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Real-Time Clock (RTC)

Overview

Glossary

Word

Definition

Real-time clock

Low power device tracking time using broken-down time

Real-time counter

Low power counter which can be used to track time

RTC

Acronym for real-time clock

An RTC is a low power device which tracks time using broken-down time. It should not be confused with low-power counters which sometimes share the same name, acronym, or both.

RTCs are usually optimized for low energy consumption and are usually kept running even when the system is in a low power state.

RTCs usually contain one or more alarms which can be configured to trigger at a given time. These alarms are commonly used to wake up the system from a low power state.

History of RTCs in Zephyr

RTCs have been supported before this API was created, using the Counter API. The unix timestamp was used to convert between broken-down time and the unix timestamp within the RTC drivers, which internally used the broken-down time representation.

The disadvantages of this approach were that hardware counters could not be set to a specific count, requiring all RTCs to use device specific APIs to set the time, converting from unix time to broken-down time, unnecessarily in some cases, and some common features missing, like input clock calibration and the update callback.

Configuration Options

Related configuration options:

API Reference

group rtc_interface

RTC Interface.

Since

3.4

Version

0.1.0

RTC Interface Alarm

int rtc_alarm_get_supported_fields(const struct device *dev, uint16_t id, uint16_t *mask)

API for getting the supported fields of the RTC alarm time.

Note

Bits in the mask param are defined here RTC_ALARM_TIME_MASK.

Parameters:

  • dev – Device instance

  • id – Id of the alarm

  • mask – Mask of fields in the alarm time which are supported

Returns:

0 if successful

Returns:

-EINVAL if id is out of range or time is invalid

Returns:

-ENOTSUP if API is not supported by hardware

Returns:

-errno code if failure

int rtc_alarm_set_time(const struct device *dev, uint16_t id, uint16_t mask, const struct rtc_time *timeptr)

API for setting RTC alarm time.

To enable an RTC alarm, one or more fields of the RTC alarm time must be enabled. The mask designates which fields of the RTC alarm time to enable. If the mask parameter is 0, the alarm will be disabled. The RTC alarm will trigger when all enabled fields of the alarm time match the RTC time.

Note

The timeptr param may be NULL if the mask param is 0

Note

Only the enabled fields in the timeptr param need to be configured

Note

Bits in the mask param are defined here RTC_ALARM_TIME_MASK

Parameters:

  • dev – Device instance

  • id – Id of the alarm

  • mask – Mask of fields in the alarm time to enable

  • timeptr – The alarm time to set

Returns:

0 if successful

Returns:

-EINVAL if id is out of range or time is invalid

Returns:

-ENOTSUP if API is not supported by hardware

Returns:

-errno code if failure

int rtc_alarm_get_time(const struct device *dev, uint16_t id, uint16_t *mask, struct rtc_time *timeptr)

API for getting RTC alarm time.

Note

Bits in the mask param are defined here RTC_ALARM_TIME_MASK

Parameters:

  • dev – Device instance

  • id – Id of the alarm

  • mask – Destination for mask of fields which are enabled in the alarm time

  • timeptr – Destination for the alarm time

Returns:

0 if successful

Returns:

-EINVAL if id is out of range

Returns:

-ENOTSUP if API is not supported by hardware

Returns:

-errno code if failure

int rtc_alarm_is_pending(const struct device *dev, uint16_t id)

API for testing if RTC alarm is pending.

Test whether or not the alarm with id is pending. If the alarm is pending, the pending status is cleared.

Parameters:

  • dev – Device instance

  • id – Id of the alarm to test

Returns:

1 if alarm was pending

Returns:

0 if alarm was not pending

Returns:

-EINVAL if id is out of range

Returns:

-ENOTSUP if API is not supported by hardware

Returns:

-errno code if failure

int rtc_alarm_set_callback(const struct device *dev, uint16_t id, rtc_alarm_callback callback, void *user_data)

API for setting alarm callback.

Setting the alarm callback for an alarm, will enable the alarm callback. When the callback for an alarm is enabled, the alarm triggered event will invoke the callback, after which the alarm pending status will be cleared automatically. The alarm will remain enabled until manually disabled using rtc_alarm_set_time().

To disable the alarm callback for an alarm, the callback and user_data parameters must be set to NULL. When the alarm callback for an alarm is disabled, the alarm triggered event will set the alarm status to “pending”. To check if the alarm status is “pending”, use rtc_alarm_is_pending().

Parameters:

  • dev – Device instance

  • id – Id of the alarm for which the callback shall be set

  • callback – Callback called when alarm occurs

  • user_data – Optional user data passed to callback

Returns:

0 if successful

Returns:

-EINVAL if id is out of range

Returns:

-ENOTSUP if API is not supported by hardware

Returns:

-errno code if failure

RTC Interface Update

int rtc_update_set_callback(const struct device *dev, rtc_update_callback callback, void *user_data)

API for setting update callback.

Setting the update callback will enable the update callback. The update callback will be invoked every time the RTC clock is updated by 1 second. It can be used to synchronize the RTC clock with other clock sources.

To disable the update callback for the RTC clock, the callback and user_data parameters must be set to NULL.

Parameters:

  • dev – Device instance

  • callback – Callback called when update occurs

  • user_data – Optional user data passed to callback

Returns:

0 if successful

Returns:

-ENOTSUP if API is not supported by hardware

Returns:

-errno code if failure

RTC Interface Calibration

int rtc_set_calibration(const struct device *dev, int32_t calibration)

API for setting RTC calibration.

Calibration is applied to the RTC clock input. A positive calibration value will increase the frequency of the RTC clock, a negative value will decrease the frequency of the RTC clock.

