Digital Electronics
Core concepts
Master number systems and codes (binary, BCD, Gray, two's complement), Boolean algebra and Karnaugh maps, and combinational building blocks: adders, multiplexers, decoders, comparators. Then sequential logic: latches, flip-flops, counters, shift registers, and state machines. Understand setup and hold times, propagation delay, and hazards. Memory types (SRAM, DRAM, ROM, flash) and programmable logic (FPGA, CPLD) appear in modern papers.
Traps examiners set
- Two's complement range for n bits is -2^(n-1) to 2^(n-1) - 1, not symmetric.
- A D flip-flop samples on the clock edge; a latch is level sensitive.
- Gray code changes only one bit between adjacent states, useful for error reduction.
- A multiplexer can implement any Boolean function of its select lines.
- Static hazards are glitches caused by unequal path delays; they are removed with redundant terms.
Worked rule of thumb
An n-input multiplexer implements a function of n select variables directly; for more variables, use the mux as a look-up with the extra variable on data inputs. Example: a 4-to-1 mux can realise any 2-variable function by wiring the four data inputs to 0, 1, or the third variable.
Practice strategy
Always minimise with a K-map before designing; then verify the truth table matches.