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230 lines
11 KiB
230 lines
11 KiB
:mod:`utime` -- time related functions
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======================================
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.. module:: utime
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:synopsis: time related functions
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The ``utime`` module provides functions for getting the current time and date,
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measuring time intervals, and for delays.
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**Time Epoch**: Unix port uses standard for POSIX systems epoch of
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1970-01-01 00:00:00 UTC. However, embedded ports use epoch of
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2000-01-01 00:00:00 UTC.
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**Maintaining actual calendar date/time**: This requires a
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Real Time Clock (RTC). On systems with underlying OS (including some
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RTOS), an RTC may be implicit. Setting and maintaining actual calendar
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time is responsibility of OS/RTOS and is done outside of MicroPython,
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it just uses OS API to query date/time. On baremetal ports however
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system time depends on ``machine.RTC()`` object. The current calendar time
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may be set using ``machine.RTC().datetime(tuple)`` function, and maintained
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by following means:
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* By a backup battery (which may be an additional, optional component for
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a particular board).
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* Using networked time protocol (requires setup by a port/user).
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* Set manually by a user on each power-up (many boards then maintain
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RTC time across hard resets, though some may require setting it again
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in such case).
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If actual calendar time is not maintained with a system/MicroPython RTC,
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functions below which require reference to current absolute time may
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behave not as expected.
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Functions
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---------
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.. function:: localtime([secs])
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Convert a time expressed in seconds since the Epoch (see above) into an 8-tuple which
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contains: (year, month, mday, hour, minute, second, weekday, yearday)
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If secs is not provided or None, then the current time from the RTC is used.
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* year includes the century (for example 2014).
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* month is 1-12
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* mday is 1-31
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* hour is 0-23
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* minute is 0-59
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* second is 0-59
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* weekday is 0-6 for Mon-Sun
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* yearday is 1-366
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.. function:: mktime()
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This is inverse function of localtime. It's argument is a full 8-tuple
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which expresses a time as per localtime. It returns an integer which is
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the number of seconds since Jan 1, 2000.
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.. only:: port_unix or port_pyboard or port_esp8266
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.. function:: sleep(seconds)
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Sleep for the given number of seconds. Seconds can be a floating-point number to
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sleep for a fractional number of seconds. Note that other MicroPython ports may
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not accept floating-point argument, for compatibility with them use ``sleep_ms()``
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and ``sleep_us()`` functions.
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.. only:: port_wipy
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.. function:: sleep(seconds)
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Sleep for the given number of seconds.
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.. only:: port_unix or port_pyboard or port_wipy or port_esp8266
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.. function:: sleep_ms(ms)
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Delay for given number of milliseconds, should be positive or 0.
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.. function:: sleep_us(us)
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Delay for given number of microseconds, should be positive or 0
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.. function:: ticks_ms()
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Returns an increasing millisecond counter with an arbitrary reference point,
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that wraps around after some value. This value is not explicitly exposed,
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but we will refer to it as `TICKS_MAX` to simplify discussion. Period of
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the values is `TICKS_PERIOD = TICKS_MAX + 1`. `TICKS_PERIOD` is guaranteed
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to be a power of two, but otherwise may differ from port to port. The same
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period value is used for all of ticks_ms(), ticks_us(), ticks_cpu() functions
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(for simplicity). Thus, these functions will return a value in range
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[0 .. `TICKS_MAX`], inclusive, total `TICKS_PERIOD` values. Note that only
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non-negative values are used. For the most part, you should treat values
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returned by these functions as opaque. The only operations available for them
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are ``ticks_diff()`` and ``ticks_add()`` functions described below.
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Note: Performing standard mathematical operations (+, -) or relational
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operators (<, <=, >, >=) directly on these value will lead to invalid
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result. Performing mathematical operations and then passing their results
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as arguments to ``ticks_diff()`` or ``ticks_add()`` will also lead to
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invalid results from the latter functions.
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.. function:: ticks_us()
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Just like ``ticks_ms`` above, but in microseconds.
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.. function:: ticks_cpu()
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Similar to ``ticks_ms`` and ``ticks_us``, but with the highest possible resolution
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in the system. This is usually CPU clocks, and that's why the function is named that
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way. But it doesn't have to a CPU clock, some other timing source available in a
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system (e.g. high-resolution timer) can be used instead. The exact timing unit
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(resolution) of this function is not specified on ``utime`` module level, but
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documentation for a specific port may provide more specific information. This
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function is intended for very fine benchmarking or very tight real-time loops.
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Avoid using it in portable code.
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Availability: Not every port implements this function.
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.. function:: ticks_add(ticks, delta)
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Offset ticks value by a given number, which can be either positive or negative.
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Given a ``ticks`` value, this function allows to calculate ticks value ``delta``
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ticks before or after it, following modular-arithmetic definition of tick values
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(see ``ticks_ms()`` above). ``ticks`` parameter must be a direct result of call
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to ``tick_ms()``, ``ticks_us()``, ``ticks_cpu()`` functions (or from previous
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call to ``ticks_add()``). However, ``delta`` can be an arbitrary integer number
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or numeric expression. ``ticks_add()`` is useful for calculating deadlines for
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events/tasks. (Note: you must use ``ticks_diff()`` function to work with
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deadlines.)
