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Sequential logic circuits are a type of digital circuit whose output depends on both the current inputs and the past history of inputs. This is different from combinational logic circuits, where the output only depends on the current input values.

In sequential circuits, memory elements (like flip-flops) are used to store information about past inputs.

Types of Sequential Circuits

1. Synchronous Sequential Circuits – Changes happen in sync with a clock signal.

2. Asynchronous Sequential Circuits – Changes happen immediately based on input changes, without a clock.

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Flip-Flops: The Building Blocks of Sequential Circuits

Flip-flops are the basic memory elements in sequential circuits. They can store 1 bit (0 or 1) and are used to build registers, counters, and memory units.

Types of Flip-Flops

1. SR Flip-Flop (Set-Reset)

   • Inputs: S (Set), R (Reset)

   • Stores a bit of data.

   • Problem: Invalid state when both S and R = 1.

2. D Flip-Flop (Data or Delay)

   • Input: D (Data)

   • Output follows input on the clock edge.

   • No invalid state; simpler and reliable.

3. JK Flip-Flop

   • Inputs: J, K

   • Like SR Flip-Flop, but the invalid state is removed.

   • When J = K = 1, it toggles the output.

4. T Flip-Flop (Toggle)

   • Input: T

   • Toggles the output when T = 1 on clock pulse.

   • Often used in counters.

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Clock Signal and Edge Triggering

• Flip-flops respond to a clock signal, which is a regular timing pulse.

• Most flip-flops are edge-triggered, meaning they change state only at the rising edge (0→1) or falling edge (1→0) of the clock.

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Applications of Flip-Flops

1. Registers – Store multi-bit data using a group of flip-flops.

2. Counters – Count pulses or events using T or JK flip-flops.

3. Memory – Store information in basic memory units.

4. State Machines – Control sequences in devices using sequential logic.

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Comparison: Combinational vs Sequential Logic

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Key Terms to Remember

• Latch – A basic memory device, not clock-driven.

• Flip-Flop – A clocked memory device.

• Clock – A timing signal that controls state changes.

• Edge Triggered – Changes state on the clock edge.

• Setup Time – Minimum time before the clock edge that input must be stable.

• Hold Time – Minimum time after the clock edge that input must remain stable.

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In Simple Words

• Sequential circuits remember things – they have memory.

• Flip-flops are like tiny memory boxes – they store 0 or 1.

• They change value based on clock signals and input values.

• We use them in computers, timers, watches, and digital devices.

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