242 lines
11 KiB
ReStructuredText
242 lines
11 KiB
ReStructuredText
:mod:`utime` -- time related functions
|
|
======================================
|
|
|
|
.. module:: utime
|
|
:synopsis: time related functions
|
|
|
|
|see_cpython_module| :mod:`python:time`.
|
|
|
|
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:: gmtime([secs])
|
|
localtime([secs])
|
|
|
|
Convert the time *secs* 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.
|
|
|
|
The `gmtime()` function returns a date-time tuple in UTC, and `localtime()` returns a
|
|
date-time tuple in local time.
|
|
|
|
The format of the entries in the 8-tuple are:
|
|
|
|
* 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.
|
|
|
|
.. function:: sleep(seconds)
|
|
|
|
Sleep for the given number of seconds. Some boards may accept *seconds* as a
|
|
floating-point number to sleep for a fractional number of seconds. Note that
|
|
other boards may not accept a floating-point argument, for compatibility with
|
|
them use `sleep_ms()` and `sleep_us()` functions.
|
|
|
|
.. 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.
|
|
|
|
The wrap-around 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 be 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 `ticks_ms()`, `ticks_us()`, or `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(ticks_add(time.ticks_ms(), -100))
|
|
|
|
# Calculate deadline for operation and test for it
|
|
deadline = ticks_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(ticks_add(0, -1))
|
|
|
|
|
|
.. function:: ticks_diff(ticks1, ticks2)
|
|
|
|
Measure ticks difference between values returned from `ticks_ms()`, `ticks_us()`,
|
|
or `ticks_cpu()` functions, as a signed value which may wrap around.
|
|
|
|
The argument order is the same as for subtraction
|
|
operator, ``ticks_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* occurred earlier in time than *ticks2*. Otherwise, it means that
|
|
*ticks1* occurred 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 two 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
|
|
behaviour: 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(scheduled_time, now) > 0:
|
|
print("Too early, let's nap")
|
|
sleep_ms(ticks_diff(scheduled_time, now))
|
|
task.run()
|
|
elif ticks_diff(scheduled_time, now) == 0:
|
|
print("Right at time!")
|
|
task.run()
|
|
elif ticks_diff(scheduled_time, now) < 0:
|
|
print("Oops, running late, tell task to run faster!")
|
|
task.run(run_faster=true)
|
|
|
|
Note: Do not pass `time()` values to `ticks_diff()`, you 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, absolute
|
|
timestamps, use `time_ns()`. If relative times are acceptable then use the
|
|
`ticks_ms()` and `ticks_us()` functions. If you need calendar time, `gmtime()` or
|
|
`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).
|
|
|
|
.. function:: time_ns()
|
|
|
|
Similar to `time()` but returns nanoseconds since the Epoch, as an integer (usually
|
|
a big integer, so will allocate on the heap).
|