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diff --git a/source/lang/constants.rst b/source/lang/constants.rst index b7521ed..bc5c894 100644 --- a/source/lang/constants.rst +++ b/source/lang/constants.rst @@ -1,11 +1,11 @@ .. _lang-constants: -constants +Constants ========= -Constants are predefined variables in the Arduino language. They -are used to make the programs easier to read. We classify constants -in groups. +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: @@ -15,288 +15,290 @@ in groups. Boolean Constants ----------------- -There are two constants used to represent truth and falsity in the -Arduino language: **true**, and **false**. +There are two constants used to represent truth and falsity: ``true``, +and ``false``. .. _lang-constants-false: false ^^^^^ -false is the easier of the two to define. false is defined as 0 -(zero). +``false`` is the false ``bool`` value. An integer which is 0 evaluates +to ``false`` as a boolean. .. _lang-constants-true: true ^^^^ -true is often said to be defined as 1, which is correct, but true -has a wider definition. Any integer which is *non-zero* is TRUE, in -a Boolean sense. So -1, 2 and -200 are all defined as true, too, in -a Boolean sense. +``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). -Note that the *true* and *false* constants are typed in lowercase -unlike HIGH, LOW, INPUT, & OUTPUT. +Pin Levels: HIGH and LOW +------------------------ -Defining Pin Levels, HIGH and LOW ---------------------------------- - -When reading or writing to a digital pin there are only two -possible values a pin can take/be-set-to: **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 a pin is set to an INPUT or OUTPUT. When a pin -is configured as an INPUT with pinMode, and read with digitalRead, -the microcontroller will report HIGH if a voltage of 3 volts or -more is present at the pin. - - +HIGH +^^^^ -A pin may also be configured as an INPUT with pinMode, and -subsequently made HIGH with digitalWrite, this will set the -internal 20K pullup resistors, which will *steer* the input pin to -a HIGH reading unless it is pulled LOW by external circuitry. +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 digitalWrite, the pin is at 5 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. +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** +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``. -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 pinMode, and read with digitalRead, the -microcontroller will report LOW if a voltage of 2 volts or less is -present at the pin. +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. - -When a pin is configured to OUTPUT with pinMode, and set to LOW -with digitalWrite, the pin is at 0 volts. In this state it can -*sink* current, e.g. light an LED that is connected through a -series resistor to, +5 volts, or to another pin configured as an -output, and set to HIGH. - - - -Defining Digital Pins, INPUT and OUTPUT ---------------------------------------- - -Digital pins can be used either as **INPUT** or **OUTPUT**. -Changing a pin from INPUT TO OUTPUT with 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: -Pins Configured as Inputs -^^^^^^^^^^^^^^^^^^^^^^^^^ +INPUT +^^^^^ -Arduino (Atmega) pins configured as **INPUT** with pinMode() 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, say equivalent to a series resistor -of 100 Megohms in front of the pin. This makes them useful for -reading a sensor, but not powering an LED. +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: -Pins Configured as Outputs -^^^^^^^^^^^^^^^^^^^^^^^^^^ - -Pins configured as **OUTPUT** with pinMode() are said to be in a -low-impedance state. This means that they can provide a substantial -amount of current to other circuits. Atmega pins can source -(provide positive current) or sink (provide negative current) up to -40 mA (milliamps) of current to other devices/circuits. This makes -them useful for powering LED's but useless for reading sensors. -Pins configured as outputs can also be damaged or destroyed if -short circuited to either ground or 5 volt power rails. The amount -of current provided by an Atmega pin is also not enough to power -most relays or motors, and some interface circuitry will be -required. - -.. _lang-constants-fp: - -Floating-Point Constants ------------------------- - -Similar to integer constants, floating point constants are used to -make code more readable. Floating point constants are swapped at -compile time for the value to which the expression evaluates. - -.. TODO explain that floating point literals are doubles - -.. _lang-constants-fp-f: - -.. TODO f modifiers - -Examples: +OUTPUT +^^^^^^ -``n = .005;`` +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. -Floating point constants can also be expressed in a variety of -scientific notation. 