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# E3_4 - E3 LOGIC CIRCUITS(A INTRODUCTION Logic circuits are...

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E3 LOGIC CIRCUITS (A) INTRODUCTION Logic circuits are becoming increasing important in today’s modern world. They are used to process information and present results in useful form eg, calculators and digital watches. Logic circuits are formed by combining various types of logic gates, such as NOR and NAND gates. There are two main types of circuit, namely analogue and digital. Analogue circuits process information by continuing fluctuating voltage which is dependant on the input signal. Digital circuits operates on two distinct states, ie “0” and “1”. For positive logic system, a higher voltage level denotes logic “1” while a lower voltage level denotes logic “0”. In a negative logic system, it is however represented in the opposite way. In this experiment, positive logic system is used. (B) EXPERIMENT OBJECTIVES There are two main objectives in this experiment and they are as follows: (i) to understand the application of J-K flip flops as a binary ripple counter ; (ii) to analyse the simple combinational circuit which consists of a sequential logic circuit in driving a seven-segment display device with useful logic gates. (C) EQUIPMENTS & COMPONENTS Equipment List 1. Digital multimeter (DMM) 2. Function generator (FG) 3. Dual display cathode-ray oscilloscope (CRO) 4. Breadboard with supply of 5V Component List 1. 5 X 1K , 7 Comm. Res. Net. SIL 330 2. IC type 74LS76 Dual J-K flip-flop 3. IC type 7408 Quad 2 input AND gate 4. IC type 74LS02 Quad 2 input NOR gate 5. IC type 74LS48 BCD-to-Seven-Segment decoder/drive 6. 7-Segement LED (common cathode) Display Device (D) THEORY Logic Gates Digital signal processing circuits are called logic gates. These gates work in a two-value logic system, the binary digit “0” for the low level voltage and “1” for high level voltage. NOT (inverter), AND, NAND, OR, NOR, XOR and XNOR are the seven types of logic gates. Their symbols and truth table can be found in Appendix-B. Combinational logic circuits are built from logic gates and they depend on present inputs. Sequential logic circuits Sequential logic circuits are different from combinational circuits as the output depends on the current inputs as well as previous output. It possesses valuable memory characteristic. A flip-flop (FF) is a bistable sequential logic circuit storage device. Flip-flops have valuable memory characteristic because it remains in one of the states until a signal changes it into the second stable state. There are the two types of triggering, namely edged-triggered and level-triggered. The flip-flop used in this experiment is negative edge triggered and Is set to toggle mode. This means that J and K inputs are both at logic level “1” and successive clock pulse will reverse the output logic state repeatedly.

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