STM

What is Ohm's law?

V = IR. Current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance (voltage pushes, resistance opposes, current is the result).

What is KCL?

The currents into a node equal the currents out of it (conservation of charge).

What is KVL?

The voltages around any closed loop sum to zero (conservation of energy).

What are the equations for power?

P = VI = V²/R = I²R

Why is power loss usually due to current rather than voltage?

Conductors have resistance, so they lose P = I²R; loss grows with the square of current, and higher voltage means lower current for the same power.

What is the voltage divider equation?

V_R2 = Vs·R2/(R1 + R2). Same current flows through both: I = Vs/(R1 + R2), then V = I × R.

Why shouldn't you use a voltage divider to power a rail?

The load sits in parallel with R2 and changes the output voltage, and the resistors waste power constantly.

When is a voltage divider fine to use?

For high-impedance loads that draw almost no current: a regulator feedback pin, an ADC input, or a reference voltage.

What is the RC time constant, and what happens after 1τ and 5τ?

τ = RC. The capacitor charges to ~63% after 1τ and ~99% (fully charged) after 5τ.

What is the impedance of a capacitor and an inductor?

Z_C = 1/(jωC), Z_L = jωL

How do a capacitor and inductor behave at DC and high frequency?

Capacitor: open at DC, short at high frequency. Inductor: short at DC, open at high frequency.

What does a capacitor resist? An inductor?

Capacitor resists sudden changes in voltage (i = C·dv/dt). Inductor resists sudden changes in current (v = L·di/dt).

What is a decoupling (bypass) capacitor?

A small cap (~0.1 µF) at an IC's power pins that supplies fast current spikes and shunts high-frequency noise to ground.

What is a bulk capacitor?

A larger cap near the regulator that supplies larger, slower changes in the load current.

Why use many capacitors on a rail instead of one big one?

Each cap only works up to its self-resonant frequency because of parasitic inductance (ESL), so mixing values covers a wider frequency range.

Why place decoupling caps close to the IC's power pins?

Trace inductance slows the cap's response, so a short loop from cap to pin to ground lets it deliver current fast.

What are pull-up and pull-down resistors?

Resistors that hold a line at a defined high or low when nothing drives it, so the input doesn't float.

How do you choose a pull-up value?

Too small wastes current; too large gives slow rise times and noise sensitivity.

What are ways to step voltage up or down?

LDOs, switching converters (buck, boost, buck-boost), charge pumps, and transformers for AC.

How does an LDO work?

A pass transistor acts as a variable resistor, controlled by an error amp that compares Vout to a reference, to hold Vout constant.

What is dropout voltage?

The minimum Vin - Vout an LDO needs to stay in regulation.

Where does an LDO's lost power go?

Heat in the pass transistor: P = (Vin - Vout) * Iout.

How does a buck converter work?

It steps DC voltage down. Switch on: inductor current ramps up, storing energy and feeding the load. Switch off: the inductor keeps current flowing through the diode/low-side FET. The LC filter averages it into steady DC.

What sets a buck's output voltage?

The duty cycle: Vo = D*Vs, so D = Vo/Vs.

What does the inductor do in a buck?

Stores energy while the switch is on and keeps current flowing to the load while it's off.

What does the output capacitor do in a buck?

Smooths the output voltage ripple so Vout is steady DC.

What does the input capacitor do in a buck?

Supplies the pulsed current the switch draws and keeps Vin stable.

What is synchronous rectification?

Replacing the buck's diode with a low-side MOSFET, which has a smaller voltage drop and higher efficiency.

What is shoot-through, and how do you prevent it?

both FETs are on at once, shorting Vin to ground. Prevent it with dead time or when both are off.

How does switching frequency affect ripple and efficiency?

Higher frequency reduces ripple (or allows smaller L and C) but increases switching losses, lowering efficiency.

What is a boost converter, and its equation?

A switching converter that steps voltage up: Vo = Vs/(1 − D).

