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Unit II Overview: Combinational Circuits and Flip-Flops

Lesson 6 of 17 in the free Computer Organization and Architecture notes on Siksha Sarovar, written by Rohit Jangra.

Combinational vs Sequential Digital Circuits

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Combinational vs Sequential Comparison

FeatureCombinationalSequential
MemoryNoneHas flip-flops / latches
FeedbackNo feedback pathHas feedback (output to input)
ClockNot requiredUsually clock-driven (synchronous)
Output depends onCurrent inputs onlyCurrent inputs + current state
ExamplesMUX, Decoder, Encoder, Adder, ALUCounters, Registers, RAM, CPUs
ComplexityLowerHigher
Analysis methodTruth table, Boolean algebraState table, state diagram

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Combinational Circuits Overview

CircuitFunctionInputs → OutputsKey Application
Multiplexer (MUX)Select 1 of N data inputs using select lines2ⁿ data + n select → 1 outputData routing, function generators
Demultiplexer (DEMUX)Route 1 input to 1 of N outputs1 data + n select → 2ⁿ outputsData distribution
EncoderConvert 2ⁿ inputs to n-bit binary code2ⁿ → nKeyboard encoding, A/D conversion
DecoderConvert n-bit binary to 2ⁿ outputsn → 2ⁿMemory address decode, display drivers
ComparatorCompare two n-bit numbers (A=B, A>B, A<B)2n → 3Sorting, conditional branches
Adder/ALUArithmetic and logic operations2n + carry → n + carryCPU arithmetic unit

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Sequential Circuits Overview

TypeDescriptionClockApplication
SR Flip-FlopSet-Reset; basic memory cellEdge/LevelStatus flags
D Flip-FlopData; captures input on clock edgeEdgeRegisters, pipeline stages
JK Flip-FlopUniversal; no invalid state; toggle modeEdgeCounters, state machines
T Flip-FlopToggle; flips state on T=1EdgeBinary counters
RegisterGroup of flip-flops storing n-bit wordEdgeCPU registers, buffers
CounterSequential state machine counting clock pulsesEdgeClocks, timers, address generators

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Study Deep: The Concept of State

A state is a snapshot of all stored values (flip-flop outputs) at a given clock cycle. Sequential circuits implement Finite State Machines (FSMs):

  • Moore machine: Output depends only on current state
  • Mealy machine: Output depends on current state AND current inputs (faster response, fewer states)

Every digital system you use — smartphone, CPU, network card — is a collection of FSMs implemented with flip-flops and combinational logic.

📝 Exam Tip: Key distinction: combinational = no memory, output = f(current inputs only); sequential = has memory, output = f(current inputs, current state). CPU internal registers make it fundamentally a sequential circuit.