See also

rtc_calibration_from_frequency()

Parameters:

  • dev – Device instance

  • calibration – Calibration to set in parts per billion

Returns:

0 if successful

Returns:

-EINVAL if calibration is out of range

Returns:

-ENOTSUP if API is not supported by hardware

Returns:

-errno code if failure

int rtc_get_calibration(const struct device *dev, int32_t *calibration)

API for getting RTC calibration.

Parameters:

  • dev – Device instance

  • calibration – Destination for calibration in parts per billion

Returns:

0 if successful

Returns:

-ENOTSUP if API is not supported by hardware

Returns:

-errno code if failure

RTC Interface Helpers

static inline struct tm *rtc_time_to_tm(struct rtc_time *timeptr)

Convenience function for safely casting a rtc_time pointer to a tm pointer.

static inline int32_t rtc_calibration_from_frequency(uint32_t frequency)

Determine required calibration to 1 Hertz from frequency.

Parameters:

  • frequency – Frequency of the RTC in nano Hertz

Returns:

The required calibration in parts per billion

RTC Alarm Time Mask

Mask for alarm time fields to enable when setting alarm time

RTC_ALARM_TIME_MASK_SECOND
RTC_ALARM_TIME_MASK_MINUTE
RTC_ALARM_TIME_MASK_HOUR
RTC_ALARM_TIME_MASK_MONTHDAY
RTC_ALARM_TIME_MASK_MONTH
RTC_ALARM_TIME_MASK_YEAR
RTC_ALARM_TIME_MASK_WEEKDAY
RTC_ALARM_TIME_MASK_YEARDAY
RTC_ALARM_TIME_MASK_NSEC

Typedefs

typedef void (*rtc_update_callback)(const struct device *dev, void *user_data)

RTC update event callback.

Param dev:

Device instance invoking the handler

Param user_data:

Optional user data provided when update irq callback is set

typedef void (*rtc_alarm_callback)(const struct device *dev, uint16_t id, void *user_data)

RTC alarm triggered callback.

Param dev:

Device instance invoking the handler

Param id:

Alarm id

Param user_data:

Optional user data passed with the alarm configuration

Functions

int rtc_set_time(const struct device *dev, const struct rtc_time *timeptr)

API for setting RTC time.

Parameters:

  • dev – Device instance

  • timeptr – The time to set

Returns:

0 if successful

Returns:

-EINVAL if RTC time is invalid or exceeds hardware capabilities

Returns:

-errno code if failure

int rtc_get_time(const struct device *dev, struct rtc_time *timeptr)

API for getting RTC time.

Parameters:

  • dev – Device instance

  • timeptr – Destination for the time

Returns:

0 if successful

Returns:

-ENODATA if RTC time has not been set

Returns:

-errno code if failure

struct rtc_time
#include <rtc.h>

Structure for storing date and time values with sub-second precision.

The structure is 1-1 mapped to the struct tm for the members tm_sec to tm_isdst making it compatible with the standard time library.

Note

Use rtc_time_to_tm() to safely cast from a rtc_time pointer to a tm pointer.

Public Members

int tm_sec

Seconds [0, 59].

int tm_min

Minutes [0, 59].

int tm_hour

Hours [0, 23].

int tm_mday

Day of the month [1, 31].

int tm_mon

Month [0, 11].

int tm_year

Year - 1900.

int tm_wday

Day of the week [0, 6] (Sunday = 0) (Unknown = -1)

int tm_yday

Day of the year [0, 365] (Unknown = -1)

int tm_isdst

Daylight saving time flag [-1] (Unknown = -1)

int tm_nsec

Nanoseconds [0, 999999999] (Unknown = 0)

RTC device driver test suite

The test suite validates the behavior of the RTC device driver. It is designed to be portable between boards. It uses the device tree alias rtc to designate the RTC device to test.

This test suite tests the following:

  • Setting and getting the time.

  • RTC Time incrementing correctly.

  • Alarms if supported by hardware, with and without callback enabled

  • Calibration if supported by hardware.

The calibration test tests a range of values which are printed to the console to be manually compared. The user must review the set and gotten values to ensure they are valid.

By default, only the mandatory setting and getting of time is enabled for testing. To test the optional alarms, update event callback and clock calibration, these must be enabled by selecting CONFIG_RTC_ALARM, CONFIG_RTC_UPDATE and CONFIG_RTC_CALIBRATION.

The following examples build the test suite for the native_sim board. To build the test suite for a different board, replace the native_sim board with your board.

To build the test application with the default configuration, testing only the mandatory features, the following command can be used for reference:

# From the root of the zephyr repository
west build -b native_sim tests/drivers/rtc/rtc_api

To build the test with additional RTC features enabled, use menuconfig to enable the additional features by updating the configuration. The following command can be used for reference:

# From the root of the zephyr repository
west build -b native_sim tests/drivers/rtc/rtc_api
west build -t menuconfig

Then build the test application using the following command:

# From the root of the zephyr repository
west build -b native_sim tests/drivers/rtc/rtc_api

To run the test suite, flash and run the application on your board, the output will be printed to the console.

Note

The tests take up to 30 seconds each if they are testing real hardware.

RTC emulated device

The emulated RTC device fully implements the RTC API, and will behave like a real RTC device, with the following limitations:

  • RTC time is not persistent across application initialization.

  • RTC alarms are not persistent across application initialization.

  • RTC time will drift over time.

Every time an application is initialized, the RTC’s time and alarms are reset. Reading the time using rtc_get_time() will return -ENODATA, until the time is set using rtc_set_time(). The RTC will then behave as a real RTC, until the application is reset.

The emulated RTC device driver is built for the compatible zephyr,rtc-emul and will be included if CONFIG_RTC is selected.