# RTC Overview - [Introduction](#section104842041574) - [Available APIs](#section16892932155715) ## Introduction The real-time clock \(RTC\) driver provides precise real time for the operating system \(OS\). If the OS is powered off, the RTC driver continues to keep track of the system time using an external battery. ## Available APIs **Table 1** APIs provided by the RTC driver

Capability	Function	Description
RTC handle	RtcOpen	Opens the RTC device to obtain its handle.
RTC handle	RtcClose	Releases a specified handle of the RTC device.
RTC time	RtcReadTime	Reads time information from the RTC driver, including the year, month, the day of the week, day, hour, minute, second, and millisecond.
RTC time	RtcWriteTime	Writes time information from the RTC driver, including the year, month, the day of the week, day, hour, minute, second, and millisecond.
RTC alarm	RtcReadAlarm	Reads the RTC alarm time that was set last time.
	RtcWriteAlarm	Writes the RTC alarm time based on the alarm index.
	RtcRegisterAlarmCallback	Registers RtcAlarmCallback that will be invoked when an alarm is generated at the specified time.
	RtcAlarmInterruptEnable	Enables or disables RTC alarm interrupts.
RTC configuration	RtcGetFreq	Reads the frequency of the external crystal oscillator connected to the RTC driver.
	RtcSetFreq	Sets the frequency of the external crystal oscillator connected to the RTC driver.
	RtcReset	Resets the RTC.
Custom register	RtcReadReg	Reads the configuration of a custom RTC register based on the register index.
Custom register	RtcWriteReg	Writes the configuration of a custom RTC register based on the register index.

>![](public_sys-resources/icon-note.gif) **NOTE:** >All functions provided in this document can be called only in kernel mode.