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diff --git a/source/arduino/constants.rst b/source/arduino/constants.rst new file mode 100644 index 0000000..49f3933 --- /dev/null +++ b/source/arduino/constants.rst @@ -0,0 +1,144 @@ +.. _arduino-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. + + + +Defining Logical Levels, true and false (Boolean Constants) +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +There are two constants used to represent truth and falsity in the +Arduino language: **true**, and **false**. + + + +false +----- + +false is the easier of the two to define. false is defined as 0 +(zero). + + + +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. + + + +Note that the *true* and *false* constants are typed in lowercase +unlike HIGH, LOW, INPUT, & OUTPUT. + + + +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**. + + + +**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. + + + +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. + + + +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. + + + +**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 pinMode, and read with 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 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. + + + +Pins Configured as Inputs +------------------------- + +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. + + + +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. + + + +See also +-------- + + +- `pinMode() <http://arduino.cc/en/Reference/PinMode>`_ +- `Integer Constants <http://arduino.cc/en/Reference/IntegerConstants>`_ +- `boolean variables <http://arduino.cc/en/Reference/BooleanVariables>`_ + + |