Logic Gates

4.6.4 (19 questions). Public Notes: AQA's A Level Computer Science

Draw a NOT gate.

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Draw a truth table for a NOT gate.

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Draw an AND gate.

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Draw a truth table for an AND gate.

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Draw an OR gate.

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Draw a truth table for an OR gate.

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Draw an XOR gate.

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Draw a truth table for an XOR gate.

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Draw a NAND gate.

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Draw a truth table for a NAND gate.

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Draw a NOR gate.

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Draw a truth table for a NOR gate.

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Draw the truth table for binary addition, ignoring the possibility of a carry in.

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Isolate the truth table for the SUM output.

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Which logic gate does this truth table resemble?

XOR gate.
Hence SUM = A(*)B

Isolate the truth table for the Carry Out Output.

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Which logic gate does this truth table resemble?

AND gate.
Hence Carry Out (CO) = A*B

Draw the logic diagram for a half adder

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S = A ⊕ B
CO = A * B

Removing the detail of half-adders draw a simplified logic diagram of a full adder.

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Describe the purpose of edge-triggered D-type flip flop.

Can store a 1 bit value, on or off.

Draw a diagram detailing the only Inputs and Outputs of a D-type flip flop.

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Explain the purpose the clock signal within a D-type flip flop.

  1. The state of output Q can only change on the rising edge of the clock signal.
  2. Therefore the clock signal can synchronise many D-type flip flops to store a larger number of bits.

Where are D type flip-flops found in a computer system.

CPU registers.

Why is a NAND gate known as a universal gate.

Because all other gates can be derived from just NAND gates, so any circuit can be built using just NAND gates.