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Examples::
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# Find out what ticks value there was 100ms ago
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print(tick_add(time.ticks_ms(), -100))
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# Calculate deadline for operation and test for it
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deadline = tick_add(time.ticks_ms(), 200)
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while ticks_diff(deadline, time.ticks_ms()) > 0:
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do_a_little_of_something()
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# Find out TICKS_MAX used by this port
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print(tick_add(0, -1))
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.. function:: ticks_diff(ticks1, ticks2)
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Measure ticks difference between values returned from ticks_ms(), ticks_us(), or ticks_cpu()
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functions. The argument order is the same as for subtraction operator,
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``tick_diff(ticks1, ticks2)`` has the same meaning as ``ticks1 - ticks2``. However, values returned by
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ticks_ms(), etc. functions may wrap around, so directly using subtraction on them will
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produce incorrect result. That is why ticks_diff() is needed, it implements modular
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(or more specifically, ring) arithmetics to produce correct result even for wrap-around
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values (as long as they not too distant inbetween, see below). The function returns
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**signed** value in the range [`-TICKS_PERIOD/2` .. `TICKS_PERIOD/2-1`] (that's a typical
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range definition for two's-complement signed binary integers). If the result is negative,
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it means that `ticks1` occured earlier in time than `ticks2`. Otherwise, it means that
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`ticks1` occured after `ticks2`. This holds `only` if `ticks1` and `ticks2` are apart from
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each other for no more than `TICKS_PERIOD/2-1` ticks. If that does not hold, incorrect
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result will be returned. Specifically, if 2 tick values are apart for `TICKS_PERIOD/2-1`
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ticks, that value will be returned by the function. However, if `TICKS_PERIOD/2` of
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real-time ticks has passed between them, the function will return `-TICKS_PERIOD/2`
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instead, i.e. result value will wrap around to the negative range of possible values.
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Informal rationale of the constraints above: Suppose you are locked in a room with no
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means to monitor passing of time except a standard 12-notch clock. Then if you look at
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dial-plate now, and don't look again for another 13 hours (e.g., if you fall for a
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long sleep), then once you finally look again, it may seem to you that only 1 hour
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has passed. To avoid this mistake, just look at the clock regularly. Your application
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should do the same. "Too long sleep" metaphor also maps directly to application
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behavior: don't let your application run any single task for too long. Run tasks
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in steps, and do time-keeping inbetween.
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``ticks_diff()`` is designed to accommodate various usage patterns, among them:
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Polling with timeout. In this case, the order of events is known, and you will deal
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only with positive results of ``ticks_diff()``::
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# Wait for GPIO pin to be asserted, but at most 500us
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start = time.ticks_us()
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while pin.value() == 0:
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if time.ticks_diff(time.ticks_us(), start) > 500:
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raise TimeoutError
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Scheduling events. In this case, ``ticks_diff()`` result may be negative
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if an event is overdue::
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# This code snippet is not optimized
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now = time.ticks_ms()
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scheduled_time = task.scheduled_time()
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if ticks_diff(now, scheduled_time) > 0:
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print("Too early, let's nap")
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sleep_ms(ticks_diff(now, scheduled_time))
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task.run()
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elif ticks_diff(now, scheduled_time) == 0:
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print("Right at time!")
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task.run()
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elif ticks_diff(now, scheduled_time) < 0:
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print("Oops, running late, tell task to run faster!")
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task.run(run_faster=true)
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Note: Do not pass ``time()`` values to ``ticks_diff()``, and should use
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normal mathematical operations on them. But note that ``time()`` may (and will)
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also overflow. This is known as https://en.wikipedia.org/wiki/Year_2038_problem .
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.. function:: time()
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Returns the number of seconds, as an integer, since the Epoch, assuming that underlying
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RTC is set and maintained as described above. If an RTC is not set, this function returns
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number of seconds since a port-specific reference point in time (for embedded boards without
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a battery-backed RTC, usually since power up or reset). If you want to develop portable
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MicroPython application, you should not rely on this function to provide higher than second
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precision. If you need higher precision, use ``ticks_ms()`` and ``ticks_us()`` functions,
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if you need calendar time, ``localtime()`` without an argument is a better choice.
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.. admonition:: Difference to CPython
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:class: attention
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In CPython, this function returns number of
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seconds since Unix epoch, 1970-01-01 00:00 UTC, as a floating-point,
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usually having microsecond precision. With MicroPython, only Unix port
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uses the same Epoch, and if floating-point precision allows,
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returns sub-second precision. Embedded hardware usually doesn't have
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floating-point precision to represent both long time ranges and subsecond
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precision, so they use integer value with second precision. Some embedded
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hardware also lacks battery-powered RTC, so returns number of seconds
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since last power-up or from other relative, hardware-specific point
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(e.g. reset).
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