Essentials

There are a number of fundamental concepts and components essential for interaction with Arduino boards. The following is a very brief description of those most universal to the successful marriage of software and hardware.

• Binary (logic) signals: a digital signal that can either be high or low, corresponding to a 1 or 0 in logic terms and 5 or 0 volts (usually) in physical terms.
• Pins and ports: a pin is an electrical lead on the MCU that is connected to the PCB, also called a bit. They are organized in bunches, called ports, and can send or receive signals to or from a given system, respectively.
• Registers: a register occupies a space in memory that data can be read from or written to. Data read from registers often represent system inputs, while data written to them represent outputs. A port is a type of register.
• Special function registers (SFR): SFR's are similar to a traditional register, except they have special bits called control and flag bits. Writing to a control bit toggles native MCU functions, while reading from a flag bit typically indicates some status regarding a triggering condition in the MCU. These register functions ship with the chip as opposed to plain registers which inherit functionality from instruction logic.

A real synthesis of these basic elements can be illustrated by an Arduino and a mechanical push button used to light up an LED. Pressing the button completes a circuit that sends a 5 volt (high) signal to a pin connected to one end. The unpressed case sends the same 5 volt signal to ground and a low signal to the pin. Software instructs the MCU to read the signal from the memory address register (MAR) corresponding to the input pin and submit the binary signal to program logic. The logic maps the input to an output, responding through a different pin using its unique MAR. The output pin sends a 5 or 0 volt signal to the LED circuit, either turning it on or off depending on the MCU's instructions.