GPIO Restrictions

• Strapping pins boot behavior — GPIO0, GPIO2, GPIO12, GPIO15 affect boot mode
• GPIO6-11 connected to flash — don't use, crashes immediately
• GPIO34-39 input only — no output, no pullup/pulldown
• ADC2 unusable with WiFi active — use ADC1 (GPIOs 32-39) when WiFi enabled

Deep Sleep

• Only RTC GPIOs for wakeup — GPIO0, 2, 4, 12-15, 25-27, 32-39
• RTC_DATA_ATTR for persistent variables — regular RAM lost in deep sleep
• esp_sleep_enable_ext0_wakeup() for single pin — ext1 for multiple pins
• WiFi reconnect takes 1-3 seconds after wake — plan for this delay

WiFi Gotchas

• Call WiFi.mode() before WiFi.begin() — mode affects behavior
• WiFi.setAutoReconnect(true) doesn't always work — implement reconnect in loop
• Event-driven with WiFi.onEvent() more reliable — don't poll WiFi.status()
• Static IP faster than DHCP — saves 2-5 seconds on connect

FreeRTOS

• Default stack too small for printf/WiFi — use 4096+ for complex tasks
• Task watchdog triggers at 5s default — call vTaskDelay() or feed watchdog
• xTaskCreatePinnedToCore() for core affinity — WiFi on core 0, your code on core 1
• delay() yields to scheduler — vTaskDelay(pdMS_TO_TICKS(ms)) in tasks

Memory

• Heap fragments over time — preallocate buffers, avoid repeated malloc/free
• ESP.getFreeHeap() for monitoring — log periodically in long-running apps
• PSRAM available on some boards — heap_caps_malloc(size, MALLOC_CAP_SPIRAM)
• String concatenation fragments heap — use reserve() or char arrays

Peripherals

• No native analogWrite() — use LEDC: ledcSetup(), ledcAttachPin(), ledcWrite()
• I2C needs external pullups usually — internal pullups too weak for fast speeds
• SPI CS pin must be managed — SPI.begin() doesn't auto-configure
• UART0 is Serial/USB — use UART1/2 for external devices

OTA Updates

• Needs two OTA partitions — default partition scheme may have only one
• Check ESP.getFreeSketchSpace() — OTA fails silently if not enough space
• ArduinoOTA blocks during update — handle in loop, not in time-critical code

Power

• Brown-out detector resets at ~2.4V — esp_brownout_disable() if using battery
• WiFi TX uses 300mA peaks — power supply must handle spikes
• Deep sleep ~10µA — but RTC peripherals add more if enabled