Language syntax
Variables
HIGH | 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 40K pullup resistors, which will steer the input pin to a HIGH reading unless it is pulled LOW by external circuitry. This is how INPUT_PULLUP works as well
When a pin is configured to OUTPUT with pinMode, and set to HIGH with 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.
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 1.5 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, +3.3 volts, or to another pin configured as an output, and set to HIGH.
INPUT, OUTPUT, INPUT_PULLUP, INPUT_PULLDOWN
Digital pins can be used as INPUT, INPUT_PULLUP, INPUT_PULLDOWN or OUTPUT. Changing a pin with pinMode() changes the electrical behavior of the pin.
Pins Configured as INPUT
The device's pins configured as INPUT with pinMode() are said to be in a high-impedance state. Pins configured as INPUT make extremely small demands on the circuit that they are sampling, 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.
If you have your pin configured as an INPUT, you will want the pin to have a reference to ground, often accomplished with a pull-down resistor (a resistor going to ground).
Pins Configured as INPUT_PULLUP or INPUT_PULLDOWN
The STM32 microcontroller has internal pull-up resistors (resistors that connect to power internally) and pull-down resistors (resistors that connect to ground internally) that you can access. If you prefer to use these instead of external resistors, you can use these argument in pinMode().
Pins Configured as OUTPUT
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. STM32 pins can source (provide positive current) or sink (provide negative current) up to 20 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 3.3 volt power rails. The amount of current provided by the pin is also not enough to power most relays or motors, and some interface circuitry will be required.
true | false
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.