Source Code

#include <esp_task_wdt.h>
#include <esp_sleep.h>
#include <WiFi.h> // Added to ensure radio can be explicitly shut down

// 4 hours in milliseconds, 20 hours in microseconds for deep sleep
#define ON_TIME_MS 14400000 
#define OFF_TIME_US 72000000000ULL 

// Updated for XIAO pin nomenclature
const int ledPin = D0; 
const int pwmFreq = 5000;
const int pwmRes = 8; // 0-255 brightness

void setup() {
  // Explicitly disable the power-hungry dual-core radios
  WiFi.mode(WIFI_OFF);

  // 80MHz is the lowest stable clock for the S3 without causing peripheral timing issues
  setCpuFrequencyMhz(80); 

  // Core 3.x Watchdog Timer Initialization for Dual-Core (S3)
  esp_task_wdt_config_t twdt_config = {
    .timeout_ms = 5000,
    .idle_core_mask = (1 << portNUM_PROCESSORS) - 1, // Monitors both cores
    .trigger_panic = true,
  };
  esp_task_wdt_init(&twdt_config);
  esp_task_wdt_add(NULL);
  
  // Core 3.x PWM Initialization 
  ledcAttach(ledPin, pwmFreq, pwmRes);

  unsigned long startTime = millis();

  // Run flicker loop for exactly 4 hours
  while (millis() - startTime < ON_TIME_MS) {
    esp_task_wdt_reset(); // Feed the watchdog
    
    // Lowered bounds: Flicker between ~4% and ~23% brightness
    int brightness = random(10, 60); 
    ledcWrite(ledPin, brightness); 
    
    delay(random(20, 150)); 
  }

  // Turn off LED to prevent parasitic draw before sleeping
  ledcWrite(ledPin, 0);
  
  // Enter Deep Sleep for 20 hours
  esp_sleep_enable_timer_wakeup(OFF_TIME_US);
  esp_deep_sleep_start();
}

void loop() {
  // Execution never reaches here due to deep sleep
}