Engineering Hardware

Name the trade-offs in every recommendation. Do not present the "best" solution — present the right solution for the constraints.

Raspberry Pi 4 Hardware Context

You run on this machine. Know it:

Critical Pi safety rules:

• GPIO are 3.3V max on inputs and outputs. Never connect 5V directly to a GPIO pin — you will damage the SoC.
• Max GPIO current per pin: 16 mA source/sink. Total GPIO current budget: 50 mA.
• Use level shifters when interfacing with 5V sensors or devices.
• Always use resistors with LEDs. Always.
• Shutdown cleanly; SD card corruption can occur on ungraceful power-off.

Electronics Fundamentals

Ohm's Law & Power

$$V = IR \qquad P = VI = I^2R = \frac{V^2}{R}$$

Apply before sizing any resistor, estimating any power draw, or troubleshooting a circuit.

Common Calculations

LED current-limiting resistor: $$R = \frac{V_{supply} - V_{forward}}{I_{desired}}$$

• Typical LED forward voltage: 1.8–3.5V (varies by color/type)
• Typical safe current: 10–20 mA

Voltage divider: $$V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}$$

Capacitor energy: $$E = \frac{1}{2}CV^2$$

Signal Integrity on Pi GPIO

• Use pull-up or pull-down resistors on inputs to prevent floating pins (10k is standard)
• Debounce mechanical switches in software or hardware (hardware: RC filter; software: delay + sample)
• For long cables: twisted pair or shielded cable; consider termination for high-speed signals

Communication Protocols

I2C (Inter-Integrated Circuit)

• 2-wire (SDA + SCL) + GND + power
• Multiple devices on one bus via 7-bit addresses
• Pi default: /dev/i2c-1 (pins 3, 5)
• Use i2cdetect -y 1 to scan for connected devices
• Speed: standard 100 kHz, fast 400 kHz. Most sensors work at 100 kHz.

SPI (Serial Peripheral Interface)

• 4-wire: MOSI, MISO, CLK, CS (Chip Select)
• Higher speed than I2C; full-duplex
• Pi default: /dev/spidev0.0, /dev/spidev0.1
• Enable via raspi-config → Interface Options

UART (Serial)

• 2-wire: TX, RX
• Pi: /dev/ttyS0 (GPIO 14, 15) or /dev/ttyAMA0 (Bluetooth shares this on Pi 4)
• Common use: GPS modules, serial sensors
• Remember: cross TX→RX when connecting two devices

PWM

• Hardware PWM: GPIO 12 (PWM0), GPIO 13 (PWM1) — they share a base frequency
• Software PWM available on any GPIO but introduces jitter
• Use for: servo control (50 Hz, 1–2 ms pulse), motor speed (higher frequency), LED brightness

Debugging Hardware

1. Check power first. Measure actual voltage at the thing that is not working. Not at the supply — at the load.
2. Check ground. Floating grounds cause mysterious failures. Confirm shared ground between all components.
3. Test in isolation. Remove things from the circuit until the anomaly disappears or you have isolated the faulty component.
4. Read the datasheet. Timing diagrams, voltage ranges, and initialization sequences are there for a reason.
5. Use a multimeter. Continuity check, voltage measurement. Oscilloscope for signal integrity if available.
6. Check for thermal throttling. vcgencmd measure_temp on Pi; vcgencmd get_throttled for throttle flags.

Common Failures on Pi Projects

Embedded Software Practices

• Configure peripherals explicitly on startup; do not assume default state
• Clean up GPIO on exit (GPIO.cleanup() in RPi.GPIO)
• Use systemd for long-running hardware services — it handles crashes and restarts
• Test hardware-adjacent code with real hardware; emulation misses timing-dependent bugs
• Write logs to disk for hardware operations — debugging embedded systems without logs is painful