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diff --git a/source/lang/api/constants.rst b/source/lang/api/constants.rst new file mode 100644 index 0000000..bc5c894 --- /dev/null +++ b/source/lang/api/constants.rst @@ -0,0 +1,304 @@ +.. _lang-constants: + +Constants +========= + +Constants are like predefined variables, whose values can't +change. They are used to make the programs easier to read and modify. +This page describes the most commonly used constants. + +.. contents:: Contents + :local: + +.. _lang-constants-bool: + +Boolean Constants +----------------- + +There are two constants used to represent truth and falsity: ``true``, +and ``false``. + +.. _lang-constants-false: + +false +^^^^^ + +``false`` is the false ``bool`` value. An integer which is 0 evaluates +to ``false`` as a boolean. + +.. _lang-constants-true: + +true +^^^^ + +``true`` is the true ``bool`` value. As an integer, ``true`` is often +said to be 1. This is correct in the sense that ``true`` evaluates to +1 as an integer. However, any integer which is *non-zero* is ``true`` +as a :ref:`bool <lang-booleanvariables>`. So -1, 2 and -200 are all +"true", in the sense that these numbers are treated the same as +``true`` in a boolean context. + +Note that the ``true`` and ``false`` constants are typed in lowercase; +unlike e.g. ``HIGH``, ``LOW``, ``INPUT``, and ``OUTPUT`` (which are +described below). + + +Pin Levels: HIGH and LOW +------------------------ + +When reading or writing to a digital pin there are only two possible +values a pin can be set to: ``HIGH`` and ``LOW``. + +.. _lang-constants-high: + +HIGH +^^^^ + +The meaning of ``HIGH`` (in reference to a pin) is somewhat different +depending on whether the pin is set to ``INPUT`` or ``OUTPUT``. When a +pin is configured as an ``INPUT`` (using :ref:`pinMode() +<lang-pinmode>`), and read with :ref:`digitalRead() +<lang-digitalread>`, the microcontroller will report ``HIGH`` if a +voltage of 3 volts or more is present at the pin. + +.. TODO? Following seems false; check it out sometime, leave out for now: + +.. A pin may also be configured as an ``INPUT`` with ``pinMode()``, and +.. subsequently made ``HIGH`` with :ref:`digitalWrite() +.. <lang-digitalwrite>`, this will set the internal pullup resistors, +.. which will *steer* the input pin to a HIGH reading unless it is pulled +.. LOW by external circuitry. + +When a pin is configured to ``OUTPUT`` with pinMode, and set to +``HIGH`` with :ref:`digitalWrite() <lang-digitalwrite>`, the pin is at +3.3 volts. In this state it can *source* current, e.g. light an LED +that is connected through a series resistor to ground, or to another +pin configured as an output and set to ``LOW``. + +.. _lang-constants-low: + +LOW +^^^ + +The meaning of ``LOW`` also has a different meaning depending on +whether a pin is set to ``INPUT`` or ``OUTPUT``. When a pin is +configured as an ``INPUT`` with :ref:`pinMode() <lang-pinmode>`, and +read with :ref:`digitalRead() <lang-digitalread>`, the microcontroller +will report ``LOW`` if a voltage of 2 volts or less is present at the +pin. + +When a pin is configured to ``OUTPUT`` with ``pinMode()``, and set to +``LOW`` with :ref:`digitalWrite() <lang-digitalwrite>`, the +microcontroller will attempt to keep that pin's voltage at 0V. In this +state it can *sink* current, e.g. light an LED that is connected +through a series resistor to +3.3V, or to another pin configured as an +output, and set to ``HIGH``. + +Pin Modes +--------- + +Digital pins can be used in a variety of modes. The basic modes, +``INPUT`` and ``OUTPUT``, have been introduced above. Changing a pin +from ``INPUT`` TO ``OUTPUT`` with :ref:`pinMode() <lang-pinmode>` +drastically changes the electrical behavior of the pin. + +This section describes the basic digital pin modes (``INPUT`` and +``OUTPUT``) only. For a detailed description of all possible pin +modes, see the :ref:`pinMode() <lang-pinmode>` reference page. + +.. _lang-constants-input: + +INPUT +^^^^^ + +Maple (STM32) pins configured as ``INPUT`` are said to be in a +high-impedance state. One way of explaining this is that pins +configured as ``INPUT`` make extremely small demands on the circuit +that they are sampling. This makes them useful for reading a sensor, +but not powering an LED. + +.. _lang-constants-output: + +OUTPUT +^^^^^^ + +Pins configured as ``OUTPUT`` with :ref:`pinMode() <lang-pinmode>` are +said to be in a low-impedance state. This means that they can provide +a substantial amount of current to other circuits. STM32 pins can +source (provide positive current) or sink (provide negative current) +up to 50 mA (milliamps) of current to other devices/circuits. This +makes them useful for powering LEDs, but useless for reading sensors. + +Pins configured as outputs can also be damaged or destroyed if short +circuited to either ground or 3.3V power rails. The amount of current +provided by an STM32 pin is also not enough to power most relays or +motors, and some interface circuitry will be required. + +.. _lang-constants-integers: + +Integer Constants +----------------- + +Integer