--- title: Common Peripheral Circuits url: https://doc.liz6.com/en/hardware/09-embedded-systems/02-common-peripheral-circuits locale: en area: hardware tags: - hardware - embedded-systems date: 2026-06-30 modified: 2026-07-16 description: After learning components and protocols, the final step is to build a functional circuit. Below are the peripheral modules most commonly encountered in projects. --- # Common Peripheral Circuits > After learning components and protocols, the final step is to build a functional circuit. Below are the peripheral modules most commonly encountered in projects. --- ## Buttons and Debouncing ### Problem ``` Mechanical button presses are not clean make/break events: V │ ┌─┐ ┌──┐ │ │ │ │ │ ← Contact bounce │ │ └──┘ └────── └──────────────── t ↑ ↑ Pressed Stable Bounce time: Typically 5~20ms No debouncing → One button press triggers dozens of events ``` ### Hardware Debouncing (RC) ``` Vcc │ Rpullup (10k) │ ├────── GPIO │ ├── R1 (1k) ──┬── Button ── GND │ C1 (0.1μF) │ GND RC time constant τ = R1×C1 = 100μs (Too short) Need τ ≈ Bounce time ≈ 10ms → R1=10k, C1=1μF → τ=10ms Disadvantages: Uses extra components, but reliable Advantages: Does not consume CPU, suitable for interrupt wake-up scenarios ``` ### Software Debouncing (Common) ```c // Simplest and most effective method if (digitalRead(BUTTON) == LOW) { delay(20); // Wait 20ms (or use a timer, avoid using delay) if (digitalRead(BUTTON) == LOW) { // Confirm press handle_press(); } } // Better: State machine + timer, non-blocking // Record the time of the last change; only consider it valid if the interval > 20ms ``` --- ## Using MOSFETs as Load Switches ### Why use a MOSFET instead of driving directly with a GPIO ``` GPIO max current: Typically 8~20mA Loads to drive: LED strips (1A), Relays (100mA), Motors (>1A) → GPIO controls MOSFET, MOSFET drives the load Low-side switch (N-MOSFET, load between Drain and Vcc): Vcc ── Load ──┬── Drain │ GPIO ── Rg ──── Gate │ Source ── GND Advantages: Simple, N-MOSFET Rds(on) is low Disadvantages: Load is not grounded (not suitable for some scenarios) High-side switch (P-MOSFET, load between Drain and GND): Source ── Vcc │ GPIO ──┬─ Rg ── Gate │ NPN/ NMOS (Level shifting, because GPIO 3.3V cannot fully turn off P-MOS) │ GND Drain ── Load ── GND Advantages: Load is grounded on one end (safer) Disadvantages: Requires level shifting, P-MOS Rds(on) is higher ``` ### Gate Protection (Mandatory!) ``` Rg (100Ω~1k) GPIO ───┤├─────── Gate │ Rgs (10k~100k) │ GND Rg: Limits gate charging/discharging current, suppresses oscillation Rgs: Ensures Gate=GND (MOSFET OFF!) during GPIO uninitialized/reset periods Without Rgs → Floating Gate at power-up → MOSFET may turn on inadvertently High voltage / inductive loads: Add a TVS or Zener diode from Gate to Ground (Prevents drain spikes from coupling back to the gate via Cgd → breaking down the gate oxide) ``` --- ## Optocoupler Isolation ### When is an optocoupler needed ``` - Between high voltage and low voltage (Mains 220V ↔ MCU 3.3V) - Long-distance signal transmission (large ground potential difference) - Need to eliminate ground loop noise - Industrial environments (surges/lightning) Scenarios where optocouplers are NOT needed: - Different voltage domains on the same board → Use level shifter ICs - On-board I2C/SPI → No isolation needed (unless industrial) ``` ### Basic Circuit ``` MCU Side (3.3V) Controlled Side (12V/24V) │ │ GPIO R (Current Limit) │ │ R (Limit LED Current) ┌──┴──┐ │ │Opto │ ┌┴┐ │Output│ │LED (Internal) │ │ └┬┘ └──┬──┘ │ │ GND GND (Controlled Side) Typical: PC817 (Cheap, Low Speed), 6N137 (High Speed, 10Mbps) LED Side: If = 5~20mA, R = (3.3V-1.2V)/If Output Side: Calculate current limit based on load ``` --- ## Relay Driving ### Flyback Diode — Absolutely Essential! ``` Vcc │ ┌─┴─┐ ▸├ (Flyback Diode) │Relay│ │ └─┬─┘ │ │ │ ┌─┴─┐ │ │MOSFET│ │ └─┬─┘ │ │ │ GND────┘ At turn-off: Coil current cannot change instantly → Generates reverse high voltage (L×dI/dt) Can exceed 100V! → Without flyback diode → MOSFET breakdown Diode Direction: Reverse parallel (Cathode to Vcc, Anode to MOSFET Drain) Normally reverse-biased and non-conducting When MOSFET turns off, provides a current path Diode Selection: 1N4148 (Small relays), 1N4007 (Large relays) Fast recovery or Schottky (Reduces EMI) ``` ### Complete Circuit ``` Vcc (5V/12V/24V) │ ┌┴┐ │▸├ (Flyback) └┬┘ │ ┌┴┐ │Relay│ └┬┘ │ Drain GPIO ──Rg─── Gate N-MOSFET │ (Select low Rds(on), Vds > Vcc×2) Source │ GND Relay Coil Current: I = Vcc/R_coil Example: 5V Relay 70Ω → 71mA MOSFET Selection: Vds > Vcc × 1.5 Id > I_coil × 1.5 Vgs(th) < GPIO High Level (For 3.3V systems, select < 2.5V) Recommended: AO3400 (30V/5.8A, Vth<1V, Cheap and easy to use) ``` --- ## LED Driving | Method | Circuit | Application | |------|------|------| | Current Limiting Resistor | R = (Vcc-Vf)/I_LED | Indicator LEDs | | Constant Current IC | e.g., CAT4101, BCR401 | High Power / Lighting | | PWM + MOS | GPIO PWM → MOS → LED | Dimming | | Constant Current LED Driver | e.g., WS2812B/SK6812 | Addressable RGB | ### Constant Current vs. Current Limiting Resistor ``` Current Limiting Resistor: I_LED = (Vcc - Vf) / R Vcc fluctuation → Current fluctuation Vf temperature drift → Current drift Severe resistor heating at high power Constant Current Source: Current remains constant, independent of Vcc/Vf changes Can connect multiple LEDs in series (Voltage adds up, current remains constant) High efficiency ``` --- ## Watchdog ### Why is it needed ``` Embedded device runs for 3 months → Freezes → User has to unplug power Causes: Memory leaks, dangling pointers, EMI interference, rare bugs Watchdog = Independent Timer Firmware normal: Periodically "feed the dog" (reset timer) Firmware stuck: No feed → Timeout → Hardware reset ``` ### Key Usage Points ``` ❌ Feeding the watchdog in the main loop (Still feeding if stuck in a sub-function) ✅ Feed the watchdog in an independent monitoring task Or: Multiple tasks must all check in before feeding the watchdog ❌ Forgetting to feed the watchdog when using delay/sleep ✅ Break long delays into multiple short delays + feed watchdog ❌ Using a very short watchdog timeout (Might not have time to feed during normal operation) ✅ Timeout > 2 times the longest task execution time Independent Watchdog (IWDG): Independent clock source, can reset even if system clock fails Window Watchdog (WWDG): Can only be fed within a specific time window (Stricter) ``` --- *Keywords: Debouncing, MOSFET Switch, Optocoupler, Flyback Diode, Relay, LED Driver, Watchdog, GPIO*