Cpp
/*
 * Wireless-controller charging LED
 * Works with MCP73831 STAT pin (active-LOW while charging)
 * Target MCU: nRF52840, Arduino-core
 */

#include <Arduino.h>

/* ---------- Pin definitions (edit to match your board) ---------- */
#define PIN_STAT  11   // nRF52840 P0.11  <-- MCP73831 STAT
#define PIN_LED   13   // nRF52840 P0.13  <-- Green LED anode (active-HIGH)
/* ---------------------------------------------------------------- */

void setup()
{
  // STAT input: pull-up because MCP73831 is open-drain
  pinMode(PIN_STAT, INPUT_PULLUP);

  // LED output (start OFF)
  pinMode(PIN_LED, OUTPUT);
  digitalWrite(PIN_LED, LOW);

  // Optional: lower CPU speed for power-save if BLE not used
  // NRF_CLOCK->TASKS_HFCLKSTOP = 1;   // stop external 64 MHz crystal
}

void loop()
{
  bool charging = (digitalRead(PIN_STAT) == LOW);  // LOW = charging
  digitalWrite(PIN_LED, charging ? HIGH : LOW);

  // Sleep 250 ms; Arduino-nRF52 core will enter low-power idle
  delay(250);
}

Bash
arduino-cli board attach arduino:mbed_nano:nano33ble
arduino-cli compile --fqbn arduino:mbed_nano:nano33ble
arduino-cli upload  -p /dev/ttyACM0 --fqbn arduino:mbed_nano:nano33ble