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diff --git a/watch-library/hal/documentation/calendar.rst b/watch-library/hal/documentation/calendar.rst new file mode 100644 index 00000000..8a3de6e8 --- /dev/null +++ b/watch-library/hal/documentation/calendar.rst @@ -0,0 +1,72 @@ +=============================== +The Calendar driver (bare-bone) +=============================== + +The Calendar driver provides means to set and get current date and time. +After enabling, an instance of the driver starts counting time from the base date with +the resolution of one second. The default base date is 00:00:00 1st of January 1970. +Only the base year of the base date can be changed via the driver API. + +The current date and time is kept internally in a relative form as the difference between +current date and time and the base date and time. This means that changing the base year changes +current date. + +The base date and time defines time "zero" or the earliest possible point in time that the calender driver can describe, +this means that current time and alarms can not be set to anything earlier than this time. + +The Calendar driver provides alarm functionality. +An alarm is a software trigger which fires on particular date and time with particular periodicity. +Upon firing the given callback function is called. + +An alarm can be in single-shot mode, firing only once at matching time; or in repeating mode, meaning that it will +reschedule a new alarm automatically based on repeating mode configuration. +In single-shot mode an alarm is removed from the alarm queue before its callback is called. It allows an application to +reuse the memory of expired alarm in the callback. + +An alarm can be triggered on the following events: match on second, minute, hour, day, month or year. +Matching on second means that the alarm is triggered when the value of seconds of the current time is equal to +the alarm's value of seconds. This means repeating alarm with match on seconds is triggered with the period of a minute. +Matching on minute means that the calendars minute and seconds values has to match the alarms, the rest of the date-time +value is ignored. In repeating mode this means a new alarm every hour. +The same logic is applied to match on hour, day, month and year. + +Each instance of the Calendar driver supports infinite amount of software alarms, only limited by the amount of RAM available. + +Features +-------- +* Initialization and de-initialization +* Enabling and disabling +* Date and time operations +* Software alarms + +Applications +------------ +* A source of current date and time for an embedded system. +* Periodical functionality in low-power applications since the driver is designed to use 1Hz clock. +* Periodical function calls in case if it is more convenient to operate with absolute time. + +Dependencies +------------ +* This driver expects a counter to be increased by one every second to count date and time correctly. +* Each instance of the driver requires separate hardware timer. + +Concurrency +----------- +The Calendar driver is an interrupt driven driver.This means that the interrupt that triggers an alarm may occur during +the process of adding or removing an alarm via the driver's API. In such case the interrupt processing is postponed +until the alarm adding or removing is complete. + +The alarm queue is not protected from the access by interrupts not used by the driver. Due to this +it is not recommended to add or remove an alarm from such interrupts: in case if a higher priority interrupt supersedes +the driver's interrupt, adding or removing an alarm may cause unpredictable behavior of the driver. + +Limitations +----------- +* Only years divisible by 4 are deemed a leap year, this gives a correct result between the years 1901 to 2099. +* The driver is designed to work outside of an operating system environment, the software alarm queue is therefore processed in interrupt context which may delay execution of other interrupts. +* If there are a lot of frequently called interrupts with the priority higher than the driver's one, it may cause delay in alarm's triggering. +* Changing the base year or setting current date or time does not shift alarms' date and time accordingly or expires alarms. + +Knows issues and workarounds +---------------------------- +Not applicable |