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