CSE-221 Digital Logic Design (DLD)

1. CSE-221Digital Logic Design (DLD) Lecture-2: Logic Operations & Digital Logic Gates P.B-Dr M.A.kashem(CS&E-221,TLDD2)

2. Analog vs. Digital V V +5 +5 1 0 1 Time Time –5 –5 Analog: values vary over a broad range continuously Digital: only discrete values P.B-Dr M.A.kashem(CS&E-221,TLDD2)

3. Analog vs. Digital • Analog devices process signal that can assume any value across a continuous range and produce results that are also in continuous form. • Digital devices process signals that take on only two discrete values such as 0 and 1 and produces output that can be represented by 0 and 1. • Examples • Analog Devices: solid-state devices TV (except for digital TV), Audio amplifier etc. • Digital Devices: Computer, CD player, digital TV, digital cellular phone, electronic calculator, and digital camera. P.B-Dr M.A.kashem(CS&E-221,TLDD2)

4. Analog vs. Digital Systems Analog systems: • Limited precision, errors accumulate, drift • Interface circuits (i.e., sensors & actuators) often analog Digital systems: • More accurate and reliable • Readily available as self-contained, easy to cascade building blocks • Computers use digital circuits internally P.B-Dr M.A.kashem(CS&E-221,TLDD2)

5. AND means that both conditions must be true in order for the conclusion to be true Let’s look at the relationship between the semantic and logical operator known as the AND operator Consider: If the car is fueled AND the engine works, then the engine will start A B Output 0 0 0 0 1 0 0 0 1 1 1 1 AND Operator AND Operator Truth Table P.B-Dr M.A.kashem(CS&E-221,TLDD2)

6. A Output A B Output Inputs 0 B 1 0 0 1 0 1 0 1 1 1 1 Digital AND For digital logic: True = 1 is 5 volts False = 0 is 0 volts • We can build an electrical device that performs the logical AND operation on voltage equivalents of logic values • An AND gate has the electrical schematic: • Practice with the Excel spreadsheet OR Operator Truth Table OR Operator • Another basic operator is the OR • Consider: If I have cash OR a credit card, then I can pay the bill • OR works such that the output is true, if either of the two inputs is true P.B-Dr M.A.kashem(CS&E-221,TLDD2)

7. A A B Output Output Inputs 0 1 B 0 0 1 0 1 0 1 1 1 0 XOR (Exclusive OR) Operator • Let’s look at the relationship between the semantic and logical operator known as the XOR operator • Consider a biological example: If gender A XOR gender B, then reproduction is possible • XOR works such that output is activated (equal to one) if both inputs are of a different value • Try the Excel spreadsheet exercise • We can build an electrical device that performs the logical XOR operation on voltage equivalents of logic values • An XOR gate has the electrical schematic: P.B-Dr M.A.kashem(CS&E-221,TLDD2)

8. AND gates can be built with any number of inputs Consider the symbol for the 4-input AND gate F is trueonly when all the inputs are true (1’s) Using the Excel workbook “EE-WISE-Digital Lab”, open the “Digital Locks” worksheet, and test this circuit A B C D F Multi-Input AND Gate • All digital computers are built using only three gate types: AND, OR, and NOT P.B-Dr M.A.kashem(CS&E-221,TLDD2)

9. 1 AND 1 1 AND 1 1 AND 1 1 Digital Combination Lock • Using 3 two-input AND gates, we could build a combination lock that requires a four-digit code, specifically: 1 1 1 1 • The number of inputs could be increased by using more and more AND gates P.B-Dr M.A.kashem(CS&E-221,TLDD2)

10. 1 0 AND 1 AND 1 1 1 AND 1 0 1 0 1 Digital Combination Lock • We could build a combination lock that only uses the AND gate, but that would be of little use since everyone would know our combination, namely 1 1 1 1 • To build a more interesting combination lock, we will utilize the NOT (inverter) gate • Let’s build a combination lock whose input (key code) combination is 0 1 1 0 • Is there any other combination that works? P.B-Dr M.A.kashem(CS&E-221,TLDD2)

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