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void TIM3_PWM_PA6_init(void) {
  // enable GPIOA, TIM3 clocks
  RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN;
  RCC->APB1ENR |= RCC_APB1ENR_TIM3EN;

  // PA6 = AF push-pull
  GPIOA->MODER &= ~GPIO_MODER_MODER6;
  GPIOA->MODER |= GPIO_MODER_MODER6_1;         // alternate function
  GPIOA->OSPEEDR |= GPIO_OSPEEDER_OSPEEDR6_1;  // 50 MHz (fast speed)

  GPIOA->AFR[0] &= ~GPIO_AFRL_AFSEL6;
  GPIOA->AFR[0] |= GPIO_AFRL_AFSEL6_1;  // 0b0010 -> AF2 = TIM3_CH1

  // TIM3 PWM -> 750 Hz
  TIM3->PSC = 1120 - 1; // 84000000 Hz / 1120 = 75000 Hz
  TIM3->ARR = 100 - 1;  // 75000 Нz / 100 = 750 Hz
  TIM3->CCR1 = 50;      // 50 % duty

  TIM3->CCMR1 |= TIM_CCMR1_OC1M_1 | TIM_CCMR1_OC1M_2;  // 0b110 -> PWM mode 1
  TIM3->CCMR1 |= TIM_CCMR1_OC1PE;
  TIM3->CCER |= TIM_CCER_CC1E;

  TIM3->EGR |= TIM_EGR_UG;   // update generation
  TIM3->CR1 |= TIM_CR1_CEN;  // counter enable
}

  while (1) {
    TIM3->CCER |= TIM_CCER_CC1E;   // tone
    delay_us(500000);

    TIM3->CCER &= ~TIM_CCER_CC1E;  // no tone
    delay_us(500000);
  }

char *cw_message = "VVV ...";

const uint8_t s[39] = {
  0b00000001,  // A
  0b00001000,  // B
  0b00001010,  // C
  0b00000100,  // D
...

const uint8_t d[39] = {
  2, 4, 4, 3,
...

const uint8_t morse_table[39] = {
  0b00000101,  // A .-
  0b00011000,  // B -...
  0b00011010,  // C -.-.
  0b00001100,  // D -..
  0b00000010,  // E .
  0b00010010,  // F ..-.
...
  0b00111110,  // 9 ----.
...

0b00000101 -> 01 -> А
...
0b00010010 -> 0010 -> F
...
0b00111110 -> 11110 -> 9
...

  uint32_t hours, minutes, seconds;
  uint32_t day, year;
  char month_str[4]; 
...
  // __TIME__  -> hh:mm:ss
  sscanf(__TIME__, "%d:%d:%d", &hours, &minutes, &seconds);

  // __DATE__  -> M mm dd yyyy
  sscanf(__DATE__, "%s %d %d", month_str, &day, &year);