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🔌 Digital Electronics (105301)

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💡 Why this subject? Every CPU, every chip, every "0 and 1" you'll ever code on is built from the gates, flip-flops, and memory circuits you learn here.

📌 Unit 1: Fundamentals of Digital Systems & Logic Families

  • Digital signal: only 2 discrete levels (0/1, LOW/HIGH) vs analog (continuous range).
  • Basic gates: AND, OR, NOT, NAND, NOR, XOR (recap from Sem-1 Electronics).
  • Number systems: Binary, Octal, Hexadecimal, Signed binary.
  • 📝 (13)₁₀ = (1101)₂ = (15)₈ = (D)₁₆
  • 1's and 2's Complement (used to represent negative numbers in binary):
  • 1's complement: flip all bits.
  • 2's complement: 1's complement + 1 → used in real computer arithmetic (subtraction = addition of negative number).
  • 📝 Example: 5 = 0101, -5 in 2's complement (4-bit) = 1011
  • Logic families: TTL, CMOS — different ways to physically build gates using transistors; CMOS is more power-efficient (used in modern chips).
  • Tri-state logic: a gate output can be 0, 1, or "high impedance" (disconnected) — used in shared data buses.

🧠 Quick Recall: 2's complement is THE method real computers use to handle negative numbers and subtraction.

📌 Unit 2: Combinational Digital Circuits

  • K-Map (Karnaugh Map): a grid-based visual trick to simplify Boolean expressions without messy algebra.
  • Don't care conditions: input combinations that never occur — can be set as 0 or 1 in K-map, whichever simplifies the circuit more.
  • Multiplexer (MUX): many inputs → one output (selected by control lines) — like a rotary switch.
  • Demultiplexer (DEMUX): one input → many outputs (opposite of MUX).
  • Adder/Subtractor:
  • Half Adder: adds 2 bits, gives Sum & Carry (no carry-in).
  • Full Adder: adds 2 bits + carry-in, gives Sum & Carry-out.
  • ALU (Arithmetic Logic Unit): the part of CPU that does all math/logic operations.
  • Decoder/Encoder: decoder converts binary code → one-hot output (selects one line); encoder does the reverse.

📝 Example — Half Adder logic: 

Sum   = A XOR B
Carry = A AND B

🧠 Quick Recall: MUX = "selector" (many→1), DEMUX = "distributor" (1→many).

📌 Unit 3: Sequential Circuits and Systems

  • Latch vs Flip-Flop: latch changes state immediately when input changes; flip-flop only changes on a clock edge (more controlled/predictable).
  • Types of Flip-Flops:
  • SR (Set-Reset): basic memory cell.
  • JK: fixes SR's "invalid state" problem.
  • D (Data): stores whatever is on the D input at clock edge — simplest to use.
  • T (Toggle): flips state every clock pulse.
  • Shift Register: a chain of flip-flops that shifts bits left/right each clock pulse — used in serial communication.
  • Counters:
  • Asynchronous (Ripple) counter: each flip-flop triggered by the previous one's output (cumulative delay).
  • Synchronous counter: all flip-flops triggered by the same clock (faster, more reliable).
  • Ring counter / Johnson counter: special shift-register-based counters with circular bit patterns.

📝 Example: A digital clock's "seconds" display is literally a synchronous counter counting 0→59 then resetting.

📌 Unit 4: A/D and D/A Converters

  • DAC (Digital to Analog Converter): converts binary number → analog voltage.
  • Weighted resistor method & R-2R ladder network — two circuit techniques to build a DAC.
  • ADC (Analog to Digital Converter): converts analog voltage → binary number.
  • Successive Approximation ADC: binary-searches the voltage level — fast & common.
  • Counting ADC: counts up until it matches — simple but slow.
  • Dual Slope ADC: very accurate, used in precision instruments.

📝 Example: Your microphone's analog sound wave is converted to digital bits using an ADC before your laptop can process/store it as an audio file.

📌 Unit 5: Semiconductor Memories

Memory Type Key feature
ROM Read-only, retains data without power (non-volatile)
RAM Read & write, loses data without power (volatile)
CAM (Content Addressable Memory) Searches by content, not address — super fast lookup
CCD Charge-based memory, used in image sensors

🧠 Quick Recall: RAM = working memory (volatile), ROM = permanent storage of fixed instructions (non-volatile) — e.g., BIOS.

📌 Unit 6: Programmable Logic Devices

  • PLA (Programmable Logic Array): both AND and OR gate arrays are programmable.
  • PAL (Programmable Array Logic): only AND array programmable, OR array fixed — cheaper, faster.
  • CPLD: more complex, multiple PAL-like blocks combined.
  • FPGA (Field Programmable Gate Array): a huge grid of configurable logic blocks — can be "rewired" in software to become almost any digital circuit. Used in custom hardware acceleration (AI chips, crypto mining, prototyping CPUs).

🔬 Lab Highlights (Digital Electronics Lab)

  1. Universal gates (build all gates using only NAND or only NOR)
  2. Code converters & parity generators/checkers
  3. Adders, Subtractors, Magnitude comparators
  4. Decoder, MUX & DEMUX circuits
  5. Latches and Flip-Flops (SR, JK, D, T, Master-Slave)
  6. Shift Registers (SISO, SIPO, PISO, PIPO)
  7. Synchronous & Asynchronous Counters
  8. 4-bit sequence generator
  9. DAC (Weighted Resistor & R-2R Ladder)
  10. ADC (Counter type & Successive Approximation)
  11. Multisim simulation practice

✅ Quick Revision Table

Topic One-line memory hook
2's complement How computers represent negative numbers
K-Map Visual shortcut to simplify Boolean logic
MUX / DEMUX Many→1 selector / 1→Many distributor
Flip-flop Changes state only on clock edge (vs latch = instant)
Synchronous counter All FFs share same clock — fast & reliable
ROM vs RAM Non-volatile read-only vs volatile read-write
FPGA Reconfigurable hardware — "rewire" a chip in software