Second Semester_Digital Logic_Sequential Circuit Design

 

5.1 Flip-Flops: RS, JK, D, and T, Latches

• Flip-Flops: Bistable devices that store 1 bit of data. They have two stable states.

  • RS (Set-Reset) Flip-Flop:

    • Inputs: R (Reset), S (Set)

    • Output Q = 1 when S=1, Q=0 when R=1

  • JK Flip-Flop:

    • Inputs: J, K

    • Toggle behavior: If J=K=1, output toggles

    • Solves invalid state problem of RS flip-flop

  • D Flip-Flop (Data/Delay):

    • Input: D

    • Output follows D at clock edge (avoids race conditions)

  • T Flip-Flop (Toggle):

    • Input: T

    • Toggles output when T=1 at clock edge

    • Used in counters

• Latches: Level-triggered storage devices (respond to input levels, not clock edges).

  • Types: SR latch, D latch

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5.2 Triggering of Flip-Flops

• Level Triggered: Flip-flop changes state as long as clock is active. (Risk of multiple transitions)

• Edge Triggered: Flip-flop changes state only at the rising or falling edge of the clock.

  • More reliable for synchronous designs.

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5.3 Master-Slave Flip-Flop

• Combines two flip-flops in series: Master and Slave

• Master responds to clock pulse, slave responds to inverted clock

• Ensures edge-triggered behavior and prevents race conditions.

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5.4 Flip-Flop Excitation Table and Design Procedure

• Excitation Table: Tells what input values are needed to make a flip-flop go from current state to next state.

  • Example: JK flip-flop excitation table:

Current Q	Next Q	J	K
0	0	0	X
0	1	1	X
1	0	X	1
1	1	X	0

• Design Procedure:

  1. Define states and inputs.

  2. Draw state table or state diagram.

  3. Select flip-flop type.

  4. Use excitation table to determine flip-flop inputs.

  5. Derive logic equations and implement circuit.

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5.5 State Diagram and Simple Sequential Circuits

• State Diagram: Graphical representation of states (circles) and transitions (arrows) based on inputs.

• Simple Sequential Circuit: Implement a design where outputs depend on current input + previous state.

  • Example: Traffic light controller, sequence detector.