Digital Electronics Cheat Sheet: Essential Logic Design

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3-Stage Carry Look Ahead Adder Using Basic Gates

  • Carry Generate: G = A · B
  • Carry Propagate: P = A ⊕ B
  • C1 = G0 + P0 · Cin
  • C2 = G1 + P1 · C1
  • C3 = G2 + P2 · C2
  • Reduces delay by avoiding ripple carry.

Realization of Boolean Function Using MUX

(a) 4×1 MUX:

  • Use 2 variables as select lines, rest for input logic.
  • Map the output as per the truth table.

(b) 8×1 MUX:

  • Use 3 variables as select lines.
  • Directly assign data lines as per minterms.

Mealy vs. Moore State Machines

  • Mealy: Output = f(state, input), faster response.
    Example: Sequence detector
  • Moore: Output = f(state), more stable.
    Example: Traffic light controller

Full Adder Circuit in VHDL

Sum <= A XOR B XOR Cin;
Cout <= (A AND B) OR (Cin AND (A XOR B));
  • Inputs: A, B, Cin
  • Outputs: Sum, Cout

3-Bit Gray Code Synchronous Counter Using J-K FFs

  • Gray sequence: 000 → 001 → 011 → 010 → 110 → 111 → 101 → 100
  • Toggle control: Use K-map to derive J, K inputs.

Boolean Function Minimization Using K-Map

  • Fill 1s for minterms.
  • Group in 1, 2, 4, 8...
  • Eliminate changing variables in groups.
  • Derive SOP or POS.

Sequential Circuit with T Flip-Flops

  • T = 1: Toggle
  • Excitation table: Q(next) = T ⊕ Q(current)
  • Draw state diagram first, then circuit.

Universal Shift Register

  • Modes: Hold, Shift Right, Shift Left, Parallel Load
  • Uses D Flip-Flops and MUX.
  • Controlled by S1, S0 lines.

Two-Bit Synchronous Binary Up/Down Counter

  • Uses T or J-K FFs.
  • Up count: 00 → 01 → 10 → 11
  • Down count: 11 → 10 → 01 → 00
  • Direction control via input line.

Boolean Expression Simplification Using K-Map

  • Same as Q7.
  • Group 1s into rectangles (powers of 2).
  • Simplify to minimize expression.

NAND and NOR as Universal Gates

  • NOT A: A NAND A
  • AND: (A NAND B)' = NAND followed by NOT
  • OR: (A' NAND B')'

Binary Subtraction Using 2's Complement

  • 2's complement of B: Invert + 1
  • Add to A.
  • Ignore carry if present.

Function Implementation Using NAND/NOR Gates

  • Convert Boolean equation to SOP/POS.
  • Apply DeMorgan's Theorem.
  • Use only NAND or only NOR equivalents.

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