void i2c_init(void) {
  rcc_periph_clock_enable(RCC_I2C1);
  rcc_periph_clock_enable(RCC_GPIOB);
  /* rcc_periph_clock_enable(RCC_AFIO); */

  gpio_set_mode(GPIOB,
                GPIO_MODE_OUTPUT_50_MHZ,
                GPIO_CNF_OUTPUT_ALTFN_OPENDRAIN,
                GPIO6 | GPIO7);

  i2c_peripheral_disable(I2C1);

  i2c_reset(I2C1);
  i2c_set_standard_mode(I2C1);

  // APB1 is running at 36MHz
  i2c_set_clock_frequency(I2C1, I2C_CR2_FREQ_36MHZ);

  // 36MHz / (100kHz * 2)
  i2c_set_ccr(I2C1, 180);

  // standard mode, freq MHz + 1
  i2c_set_trise(I2C1, 37);

  i2c_set_dutycycle(I2C1, I2C_CCR_DUTY_DIV2);

  /* nvic_enable_irq(NVIC_I2C1_EV_IRQ); */
  /* nvic_enable_irq(NVIC_I2C1_ER_IRQ); */
  /* i2c_enable_interrupt(I2C1, I2C_CR2_ITERREN | I2C_CR2_ITEVTEN); */

  i2c_peripheral_enable(I2C1);

  printf("i2c: intialized\n");
}