Abstraction & Digital Logic: From Transistors to Gates
Wave Diagram concept
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Building functions from logic gates
Combinational Logic Circuit
Output depends only on the current inputs Stateless (memoryless)
Sequential Logic Circuit
Output depends on the sequence of inputs (past
and present)
Stores information (state) from past inputs
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Sequential Circuits and Memory
Combinational vs. Sequential
■ Combinational circuit
◆ Always gives the same output for a given set of
■ Sequential circuit
◆ Remembers previous input
◆ Output depends on state and input
inputs
◆ Example: Adder always generates sum and carry, regardless of previous inputs
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Sequential Circuits
■ Store information
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Reset-Set (RS) Latch – NAND SR Latch
■ Two inputs: Set and Reset
■ Setto0oneofthetwoinputsatatimetostore
a value, S sets, R clears
■ The transition to 00 generates an undefined output
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NAND SR Latch
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R-S Latch Nor Gates
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NAND SR Latch wave diagram
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NOR SR Latch wave diagram
Make up your own examples and practice!
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D Latch
E/cl D k
0 0 0 1 1
1
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S’
1
R’
R’
S’
Q
Q’ Comment
Keep state Keep state D=Q D=Q
0 1
11 11 01 10
Q Q’ Q Q’
1 0
0 1
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D Latch Wave diagram
E/cl D k
0 0 0 1
Q Q’
Keep state Keep state D D’
D D’
1
0
1
1
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D Flip Flop
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D FF positive edge wave
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