diff --git a/docs/library/utime.rst b/docs/library/utime.rst index f3a067cdef..933e70ac57 100644 --- a/docs/library/utime.rst +++ b/docs/library/utime.rst @@ -57,10 +57,10 @@ Functions .. function:: sleep(seconds) - Sleep for the given number of seconds. Some boards may accept `seconds` as a + 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. + them use `sleep_ms()` and `sleep_us()` functions. .. function:: sleep_ms(ms) @@ -73,30 +73,32 @@ Functions .. 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 + 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 + 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. + `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 + 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. + Just like `ticks_ms()` above, but in microseconds. .. function:: ticks_cpu() - Similar to ``ticks_ms()`` and ``ticks_us()``, but with the highest possible resolution + 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 @@ -111,13 +113,13 @@ Functions .. 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`` + 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 + (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:: @@ -136,23 +138,25 @@ Functions .. 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 + 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 + 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) + `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 + [*-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 + *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 + 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 @@ -164,10 +168,10 @@ Functions 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: + `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()``:: + * 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() @@ -175,8 +179,8 @@ Functions 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:: + * 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() @@ -192,8 +196,8 @@ Functions 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) + 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 . @@ -205,8 +209,8 @@ Functions 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. + `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