constants (or more properly speaking, integer *literals*) are +numbers used directly in a sketch, like ``123``. By default, an +integer literal is treated as a (signed) :ref:`int <lang-int>`, but +you can change this with the U and L modifiers (see :ref:`below +<lang-constants-integers-u-l>`). You can specify negative numbers by +putting a minus sign in front, like ``-123``. + +Normally, integer literals are treated as base 10 (decimal) integers, +but special notation (formatters) may be used to enter numbers in +other bases. These are summarized in the following table: + +.. list-table:: + :header-rows: 1 + + * - Base + - Example + - Formatter + - Comment + + * - 10 (decimal) + - ``123`` + - None + - + + * - 2 (binary) + - ``0b1111011`` + - Leading "0b" + - GCC extension; not standard C++ + + * - 8 (octal) + - ``0173`` + - Leading "0" + - Characters 0-7 valid + + * - 16 (hexadecimal) + - ``0x7B`` + - Leading "0x" + - Characters 0-9, A-F (or a-f) valid + +Binary constants (like ``B1111011``) for values between 0 and 255 are +supported for compatibility with Arduino only. Their use in new +programs is discouraged. + +.. _lang-constants-integers-dec: + +**Decimal** is base 10. This is the common number system we learn in +school. Integer literals without other prefixes are assumed to be in +decimal format. + +For example, the decimal literal ``101`` is one hundred and one: 1×10\ +:sup:`2` + 0×10\ :sup:`1` + 1×10\ :sup:`0` = 101. + +.. _lang-constants-integers-bin: + +**Binary** is base two. Only characters 0 and 1 are valid. Binary +literals are indicated by the prefix ``0b`` (this is a :ref:`GCC +<arm-gcc>` extension; it's not standard C++). + +For example, the binary literal ``0b101`` is five: 1×2\ :sup:`2` + +0×2\ :sup:`1` + 1×2\ :sup:`0` = 5. + +.. _lang-constants-integers-oct: + +**Octal** is base eight. Only characters 0 through 7 are valid. Octal +literals are indicated by the prefix ``0``. + +For example, the octal literal ``0101`` is sixty five: 1×8\ :sup:`2` + +0×8\ :sup:`1` + 1×8\ :sup:`0` = 65. + +.. warning:: Bugs sometimes result by (unintentionally) including a + leading "0" before an integer literal, which makes the compiler + interpret it in octal. + +.. _lang-constants-integers-hex: + +**Hexadecimal** (or "hex") is base sixteen. Valid characters are 0 +through 9 and letters A through F; A has the value 10, B is 11, up to +F, which is 15. Hex values are indicated by the prefix ``0x``. A-F +may be typed in upper or lower case (a-f). + +For example, the hexadecimal literal ``0x101`` is two hundred fifty +seven: 1×16\ :sup:`2` + 0×16\ :sup:`1` + 1×16\ :sup:`0` = 257. + +The hexadecimal literal ``0xCF2`` is three thousand, three hundred +fourteen: 12×16\ :sup:`2` + 15×16\ :sup:`1` + 2×16\ :sup:`0` = 3314. + +(Remember that in hex, ``A`` means 10, and counting up, ``B``\ =11, so +``C``\ =12 and ``F``\ =15). + +.. _lang-constants-integers-u-l: + +U and L Suffixes +^^^^^^^^^^^^^^^^ + +By default, an integer constant is treated as an :ref:`int +<lang-int>`, with the attendant :ref:`limitations in values +<lang-int-overflow>`. To specify an integer constant with another data +type, follow it with: + +- a ``u`` or ``U`` to interpret the constant as an unsigned value. + For example, ``33U`` is an :ref:`unsigned int <lang-unsignedint>`. + +- an ``l`` or ``L`` to interpret the constant as a long value. For + example, ``100000L`` is a :ref:`long <lang-long>`. + +- a ``ul`` or ``UL`` to do both. For example, ``32767UL`` is an + :ref:`unsigned long <lang-unsignedlong>`. + +.. _lang-constants-fp: + +Floating-Point Constants +------------------------ + +Similar to integer literals, floating point constants (properly: +floating-point *literals*) are used to make code more readable. +Floating point literals are swapped at compile time for the value to +which the expression evaluates. + +A floating point literal is any number which includes a decimal point. +For instance, ``3.0`` is a floating-point literal for the number 3. +By default, a floating-point literal is a :ref:`double <lang-double>`. +In order for the literal to be interpreted as a :ref:`float +<lang-float>`, you can write ``f`` directly after it. For example, +``3.0f`` is a floating-point literal with type ``float``. + +Floating point constants can also be expressed in a variety of +scientific notation. ``E`` and ``e`` are both accepted as valid +exponent indicators. Some examples are given in the following table: + + +.. list-table:: + :header-rows: 1 + + * - Floating-point literal + - Evaluates to + - Alternate expression + + * - ``10.0`` + - 10 + - + + * - ``2.34E5`` + - 2.34×10\ :sup:`5` + - ``234000.0`` + + * - ``67e-12`` + - 67.0×10\ :sup:`-12` + - ``0.000000000067`` + +See Also +-------- + +- :ref:`pinMode() <lang-pinmode>` +- :ref:`Boolean Variables <lang-booleanvariables>` +- :ref:`#define <lang-define>` +- :ref:`int <lang-int>` +- :ref:`unsigned int <lang-unsignedint>` +- :ref:`long <lang-long>` +- :ref:`unsigned long <lang-unsignedlong>` +- :ref:`float <lang-float>` +- :ref:`double <lang-double>` + +.. include:: cc-attribution.txt |