circuitpython/docs/library/utime.rst

:mod:`utime` -- time related functions
======================================

.. module:: utime
   :synopsis: time related functions

The ``utime`` module provides functions for getting the current time and date,
measuring time intervals, and for delays.

**Time Epoch**: Unix port uses standard for POSIX systems epoch of
1970-01-01 00:00:00 UTC. However, embedded ports use epoch of
2000-01-01 00:00:00 UTC.

**Maintaining actual calendar date/time**: This requires a
Real Time Clock (RTC). On systems with underlying OS (including some
RTOS), an RTC may be implicit. Setting and maintaining actual calendar
time is responsibility of OS/RTOS and is done outside of MicroPython,
it just uses OS API to query date/time. On baremetal ports however
system time depends on ``machine.RTC()`` object. The current calendar time
may be set using ``machine.RTC().datetime(tuple)`` function, and maintained
by following means:

* By a backup battery (which may be an additional, optional component for
  a particular board).
* Using networked time protocol (requires setup by a port/user).
* Set manually by a user on each power-up (many boards then maintain
  RTC time across hard resets, though some may require setting it again
  in such case).

If actual calendar time is not maintained with a system/MicroPython RTC,
functions below which require reference to current absolute time may
behave not as expected.

Functions
---------

.. function:: localtime([secs])

   Convert a time expressed in seconds since the Epoch (see above) into an 8-tuple which
   contains: (year, month, mday, hour, minute, second, weekday, yearday)
   If secs is not provided or None, then the current time from the RTC is used.

   * year includes the century (for example 2014).
   * month   is 1-12
   * mday    is 1-31
   * hour    is 0-23
   * minute  is 0-59
   * second  is 0-59
   * weekday is 0-6 for Mon-Sun
   * yearday is 1-366

.. function:: mktime()

   This is inverse function of localtime. It's argument is a full 8-tuple
   which expresses a time as per localtime. It returns an integer which is
   the number of seconds since Jan 1, 2000.

.. only:: port_unix or port_pyboard or port_esp8266

    .. function:: sleep(seconds)
    
       Sleep for the given number of seconds.  Seconds can be a floating-point number to
       sleep for a fractional number of seconds. Note that other MicroPython ports may
       not accept floating-point argument, for compatibility with them use ``sleep_ms()``
       and ``sleep_us()`` functions.

.. only:: port_wipy

    .. function:: sleep(seconds)
    
       Sleep for the given number of seconds.

.. only:: port_unix or port_pyboard or port_wipy or port_esp8266

    .. function::  sleep_ms(ms)

       Delay for given number of milliseconds, should be positive or 0.

    .. function::  sleep_us(us)

       Delay for given number of microseconds, should be positive or 0

    .. function::  ticks_ms()

        Returns an increasing millisecond counter with an arbitrary reference point,
        that wraps around after some value. This value is not explicitly exposed,
        but we will refer to it as `TICKS_MAX` to simplify discussion. Period of
        the values is `TICKS_PERIOD = TICKS_MAX + 1`. `TICKS_PERIOD` is guaranteed
        to be a power of two, but otherwise may differ from port to port. The same
        period value is used for all of ticks_ms(), ticks_us(), ticks_cpu() functions
        (for simplicity). Thus, these functions will return a value in range
        [0 .. `TICKS_MAX`], inclusive, total `TICKS_PERIOD` values. Note that only
        non-negative values are used. For the most part, you should treat values
        returned by these functions as opaque. The only operations available for them
        are ``ticks_diff()`` and ``ticks_add()`` functions described below.

        Note: Performing standard mathematical operations (+, -) or relational
        operators (<, <=, >, >=) directly on these value will lead to invalid
        result. Performing mathematical operations and then passing their results
        as arguments to ``ticks_diff()`` or ``ticks_add()`` will also lead to
        invalid results from the latter functions.

    .. function::  ticks_us()

       Just like ``ticks_ms`` above, but in microseconds.

.. function::  ticks_cpu()

   Similar to ``ticks_ms`` and ``ticks_us``, but with the highest possible resolution
   in the system. This is usually CPU clocks, and that's why the function is named that
   way. But it doesn't have to a CPU clock, some other timing source available in a
   system (e.g. high-resolution timer) can be used instead. The exact timing unit
   (resolution) of this function is not specified on ``utime`` module level, but
   documentation for a specific port may provide more specific information. This
   function is intended for very fine benchmarking or very tight real-time loops.
   Avoid using it in portable code.

   Availability: Not every port implements this function.


.. function::  ticks_add(ticks, delta)

   Offset ticks value by a given number, which can be either positive or negative.
   Given a ``ticks`` value, this function allows to calculate ticks value ``delta``
   ticks before or after it, following modular-arithmetic definition of tick values
   (see ``ticks_ms()`` above). ``ticks`` parameter must be a direct result of call
   to ``tick_ms()``, ``ticks_us()``, ``ticks_cpu()`` functions (or from previous
   call to ``ticks_add()``). However, ``delta`` can be an arbitrary integer number
   or numeric expression. ``ticks_add()`` is useful for calculating deadlines for
   events/tasks. (Note: you must use ``ticks_diff()`` function to work with
   deadlines.)

