Real-Time Clock

This section covers functions related to the SAM L22's real-time clock peripheral, including date, time and alarm functions. More...

Data Structures
union	watch_date_time

Macros
#define	WATCH_RTC_REFERENCE_YEAR   (2020)

Typedefs
typedef enum watch_rtc_alarm_match	watch_rtc_alarm_match

Enumerations
enum	watch_rtc_alarm_match { ALARM_MATCH_DISABLED = 0 , ALARM_MATCH_SS , ALARM_MATCH_MMSS , ALARM_MATCH_HHMMSS }

Functions
bool	_watch_rtc_is_enabled (void)
	Called by main.c to check if the RTC is enabled. You may call this function, but outside of app_init, it should always return true.
void	watch_rtc_set_date_time (watch_date_time date_time)
	Sets the date and time.
watch_date_time	watch_rtc_get_date_time (void)
	Returns the date and time.
void	watch_rtc_register_alarm_callback (ext_irq_cb_t callback, watch_date_time alarm_time, watch_rtc_alarm_match mask)
	Registers an alarm callback that will be called when the RTC time matches the target time, as masked by the provided mask.
void	watch_rtc_disable_alarm_callback (void)
	Disables the alarm callback.
void	watch_rtc_register_tick_callback (ext_irq_cb_t callback)
	Registers a "tick" callback that will be called once per second.
void	watch_rtc_disable_tick_callback (void)
	Disables the tick callback for the given period.
void	watch_rtc_register_periodic_callback (ext_irq_cb_t callback, uint8_t frequency)
	Registers a callback that will be called at a configurable period.
void	watch_rtc_disable_periodic_callback (uint8_t frequency)
	Disables the tick callback for the given period.
void	watch_rtc_disable_matching_periodic_callbacks (uint8_t mask)
	Disables tick callbacks for the given periods (as a bitmask).
void	watch_rtc_disable_all_periodic_callbacks (void)
	Disables all periodic callbacks, including the once-per-second tick callback.
void	watch_rtc_enable (bool en)
	Enable/disable RTC while in-flight. This is quite dangerous operation, so we repeat writing register twice. Used when temporarily pausing RTC when adjusting subsecond, which are not accessible otherwise.
void	watch_rtc_freqcorr_write (int16_t value, int16_t sign)
	Adjusts frequency correction in single register write. Not waiting for syncronisation to save power - if you won't write new correction value in the same ~millisecond - will not cause issue.

Detailed Description

This section covers functions related to the SAM L22's real-time clock peripheral, including date, time and alarm functions.

The real-time clock is the only peripheral that main.c enables for you. It is the cornerstone of low power operation on the watch, and it is required for several key functions that we assume will be available, namely the wake from BACKUP mode and the callback on the ALARM button. It is also required for the operation of the 1 Hz tick interrupt, which you will most likely use to wake from STANDBY mode.

Function Documentation

watch_rtc_disable_matching_periodic_callbacks()

void watch_rtc_disable_matching_periodic_callbacks

(

uint8_t

mask

)

Disables tick callbacks for the given periods (as a bitmask).

Parameters

mask	The frequencies of tick callbacks you wish to disable, in Hz. The 128 Hz callback is 0b1, the 64 Hz callback is 0b10, the 32 Hz callback is 0b100, etc.

watch_rtc_disable_periodic_callback()

void watch_rtc_disable_periodic_callback

(

uint8_t

frequency

)

Disables the tick callback for the given period.

Parameters

frequency

The frequency of the tick you wish to disable, in Hz. Must be a power of 2, from 1 to 128.

watch_rtc_get_date_time()

watch_date_time watch_rtc_get_date_time

(

void

)

Returns the date and time.

Returns

A watch_date_time with the current date and time, with a year value from 0-63 representing 2020-2083.

See also

watch_rtc_set_date_time for notes about how the year is stored.

watch_rtc_register_alarm_callback()

void watch_rtc_register_alarm_callback	(	ext_irq_cb_t	callback,
		watch_date_time	alarm_time,
		watch_rtc_alarm_match	mask
	)

Registers an alarm callback that will be called when the RTC time matches the target time, as masked by the provided mask.

Parameters

callback	The function you wish to have called when the alarm fires. If this value is NULL, the alarm interrupt will still be enabled, but no callback function will be called.
alarm_time	The time that you wish to match. The date is currently ignored.
mask	One of the values in watch_rtc_alarm_match indicating which values to check.

The alarm interrupt is a versatile tool for scheduling events in the future, especially since it can wake the device from all sleep modes. The key to its versatility is the mask parameter. Suppose we set an alarm for midnight, 00:00:00.

• if mask is ALARM_MATCH_SS, the alarm will fire every minute when the clock ticks to seconds == 0.
• with ALARM_MATCH_MMSS, the alarm will once an hour, at the top of each hour.
• with ALARM_MATCH_HHMMSS, the alarm will fire at midnight every day. In theory the SAM L22's alarm function can match on days, months and even years, but I have not had success with this yet; as such, I am omitting these options for now.

watch_rtc_register_periodic_callback()

void watch_rtc_register_periodic_callback	(	ext_irq_cb_t	callback,
		uint8_t	frequency
	)

Registers a callback that will be called at a configurable period.

Parameters

callback	The function you wish to have called at the specified period. If you pass in NULL, the periodic interrupt will still be enabled, but no callback function will be called.
frequency	The frequency of the tick in Hz. Must be a power of 2, from 1 to 128 inclusive.

Note

A 1 Hz tick (

See also

watch_rtc_register_tick_callback) is suitable for most applications, in that it gives you a chance to update the display once a second — an ideal update rate for a watch! If however you are displaying a value (such as an accelerometer output) that updates more frequently than once per second, you may want to tick at 16 or 32 Hz to update the screen more quickly. Just remember that the more frequent the tick, the more power your Application Framework will consume. Ideally you should enable the fast tick only when the user requires it (i.e. in response to an input event), and move back to the slow tick after some time.

Also note that the RTC peripheral does not have sub-second resolution, so even if you set a 2 or 4 Hz interval, the system will not have any way of telling you where you are within a given second; watch_rtc_get_date_time will return the exact same timestamp until the second ticks over.

watch_rtc_register_tick_callback()

void watch_rtc_register_tick_callback

(

ext_irq_cb_t

callback

)

Registers a "tick" callback that will be called once per second.

Parameters

callback

The function you wish to have called when the clock ticks. If you pass in NULL, the tick interrupt will still be enabled, but no callback function will be called.

Note

this is equivalent to calling watch_rtc_register_periodic_callback with a frequency of 1. It can be disabled with either watch_rtc_disable_tick_callback() or watch_rtc_disable_periodic_callback(1), and will also be disabled when watch_rtc_disable_all_periodic_callbacks is called.

watch_rtc_set_date_time()

void watch_rtc_set_date_time

(

watch_date_time

date_time

)

Sets the date and time.

Parameters

date_time

The date and time you wish to set, with a year value from 0-63 representing 2020-2083.

Note

The SAM L22 stores the year as six bits representing a value from 0 to 63. It treats this as a year offset from a reference year, which must be a leap year. Since 2020 was a leap year, and it allows useful dates through 2083, it is assumed that watch apps will use 2020 as the reference year; thus 1 means 2021, 2 means 2022, etc. You will be responsible for handling this offset in your code, if the calendar year is needed for timestamp calculation logic or display purposes.