While the Arduino family includes a diverse range of boards, most share a common set of core components and architectural features. This section details these common elements, using the popular Arduino UNO board as a reference model. Understanding this foundational architecture provides a baseline of knowledge applicable across the broader product family.

2.1 Power System

The power system provides flexible options for powering the board and external components.

• Power USB: This standard USB connection serves a dual purpose. It provides 5V power directly from a host computer and is the primary interface for programming the microcontroller.
• Power (Barrel Jack): This connector allows the board to be powered from an external DC power supply, typically an AC-to-DC adapter, eliminating the need for a host computer during standalone operation.
• Voltage Regulator: This component controls the voltage supplied to the board, ensuring a stable and consistent DC voltage for the microcontroller and other on-board elements.
• Power Pins: A set of pins provides direct access to regulated power for external circuits and components.
  • 3.3V: Supplies a regulated 3.3V output.
  • 5V: Supplies a regulated 5V output.
  • GND: Ground pins for completing electrical circuits. Multiple GND pins are provided for convenience.
  • Vin: This pin allows the board to be powered from an external, unregulated power source.

2.2 Core Processing and Timing

These components form the operational core of the board.

• Main Microcontroller: Functioning as the “brain” of the board, this Integrated Circuit (IC) executes the user-loaded program. The microcontrollers are typically sourced from the ATMEL Company, with the specific model varying by board.
• Crystal Oscillator: This component provides the primary timing reference for the microcontroller, enabling precise calculation of time-based operations. On an Arduino UNO, the oscillator has a frequency of 16 MHz.

2.3 Input/Output (I/O) and Connectivity

These features allow the Arduino to interact with sensors, actuators, and other external devices.

• Digital I/O Pins: These pins can be configured as either inputs to read logic values (0 or 1) or as outputs to drive external modules such as LEDs and relays. A subset of these pins, marked with a tilde symbol (~), are capable of generating Pulse Width Modulation (PWM) signals.
• Analog Pins: These pins are used to read signals from analog sensors, such as temperature or humidity sensors. They convert the incoming analog voltage into a digital value that the microcontroller can process.
• AREF (Analog Reference): This pin is used to set an external reference voltage (between 0 and 5 Volts) as the upper limit for the analog input pins, allowing for more precise analog-to-digital conversions.
• ICSP (In-Circuit Serial Programming) Pin: This is a programming header that exposes the Serial Peripheral Interface (SPI) bus. It allows for direct programming of the microcontroller, bypassing the standard USB interface if required.

2.4 On-Board Indicators and Controls

These components provide direct user feedback and control.

• Power LED Indicator: This LED illuminates when the board is correctly connected to a power source, providing a clear visual indication of its power status.
• TX and RX LEDs: These LEDs indicate serial communication activity. The TX (transmit) LED flashes when the board is sending data, and the RX (receive) LED flashes when it is receiving data.
• Reset Control: The board can be reset, restarting the program from the beginning, via two methods: a physical on-board reset button or by connecting an external control to the RESET pin.

Having established the common architectural features, the following sections will provide detailed specifications for boards based on specific microcontroller families, starting with the ATMEGA328.