JK Flip-Flop

The JK flip-flop is the most versatile sequential element, supporting four operating modes: hold (J=K=0), reset (J=0,K=1), set (J=1,K=0), and toggle (J=K=1). Unlike the SR latch, the J=K=1 state is well-defined - the output toggles. This makes it useful for counters and state machines.

Objectives

• Write the JK flip-flop truth table with all four modes
• Compare JK to SR (toggle instead of forbidden)
• Build counters using JK flip-flops in toggle mode
• Use JK excitation tables for state machine design

Key Takeaways

• JK flip-flop: 4 modes - Hold (00), Reset (01), Set (10), Toggle (11)
• J=K=1 toggles the output (no forbidden state like SR)
• Excitation table has don't cares, simplifying state machine design
• D flip-flop = JK with J=D, K=D̄ (D is simpler and more common today)
• Toggle mode makes JK ideal for counter circuits
• Common ICs: 74HC76, 74HC112

Applications

• Counters: Toggle mode naturally implements binary counting.
• State Machines: Excitation table don't cares simplify next-state logic.
• Frequency Division: Toggle mode divides clock frequency by 2.

Practice Problems

Problem 1: A JK flip-flop has Q=1. On the next clock edge with J=1, K=1, what is the new Q?

Problem 2: How do you make a D flip-flop from a JK flip-flop?

Problem 3: Design a 2-bit binary counter using JK flip-flops in toggle mode.

Problem 4: Using the excitation table, what J,K values cause Q to go from 1 to 1?