• MCU: STM32L433CBT6
• Framework: Arduino STM32 core via PlatformIO
• Power model: sleep most of the time, wake on button, show LEDs for a short timeout
• Clock: 32.768 kHz LSE crystal on PC14/PC15 for RTC timekeeping

Ini
[env:genericSTM32L433CB]
platform = ststm32
board = genericSTM32L433CB
framework = arduino
upload_protocol = stlink
debug_tool = stlink
monitor_speed = 115200
build_flags =
    -D PIO_FRAMEWORK_ARDUINO_ENABLE_CDC

Cpp
#include <Arduino.h>
#include <STM32RTC.h>

STM32RTC& rtc = STM32RTC::getInstance();

#define LED_H1       PA0
#define LED_H2       PA1
#define LED_H4       PA2
#define LED_H8       PA3
#define LED_M1       PA4
#define LED_M2       PA5
#define LED_M4       PA6
#define LED_M8       PA7
#define LED_M16      PB0
#define LED_M32      PB1
#define LED_PM       PB10
#define LED_DECOR    PB11

#define BTN_DISPLAY  PB12
#define BTN_MODE     PB13
#define BTN_SET      PB14

const uint8_t hourLedPins[4] = {LED_H1, LED_H2, LED_H4, LED_H8};
const uint8_t minuteLedPins[6] = {LED_M1, LED_M2, LED_M4, LED_M8, LED_M16, LED_M32};
const uint8_t allLedPins[12] = {
  LED_H1, LED_H2, LED_H4, LED_H8,
  LED_M1, LED_M2, LED_M4, LED_M8, LED_M16, LED_M32,
  LED_PM, LED_DECOR
};

const uint32_t DISPLAY_TIMEOUT_MS = 5000;
uint32_t displayUntil = 0;
bool showing = false;

void allLedsOff() {
  for (uint8_t pin : allLedPins) {
    digitalWrite(pin, LOW);
  }
}

void showBinaryTime() {
  uint8_t hour24 = rtc.getHours();
  uint8_t minute = rtc.getMinutes();
  bool pm = hour24 >= 12;
  uint8_t hour12 = hour24 % 12;
  if (hour12 == 0) hour12 = 12;

  for (int i = 0; i < 4; i++) {
    digitalWrite(hourLedPins[i], (hour12 & (1 << i)) ? HIGH : LOW);
  }
  for (int i = 0; i < 6; i++) {
    digitalWrite(minuteLedPins[i], (minute & (1 << i)) ? HIGH : LOW);
  }
  digitalWrite(LED_PM, pm ? HIGH : LOW);
  digitalWrite(LED_DECOR, HIGH);

  showing = true;
  displayUntil = millis() + DISPLAY_TIMEOUT_MS;
}

bool pressed(uint8_t pin) {
  static uint32_t lastMs = 0;
  if (digitalRead(pin) == LOW && millis() - lastMs > 180) {
    lastMs = millis();
    return true;
  }
  return false;
}

void incrementHour() {
  uint8_t h = rtc.getHours();
  h = (h + 1) % 24;
  rtc.setHours(h);
}

void incrementMinute() {
  uint8_t m = rtc.getMinutes();
  m = (m + 1) % 60;
  rtc.setMinutes(m);
  rtc.setSeconds(0);
}

void setup() {
  for (uint8_t pin : allLedPins) {
    pinMode(pin, OUTPUT);
    digitalWrite(pin, LOW);
  }

  pinMode(BTN_DISPLAY, INPUT_PULLUP);
  pinMode(BTN_MODE, INPUT_PULLUP);
  pinMode(BTN_SET, INPUT_PULLUP);

  // Use external 32.768 kHz LSE crystal where supported by the board core.
  rtc.setClockSource(STM32RTC::LSE_CLOCK);
  rtc.begin();

  // First-program default. User can set using MODE/SET buttons.
  if (!rtc.isTimeSet()) {
    rtc.setTime(12, 0, 0);
    rtc.setDate(1, 1, 24);
  }
}

void loop() {
  if (pressed(BTN_DISPLAY)) {
    showBinaryTime();
  }

  // Simple setting mode while LEDs are awake:
  // MODE increments hours, SET increments minutes.
  if (showing && pressed(BTN_MODE)) {
    incrementHour();
    showBinaryTime();
  }
  if (showing && pressed(BTN_SET)) {
    incrementMinute();
    showBinaryTime();
  }

  if (showing && (int32_t)(millis() - displayUntil) >= 0) {
    allLedsOff();
    showing = false;
  }

  // For the first revision, keep this simple. Later firmware can enter STOP mode
  // and wake from EXTI on BTN_DISPLAY for much longer battery life.
  delay(20);
}

1. Install PlatformIO.
2. Connect ST-Link to SWD header: VDD, GND, SWDIO, SWCLK, NRST.
3. Build: pio run
4. Flash: pio run -t upload
5. If low-power optimization is needed, replace the simple delay(20) loop with STM32 STOP mode and wake on the display button EXTI interrupt.