Usually, the gpio16 is not used as gpio.
But we can read the state or change the state by writing directly to the registers.
WRITE_PERI_REG(PAD_XPD_DCDC_CONF, (READ_PERI_REG(PAD_XPD_DCDC_CONF) & 0xffffffbc) | (uint32)0x1);
WRITE_PERI_REG(RTC_GPIO_CONF, (READ_PERI_REG(RTC_GPIO_CONF) & (uint32)0xfffffffe) | (uint32)0x0);
WRITE_PERI_REG(RTC_GPIO_ENABLE, READ_PERI_REG(RTC_GPIO_ENABLE) & (uint32)0xfffffffe);
(READ_PERI_REG(RTC_GPIO_IN_DATA) & 1);
Is there a way to enable interrupts on this pin by writing directly to the registers?