D latch symbol clk d q q two inputs clk d clk

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D Latch Symbol CLK D Q Q Two inputs: CLK , D CLK : controls when the output changes D (the data input): controls what the output changes to Function When CLK = 1, D passes through to Q (the latch is transparent ) When CLK = 0, Q holds its previous value (the latch is opaque ) Avoids invalid case when Q ≠ NOT Q 16
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D Latch Internal Circuit S R Q Q Q Q D CLK D R S CLK D Q Q S R Q Q CLK D 0 X 1 0 1 1 D 17
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D Latch Internal Circuit S R Q Q Q Q D CLK D R S CLK D Q Q S R Q 0 0 Q prev 0 1 0 1 0 1 Q 1 0 CLK D 0 X 1 0 1 1 D X 1 0 Q prev 18
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D Flip-Flop Two inputs: CLK , D Function The flip-flop “samples” D on the rising edge of CLK When CLK rises from 0 to 1, D passes through to Q Otherwise, Q holds its previous value Q changes only on the rising edge of CLK A flip-flop is called an edge-triggered device because it is activated on the clock edge D Flip-Flop Symbols D Q Q 19
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D Flip-Flop Internal Circuit CLK CLK Q Q D N1 CLK Two back-to-back latches (L1 and L2) controlled by complementary clocks When CLK = 0 L1 is transparent, L2 is opaque D passes through to N1 When CLK = 1 L2 is transparent, L1 is opaque N1 passes through to Q Thus, on the edge of the clock (when CLK rises from 0 1 ) D passes through to Q D Q Q D Q Q L1 L2 20
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D Flip-Flop vs. D Latch CLK D Q Q D Q Q CLK D Q (latch) Q (flop) 21
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D Flip-Flop vs. D Latch CLK D Q Q D Q Q CLK D Q (latch) Q (flop) 22
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