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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