What is the buck-boost equation?

Vo = Vs*D/(1 − D) (inverted polarity)

What is a charge pump?

A converter using only capacitors and switches, no inductor, to double, invert, or step voltage at low current. A flying capacitor stores charge and the switch flips to either connect or disconnect it from the output, enabling voltage transformation.

How does an inverting charge pump make a negative voltage?

The flying cap charges to Vin, then switches ground its top terminal so the bottom sits at −Vin and transfers charge to the output cap. Repeating this holds the output negative.

What is the tradeoff between an LDO and a buck?

LDO: simple and quiet but inefficient for big drops. Buck: efficient but noisier and needs more parts.

When can an LDO be more efficient than a buck?

When Vin is close to Vout and the load is light, where a buck's switching and quiescent losses dominate.

What is efficiency, and how do you find Iin?

η = Pout/Pin. Pout = Vout × Iout, Pin = Pout/η, Iin = Pin/Vin.

What is quiescent current?

The current a regulator consumes to run itself, not delivered to the load; matters at light loads and in batteries.

What is power sequencing?

Turning rails on in a set order so ICs aren't damaged or latched up by one rail coming up before another.

How does a diode work, and what is its forward drop?

It conducts one way (anode → cathode) once forward-biased past its threshold and blocks reverse current. ~0.7 V silicon, 0.2–0.4 V Schottky.

What is a Zener diode used for?

It conducts in reverse at a set breakdown voltage, used for simple voltage references and clamping.

What is a Schottky diode used for?

Low forward drop and fast switching, used in power converters and reverse-polarity protection.

What is a flyback diode?

a diode across an inductive load that gives the inductor's current a path at turn-off, preventing a damaging voltage spike

What are the three terminals of a MOSFET?

Gate, drain, and source. The gate voltage relative to the source controls current from drain to source.

What is the difference between an NFET (NMOS) and a PFET (PMOS)?

NFET (NMOS) turns on when gate > source, used as a low-side switch. PFET (PMOS) turns on when gate < source, used as a high-side switch.

What is Vgs(th)?

The gate-to-source voltage where the MOSFET just starts to turn on (not fully on).

What is Rds(on)?

The drain-to-source resistance when the MOSFET is fully on; sets conduction loss P = I²·Rds(on).

What is the body diode in a MOSFET?

A built-in diode from source to drain in an NFET that conducts if the drain goes below the source (e.g. during dead time).

What is the difference between a BJT and a MOSFET?

BJT is current-controlled (base current). MOSFET is voltage-controlled (gate voltage, almost no gate current), so it's easier to drive and better as a power switch.

What is an op-amp, and what are the ideal rules?

A high-gain amplifier of the difference between its inputs. Rules: no current into the inputs; with negative feedback, V+ = V− (virtual short).

What is the gain of an inverting op-amp?

Gain = −Rf/Rin

What is the gain of a non-inverting op-amp?

Gain = 1 + Rf/Rg

What is gain-bandwidth product?

Gain × bandwidth is roughly constant, so more gain means less bandwidth.

What is slew rate?

The maximum rate the output voltage can change (V/µs).

What is the difference between an op-amp and a comparator?

An op-amp is built for linear operation with feedback; a comparator has no feedback and snaps its output high or low.

What is the cutoff frequency of an RC low-pass filter?

fc = 1/(2πRC)

What is the 3 dB point, and the first-order roll-off?

The cutoff frequency, where output drops to half power (~70.7% voltage, −45° phase). First-order filters roll off at −20 dB/decade after it.

How does a sense resistor measure current?

A small resistor in the current path; measure the voltage across it and use I = V/R. Keep it small (milliohms) to limit I²R loss.

What is high-side vs. low-side current sensing?

High-side: resistor between supply and load (catches shorts, needs a current-sense amp for high common-mode voltage). Low-side: between load and ground (simpler, but shifts the load's ground).

What does ADC stand for, and what does it do?

