Digital Electronics

1. Digital Electronics

2. Chapter 5 Synchronous Sequential Logic

3. SR NAND Latch Set up the Truth Table

4. SR NAND Latch Truth Table

5. SR NOR Latch Set up the Truth Table

6. SR NOR Latch Truth Table

7. D Latch Eliminates the indeterminate S=R=1 state of the NAND Latch in addition to a control input C.

8. Graphic Symbols for Latches Note: 74LS75 is D Latch

9. Flip-Flops Flip-Flops are triggered by a clock transition in order to make the operation reliable Latch FF FF

10. Master-Slave D Flip-Flop Master reads while the clock is high but Q records the last data when the clock is low.

11. Positive Edge-Triggered D Flip-Flop

12. Graphic Symbol for 74LS74 Edge-triggered design is superior to master-slave because reading and recording occur in a flash during the clock transition.

13. T Flip-Flop Determine the Truth Table of the T FF

14. D Flip-Flop and T Flip-Flop Characteristic Tables D Flip-Flop T Flip-Flop D Q(t+1) T Q(t+1) 0 0 0 No Change 1 1 1 Toggles

15. Frequency Divider T Flip-Flop can be used to divide the frequency of a clock by 2. Sketch the circuit. How can you divide the frequency by 4?

16. JK Flip-Flop Draw the Characteristic Table

17. JK Flip-Flop JK Flip-Flop J K Q(t+1) 0 0 No Change 0 1 0 (reset) 1 0 1(set) 1 1 Toggles

18. JK Flip-Flop Equation Q(t+1) = JQ' + K'Q 74LS76

19. What’s wrong with this picture?

20. What’s wrong with this picture? Connect a wire fom the AND gate to the D Flip-Flop. P.S. This is figure 5-15 in your textbook! P.P.S. Analyze the given sequential circuit. In other words, write the equations for A(t+1), B(t+1), and y, draw a state table, and sketch a state diagram.

21. State Equations A(t+1) = A x + B x B(t+1) = A' x y = (A + B) x'

22. State Table Present State Next State Output x = 0 x =1 x = 0 x =1 A B A B A B y y 00 00 01 0 0 01 00 11 1 0 10 00 10 1 0 11 00 10 1 0

23. State Diagram

24. Design of Sequential Circuits Design a circuit that detects three or more consecutive 1’s in a string of bits coming through an input line

25. Planning, Planning, Planning! Our circuit should start off in a “state” S(0). If a 0 comes along it should stay put in S(0). If a 1 comes along it should jump to state S(1). Now if a 0 comes along it should go right back to S(0) but if a second 1 comes along it should jump to S(2). At this point if a third 1 comes along it should jump to S(3) and also set a flag. Otherwise start all over again in S(0).

26. State Diagram for Sequence Detector

27. State Table for Sequence Detector Present State Next State Output x = 0 x =1 x = 0 x =1 A B A B A B y y 00 00 01 0 0 01 00 10 0 0 10 00 11 0 0 11 00 11 1 1

28. K-Map for DA of Sequence Detector

29. K-Map for DB of Sequence Detector

30. K-Map for y of Sequence Detector

31. Logic Diagram of Sequence Detector

32. Some Terminology ... FSM:A Sequential Circuit is also called a Finite State Machine (FSM) Mealy Model: The output (y) of an FSM depends on the input (x) as well as the present state of A and B [e.g. Fig 5-15 where y = (A+B)x'] Moore Model: The output (y) of an FSM depends on the present state of A and B but not on the input (x). [e.g. Sequence Detector where y = AB]

33. VHDL for JK Flip-Flop // Functional description of JK flip-flop module My_JKFlipFlop (J,K,CLK,Q,Qnot); output Q,Qnot; input J,K,CLK; reg Q; assign Qnot = ~ Q ; always @ (posedge CLK) case ({J,K}) 2'b00: Q = Q; 2'b01: Q = 1'b0; 2'b10: Q = 1'b1; 2'b11: Q = ~ Q; endcase endmodule

34. That’s All Folks!