   Examples::

        # Find out what ticks value there was 100ms ago
        print(tick_add(time.ticks_ms(), -100))

        # Calculate deadline for operation and test for it
        deadline = tick_add(time.ticks_ms(), 200)
        while ticks_diff(deadline, time.ticks_ms()) > 0:
            do_a_little_of_something()

        # Find out TICKS_MAX used by this port
        print(tick_add(0, -1))


.. function::  ticks_diff(ticks1, ticks2)

   Measure ticks difference between values returned from ticks_ms(), ticks_us(), or ticks_cpu()
   functions. The argument order is the same as for subtraction operator,
   ``tick_diff(ticks1, ticks2)`` has the same meaning as ``ticks1 - ticks2``. However, values returned by
   ticks_ms(), etc. functions may wrap around, so directly using subtraction on them will
   produce incorrect result. That is why ticks_diff() is needed, it implements modular
   (or more specifically, ring) arithmetics to produce correct result even for wrap-around
   values (as long as they not too distant inbetween, see below). The function returns
   **signed** value in the range [`-TICKS_PERIOD/2` .. `TICKS_PERIOD/2-1`] (that's a typical
   range definition for two's-complement signed binary integers). If the result is negative,
   it means that `ticks1` occured earlier in time than `ticks2`. Otherwise, it means that
   `ticks1` occured after `ticks2`. This holds `only` if `ticks1` and `ticks2` are apart from
   each other for no more than `TICKS_PERIOD/2-1` ticks. If that does not hold, incorrect
   result will be returned. Specifically, if 2 tick values are apart for `TICKS_PERIOD/2-1`
   ticks, that value will be returned by the function. However, if `TICKS_PERIOD/2` of
   real-time ticks has passed between them, the function will return `-TICKS_PERIOD/2`
   instead, i.e. result value will wrap around to the negative range of possible values.

   Informal rationale of the constraints above: Suppose you are locked in a room with no
   means to monitor passing of time except a standard 12-notch clock. Then if you look at
   dial-plate now, and don't look again for another 13 hours (e.g., if you fall for a
   long sleep), then once you finally look again, it may seem to you that only 1 hour
   has passed. To avoid this mistake, just look at the clock regularly. Your application
   should do the same. "Too long sleep" metaphor also maps directly to application
   behavior: don't let your application run any single task for too long. Run tasks
   in steps, and do time-keeping inbetween.

   ``ticks_diff()`` is designed to accommodate various usage patterns, among them:

   Polling with timeout. In this case, the order of events is known, and you will deal
   only with positive results of ``ticks_diff()``::

        # Wait for GPIO pin to be asserted, but at most 500us
        start = time.ticks_us()
        while pin.value() == 0:
            if time.ticks_diff(time.ticks_us(), start) > 500:
                raise TimeoutError

   Scheduling events. In this case, ``ticks_diff()`` result may be negative
   if an event is overdue::

        # This code snippet is not optimized
        now = time.ticks_ms()
        scheduled_time = task.scheduled_time()
        if ticks_diff(now, scheduled_time) > 0:
            print("Too early, let's nap")
            sleep_ms(ticks_diff(now, scheduled_time))
            task.run()
        elif ticks_diff(now, scheduled_time) == 0:
            print("Right at time!")
            task.run()
        elif ticks_diff(now, scheduled_time) < 0:
            print("Oops, running late, tell task to run faster!")
            task.run(run_faster=true)

   Note: Do not pass ``time()`` values to ``ticks_diff()``, and should use
   normal mathematical operations on them. But note that ``time()`` may (and will)
   also overflow. This is known as https://en.wikipedia.org/wiki/Year_2038_problem .


.. function:: time()

   Returns the number of seconds, as an integer, since the Epoch, assuming that underlying
   RTC is set and maintained as described above. If an RTC is not set, this function returns
   number of seconds since a port-specific reference point in time (for embedded boards without
   a battery-backed RTC, usually since power up or reset). If you want to develop portable
   MicroPython application, you should not rely on this function to provide higher than second
   precision. If you need higher precision, use ``ticks_ms()`` and ``ticks_us()`` functions,
   if you need calendar time, ``localtime()`` without an argument is a better choice.

   .. admonition:: Difference to CPython
      :class: attention

      In CPython, this function returns number of
      seconds since Unix epoch, 1970-01-01 00:00 UTC, as a floating-point,
      usually having microsecond precision. With MicroPython, only Unix port
      uses the same Epoch, and if floating-point precision allows,
      returns sub-second precision. Embedded hardware usually doesn't have
      floating-point precision to represent both long time ranges and subsecond
      precision, so they use integer value with second precision. Some embedded
      hardware also lacks battery-powered RTC, so returns number of seconds
      since last power-up or from other relative, hardware-specific point
      (e.g. reset).