'E' and 'e' are both accepted as valid -exponent indicators. - -:: - - - floating-point evaluates to: also evaluates to: - constant - - 10.0 10 - 2.34E5 2.34 * 10^5 234000 - 67e-12 67.0 * 10^-12 .000000000067 +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 are numbers used directly in a sketch, like -``123``. By default, these numbers are treated as -`int <http://arduino.cc/en/Reference/Int>`_'s but you can change -this with the U and L modifiers (see below). - - - -Normally, integer constants are treated as base 10 (decimal) -integers, but special notation (formatters) may be used to enter -numbers in other bases. - - - -:: - - Base Example Formatter Comment - - 10 (decimal) 123 none - - 2 (binary) B1111011 leading 'B' only works with 8 bit values (0 to 255) - characters 0-1 valid - - 8 (octal) 0173 leading "0" characters 0-7 valid - - 16 (hexadecimal) 0x7B leading "0x" characters 0-9, A-F, a-f valid +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-sense math with which -you are acquainted. Constants without other prefixes are assumed to -be in decimal format. +**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. - - -Example: -:: - - 101 // same as 101 decimal ((1 * 10^2) + (0 * 10^1) + 1) +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. - - - -Example: -:: - - B101 // same as 5 decimal ((1 * 2^2) + (0 * 2^1) + 1) - -The binary formatter only works on bytes (8 bits) between 0 (B0) -and 255 (B11111111). If it is convenient to input an int (16 bits) -in binary form you can do it a two-step procedure such as: +**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++). - - -:: - - myInt = (B11001100 * 256) + B10101010; // B11001100 is the high byte +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 -values are indicated by the prefix "0". +literals are indicated by the prefix ``0``. -Example: +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. - 0101 // same as 65 decimal ((1 * 8^2) + (0 * 8^1) + 1) +.. _lang-constants-integers-hex: -Warning -It is possible to generate a hard-to-find bug by (unintentionally) -including a leading zero before a constant and having the compiler -unintentionally interpret your constant as octal. +**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). -.. _lang-constants-integers-hex: +For example, the hexadecimal literal ``0x101`` is two hundred fifty +seven: 1×16\ :sup:`2` + 0×16\ :sup:`1` + 1×16\ :sup:`0` = 257. -**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". -Note that A-F may be syted in upper or lower case (a-f). +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: -Example: +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: - 0x101 // same as 257 decimal ((1 * 16^2) + (0 * 16^1) + 1) +- a ``u`` or ``U`` to interpret the constant as an unsigned value. + For example, ``33U`` is an :ref:`unsigned int <lang-unsignedint>`. -.. _lang-constants-integers-u-l: +- an ``l`` or ``L`` to interpret the constant as a long value. For + example, ``100000L`` is a :ref:`long <lang-long>`. -U & L formatters -^^^^^^^^^^^^^^^^ +- a ``ul`` or ``UL`` to do both. For example, ``32767UL`` is an + :ref:`unsigned long <lang-unsignedlong>`. + +.. _lang-constants-fp: -By default, an integer constant is treated as an -`int <http://arduino.cc/en/Reference/Int>`_ with the attendant -limitations in values. To specify an integer constant with another -data type, follow it with: +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: -- a 'u' or 'U' to force the constant into an unsigned data format. - Example: ``33u`` -- a 'l' or 'L' to force the constant into a long data format. - Example: ``100000L`` -- a 'ul' or 'UL' to force the constant into an unsigned long - constant. Example: ``32767ul`` +.. list-table:: + :header-rows: 1 + * - Floating-point literal + - Evaluates to + - Alternate expression + * - ``10.0`` + - 10 + - -See also --------- + * - ``2.34E5`` + - 2.34×10\ :sup:`5` + - ``234000.0`` + * - ``67e-12`` + - 67.0×10\ :sup:`-12` + - ``0.000000000067`` -- `pinMode() <http://arduino.cc/en/Reference/PinMode>`_ -- `Integer Constants <http://arduino.cc/en/Reference/IntegerConstants>`_ -- `boolean variables <http://arduino.cc/en/Reference/BooleanVariables>`_ -- `#define <http://arduino.cc/en/Reference/Define>`_ -- `byte <http://arduino.cc/en/Reference/Byte>`_ -- `int <http://arduino.cc/en/Reference/Int>`_ -- `unsigned int <http://arduino.cc/en/Reference/UnsignedInt>`_ -- `long <http://arduino.cc/en/Reference/Long>`_ -- `unsigned long <http://arduino.cc/en/Reference/UnsignedLong>`_ +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 |