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Logic gates

Official AQA 7517 section 4.6.4.

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36 min

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AqaA LevelComputer ScienceFundamentals of computer systems

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Logic gates1 objectives

Logic gates: Construct truth tables for the following logic gates: • NOT • AND • OR • XOR • NAND • NOR. Students should know and be able to use ANSI/ IEEE standard 91-1984 Distinctive shape logic gate symbols for these logic gates. Be familiar with drawing and interpreting logic gate circuit diagrams involving one or more of the above gates. Complete a truth table for a given logic gate circuit. Write a Boolean expression for a given logic gate circuit. Draw an equivalent logic gate circuit for a given Boolean expression. Recognise and trace the logic of the circuits of a half-adder and a full-adder. Construct the circuit for a half-adder. Be familiar with the use of the edge-triggered D- type flip-flop as a memory unit. Knowledge of internal operation of this flip-flop is not required.

Truth tableHalf-adder

Trace circuits in stages: For a circuit containing several gates, label each intermediate output and evaluate the gates in the order of the connections.

Confusing OR with XOR: Remember that OR allows one or more 1 inputs, whereas XOR outputs 1 only when exactly one input is 1. Thus, 1 XOR 1 = 0.

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