Constants are predefined variables in the Energia 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 Energia language: true, and false.
false is the easier of the two to define. false is defined as 0 (zero).
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.
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 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.
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, +3 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
LaunchPad 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. LaunchPad 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 3 volt power rails. The amount of current provided by a LaunchPad pin is also not enough to power most relays or motors, and some interface circuitry will be required.
Pins Configured as Input Pullup
LaunchPad pins can be configured as INPUT_PULLUP. Often it is useful to steer an input pin to a known state if no input is present. This can be done by adding a pullup resistor (to ~3V), or a pulldown resistor (resistor to ground) on the input, with 10K being a common value. There are also convenient pullup resistors built into the LaunchPad that can be accessed from software. These built-in pullup resistors are accessed in the following manner.
pinMode(pin, INPUT_PULLUP); // set pin to input and enable internal pullup resistor
Pins Configured as Input Pulldown
Pins can be configured to support pulldown resistors.
NOTE: Launchpad on-board button does not support INPUT_PULLDOWN for PinMode().
pinMode(pin, INPUT_PULLDOWN); // set pin to input and enable internal pulldown resistor