Analog-to-digital converter: samples an analog voltage and converts it to a digital code relative to Vref.

How does a SAR ADC work?

It finds the code bit by bit, MSB first, comparing the input to an internal DAC like a binary search.

What is sample-and-hold?

A switch and capacitor that capture the input voltage and hold it steady while the ADC converts.

How does a DAC work?

It converts a digital code to an analog voltage: Vout = (code/2^N) × Vref.

What is the Nyquist rate?

You must sample at least 2× the highest frequency in the signal.

What is aliasing, and how do you prevent it?

Frequencies above half the sample rate show up as false lower frequencies; prevent it with an anti-aliasing low-pass filter before the ADC.

What is the difference between a microcontroller and a microprocessor?

MCU: CPU, memory, and peripherals on one chip for dedicated tasks (STM32). MPU: mainly a CPU needing external memory and peripherals (laptop CPU).

What is the difference between I2C, SPI, and UART?

I2C: 2 wires, addressed, slower

SPI: 4 wires, chip select per device, fastest.

UART: 2 wires, no clock, point-to-point

Why does I2C need pull-up resistors?

The lines are open-drain, so devices only pull low; resistors pull the line high. Too big = slow rise, too small = too much sink current.

What is the difference between push-pull and open-drain outputs?

Push-pull drives both high and low; open-drain only pulls low and needs a pull-up, letting several devices share a line.

How do you drive a motor or LED from a GPIO?

Use the GPIO to switch a low-side NFET that carries the load current; add a gate resistor (limits spikes), a gate pulldown (keeps it off at boot), and a flyback diode for inductive loads.

What is an interrupt?

A hardware signal that pauses the main code to run a handler (ISR) right away; keep ISRs short and use volatile shared variables.

What is PWM?

A square wave whose duty cycle controls average power, e.g. LED brightness or motor speed; generated by a timer.

What does volatile do in C?

Tells the compiler the variable can change outside normal code flow (ISR or hardware register), so it re-reads it every time.

What are the layers of a PCB?

Silkscreen (labels), soldermask (green coating), copper layers (traces and planes), and the FR4 dielectric core between them. Taurus was 2 layers.

Why is a ground plane important?

A large copper area tied to ground that gives signals a short, low-impedance return path, reduces noise and EMI, and spreads heat.

What are layout rules for a switching regulator?

Input cap right at the regulator, short switch node away from sensitive traces, feedback trace away from the inductor, solid ground underneath.

How do you route high-current traces?

Wide and short, copper pours for big currents, and multiple vias when changing layers.

What is a DRC?

A design rule check that flags spacing, clearance, short, and unconnected-net violations before fabrication.

What are test points?

Exposed pads on key signals and rails for easy scope or meter probing during bring-up, debugging, and automated testing.

When do you use a DMM vs. an oscilloscope?

DMM for steady values (DC voltage, resistance, continuity); scope for signals over time (waveforms, ripple, switching edges).

What does a 10x probe do?

Attenuates the signal 10× in exchange for higher input impedance (~10 MΩ), less circuit loading, and more bandwidth.

What is a probe ground loop?

The loop from probe tip to ground lead; a big loop picks up noise, so use the short ground spring for fast or noisy signals.

How do you measure current with a scope?

Put a sense resistor in series with the current path, probe the voltage across it, and use I = V/R; or use a current probe.

How do you safely bring up a new board?

Inspect visually, check rails for shorts to ground, power with a current-limited supply, verify rail voltages, check clocks, then test one block at a time.

A board won't power on. What do you check?

Check for shorts, measure rails to see where power stops, check enable and power-good pins, look for backward parts, and follow traces against the schematic.

How do you find a short on a PCB?

DMM continuity from rail to ground (near 0 Ω = short), inject limited current and find the part that heats up, inspect for solder bridges, lift suspect parts.

What is your general approach to debugging?

Isolate it into small sections: reproduce it, start at the input side and work forward, check each block, change one thing at a time, and document findings.