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Latches Module M10.1 Section 7.1. Sequential Logic Combinational Logic –Output depends only on current input Sequential Logic –Output depends not only.

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Presentation on theme: "Latches Module M10.1 Section 7.1. Sequential Logic Combinational Logic –Output depends only on current input Sequential Logic –Output depends not only."— Presentation transcript:

1 Latches Module M10.1 Section 7.1

2 Sequential Logic Combinational Logic –Output depends only on current input Sequential Logic –Output depends not only on current input but also on past input values –Need some type of memory to remember the past input values

3 Cross-coupled Inverters State 1 State 2

4 !S-!R Latch !S !R Q !Q 0 0 1 1 0 1 !S !R Q !Q 1 1 0 1 0 1 0 0 1 0 1 1 1 0 1 1 1 0 X Y nand

5 !S-!R Latch !S !R Q !Q 0 0 1 1 0 1 !S !R Q !Q 0 1 0 1 0 1 0 0 1 0 1 1 1 0 1 1 1 0 X Y nand

6 !S-!R Latch !S !R Q !Q 0 0 1 1 0 1 !S !R Q !Q 0 1 1 1 0 1 0 0 1 0 1 1 1 0 1 1 1 0 X Y nand

7 !S-!R Latch !S !R Q !Q 0 0 1 1 0 1 !S !R Q !Q 0 1 1 0 0 1 0 0 1 0 1 1 1 0 1 1 1 0 X Y nand 1 0 Set

8 !S-!R Latch !S !R Q !Q 0 0 1 1 0 1 !S !R Q !Q 1 1 1 0 0 1 0 0 1 0 1 1 1 0 1 1 1 0 X Y nand 1 0 Set 1 0 Store

9 !S-!R Latch !S !R Q !Q 0 0 1 1 0 1 !S !R Q !Q 1 0 1 0 0 1 0 0 1 0 1 1 1 0 1 1 1 0 X Y nand 1 0 Set 1 0 Store

10 !S-!R Latch !S !R Q !Q 0 0 1 1 0 1 !S !R Q !Q 1 0 1 1 0 1 0 0 1 0 1 1 1 0 1 1 1 0 X Y nand 1 0 Set 1 0 Store

11 !S-!R Latch !S !R Q !Q 0 0 1 1 0 1 !S !R Q !Q 1 0 0 1 0 1 0 0 1 0 1 1 1 0 1 1 1 0 X Y nand 1 0 Set 1 0 Store 0 1 Reset

12 !S-!R Latch !S !R Q !Q 0 0 1 1 0 1 !S !R Q !Q 1 1 0 1 0 1 0 0 1 0 1 1 1 0 1 1 1 0 X Y nand 1 0 Set 1 0 Store 0 1 Reset

13 !S-!R Latch !S !R Q !Q 0 0 1 1 0 1 !S !R Q !Q 0 0 1 1 0 1 0 0 1 0 1 1 1 0 1 1 1 0 X Y nand 1 0 Set 1 0 Store 0 1 Reset 1 1 Disallowed Q 0 !Q 0

14 !S-!R Latch !S !R Q !Q 0 0 1 1 0 1 !S !R Q !Q 1 1 0 1 0 1 0 0 1 0 1 1 1 0 1 1 1 0 X Y nand 1 0 Set 1 0 Store 0 1 Reset 1 1 Disallowed Q 0 !Q 0

15 S-R Latch !S !R Q !Q S R CLK S R CLK !S !R Q !Q 0 0 1 1 1 Q 0 !Q 0 Store 0 1 1 1 0 0 1 Reset 1 0 1 0 1 1 0 Set 1 1 1 0 0 1 1 Disallowed X X 0 1 1 Q 0 !Q 0 Store

16 D Latch Q !Q CLK D !S !R S R S R CLK Q !Q 0 0 1 Q 0 !Q 0 Store 0 1 1 0 1 Reset 1 0 1 1 0 Set 1 1 1 1 1 Disallowed X X 0 Q 0 !Q 0 Store 0 1 1 1 1 0 X 0 Q 0 !Q 0 D CLK Q !Q

17 D Latch Q !Q CLK D !S !R S R 0 1 1 1 1 0 X 0 Q 0 !Q 0 D CLK Q !Q Note that Q follows D when the clock in high, and is latched when the clock goes to zero.

18 D Latch CLK D Q E x y z x y z Does NOT latch z = z $ x = 0 $ 1 = 1 Latches on following edge of clock

19 D Latch CLK D Q E x y z x y z Does latch z = z $ x = 0 $ 1 = 1 Use narrow pulse If x remains high, successive clock pulses will toggle z

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