The formulas, component behaviour, and logic tables you should know
cold — type them until they're reflex.
Fundamentals · Ohm's law, KVL/KCL, power, dividers, Thevenin, RC/RL
Passives · resistors, capacitors, inductors, transformers, derating
Parts · packages, markings, dielectrics, connectors, datasheets
Diodes · PN junction, rectifiers, Zener, LED, Schottky, flyback
Transistors · BJT regions, MOSFET, biasing, switch vs amplifier
Op-Amps · ideal rules, gain, comparators, filters, saturation
Digital · number systems, two's complement, boolean, gates, K-maps
Sequential · latches vs flip-flops, setup/hold, counters, FSM
Signals · RC/RL filters, cutoff, Bode slope, sampling, the scope
Power · regulators, switchers, batteries, thermals, protection
Micro · GPIO, pull-ups, debounce, ADC/PWM, I2C/SPI/UART, decoupling
Firmware · registers, interrupts, memory, timing, toolchain
Interfacing · level shifting, driving loads, sensors, noise, protection
Comms · USB, CAN, RS-485, 1-Wire, radio, framing and CRC
Bench · meters, scopes, soldering, prototyping, PCB workflow
Everything · mixed endless decks across all layers + fill-in-the-blank