truth table shown. XZ0 1 From analysis of the truth table, why do you think this is called an “INVERTER” gate? Getting Started Note: When the switch is at GND, the input is a (0). When it is at +5V, the input is a (1). Likewise, when the probe is on, the output is a (1). When the probe is off, the output is a (0).Use switches for the inputs X & Y and a probe for the output Z. Toggle the switches to complete the truth table shown. XYZ00 01 10 11 From analysis of the truth table, why do you think this is called an “AND” gate? Use switches for the inputs X & Y and a probe for the output Z. Toggle the switches to complete the truth table shown. XYZ00 01 10 11 From analysis of the truth table, why do you think this is called a “NAND” gate? (NOT AND)In this course we will use 2-input gates predominantly. There are gates available with more than 2 inputs.3-Input ANDUse switches for the inputs X & Y and a probe for the output Z. Toggle the switches to complete the truth table shown. XYZ00 01 10 11 From analysis of the truth table, why do you think this is called an “OR” gate? Use switches for the inputs X & Y and a probe for the output Z. Toggle the switches to complete the truth table shown. XYZ00 01 10 11 From analysis of the truth table, why do you think this is called a “NOR” gate? (NOT OR)Use switches for the inputs X & Y and a probe for the output Z. Toggle the switches to complete the truth table shown. XYZ00 01 10 11 From analysis of the truth table, why do you think this is called an “XOR” gate? (EXCLUSIVE OR)Sequential LogicIn the previous combinational logic circuits, YOU made all the outputs change based on the inputs YOU entered with switches. What if you wanted the changes to happen without YOU needing to flip a switch? There are two ICs that allow us to do this.Using the Circuit Design Software (CDS), enter the 74LS74N test circuit shown below. Use a switch for the input T and probes for the outputs Que and NOT_Que.The 74LS74N has two inputs. (Data in and a clock signal CLK). In this circuit, the clock signal input will be a switch T that you flip. The Data in is tied to the NOT_Q.The 74LS74 also has preset and clear inputs. In this circuit the PR (preset) and CLR (clear) are connected to 5v (high), which makes them both inactive. These PR and CLR inputs on the 74LS74 said to be “active low” inputs. (It takes a low signal to activate them)74LS74N Test CircuitTo help with the class discussion (and make sure everyone is seeing the same thing), toggle the input T several times until T is low and NOT_Que is low. (Toggle means to change the state of the switch. One “toggle” of the switch would change the state of the switch from low to high. Another toggle of the switch would change the state of the switch from high to low.) Starting with the switch on GROUND, what happens to output Que when the switch is moved from GROUND to 5v? One toggle of the switch. What happens to output Que when the switch is moved from 5v to GROUND? One toggle of the switch.Toggle the switch one more time. What happened to output Que? What does this tell you about when Que changes (toggles) in relation to when the input at the CLK Changes. (Helpful hint: It might help to cycle though a number of changes to spot this relationship of when Que changes.)What is the relationship of D Data in to Que always?What is the relationship of Que to NOT_Que always?The 74LS74N is called a “flip-flop” Based on your observations can you explain the relationship between the D Data in, Que, NOT_Q, and the CLK signal? What does a flip-flop do?Later in this course we will learn how to combine flip-flops to make transitions to desired outputs. The flip-flop is holding bit of information, waiting for a signal to change (memory). But in this example YOU are still making the transition by flipping a switch. How can we make the flip-flop change without us providing the input directly?Clock Signals (The 555 Timer)Using the Circuit Design Software (CDS), create the three LM555CN test circuits shown below on the same sheet. The 555 Timer Oscillator is one of the most common circuits used in introductory electronics. We will use this design to generate a clock signal in the Random Number Generator (our first full circuit design). We will look more closely at this circuit in the next lesson. In each of the three circuits, the only value that is changed is capacitor C2. Circuit A Circuit B Circuit CWhen C2 is changed from 12μF to 6μF. What happens?When C2 is changed from 12μF to 24μF. What happens?ConclusionCan you combine the 555 Timer circuit with the flip-flop circuit so the changes on the flip-flop happen without your input at a rate you desire? Share this simulation with your instructor. ................
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