ESP32

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