Arduino General Overview

No doubt, Arduino became the most widespread name in the development boards world, particularly among enthusiasts, educators, amateurs, and hobbyists, driving de-facto the embedded systems market for years.

Using cheap Atmel AVR microcontrollers, delivered along with development board and peripherals of almost any kind, including sensors and actuators, where you do not need to develop your PCB nor solder to obtain the fully functional device, all that triggered a new era where almost anyone can afford to have a development set and start playing the way only professionals used to do. Moreover, Arduino was not only the hardware but also the programming idea, delivering a simple development environment that is easy to use for beginners. Perhaps the most crucial impact of the Arduino on daily use was to spread the idea of taking automation control from the industry and bringing it on a massive scale to regular life, homes, cars, and toys to automate daily life.

The beginnings of the Arduino are dated to the year 2003 in Italy. Their most popular development board was delivered to the market in the fall of 2010. While AVRs microcontrollers are considered to be embedded systems more than IoT, and most of the early Arduino boards didn't offer any network interface, even then, it is essential to understand the idea of how to work with SoCs, so we start our guide here. However, many extension boards are suitable for the standard development boards (so-called shields) that offer wired and wireless networking for Arduino. Also, their clones, made mainly by Chinese manufacturers, evolved into more sophisticated products, integrating, e.g. Arduino Mega 2560 and ESP8266 SoC into one development board.

Initially, all Arduino development boards were using ATMEL's MCUs. It is no longer the case due to the demand for integrated radio communication that ATMEL's MCUs lack.

At the moment of writing this book, the Arduino family contains 4 main branches:

• Nano: the tiniest yet powerful boards, newer models containing integrated radio modules such as Bluetooth and WiFi. Many 3rd party clones are available worldwide. A wide choice of shields provides the capability to extend the system with sensors and actuators. Depending on the board, you can find ATMEL's ATmegas, RP2040 or ARM Cortex-based ones.
• MKR: much bigger than Nano, providing broader wireless connectivity capabilities, including LoRa, Sigfox and NB-IoT. All of those boards use ARM Cortex M0, 32-bit MCU.
• Mega: the biggest development boards with many GPIO pins, efficiently allocating, e.g. dot matric displays that need to be connected with parallel interface. There are currently 3 family members, each using a different MCU: the original ATmega2560, ARM Cortex M3 and STM32.
• Classic: the most recognisable shape of the development boards still driving the look of the embedded systems and IoT: Arduino Uno's development board shape. The family uses ATmegas, Reneas and ARM Cortex M0+ MCUs.

There are also a dozen retired products that are still present on the market, such as the LilyPad series, which was intended to become intelligent jewellery and smart clothing, or the Yun series - the first of real IoT devices made by Arduino, that were designed to run Linux distribution.

The Arduino boards work by reacting on signals at inputs that are received from various sensors, and after executing a set of instructions, an output is generated to respond to the environment. The input signal can be generated by pressing a button, receiving the radio or light signal, hearing the sound, perceiving an image of the situation using a camera resulting from the environmental sensor measurement, and many others. The output actions in the environment use output elements like actuators, blinking LEDs, audio devices, and others. The set of instructions executed to handle both sensors and actuators is created using the Arduino programming language based on an open-source programming framework called Wiring and the Arduino Software (IDE) based on Processing. The microcontroller or System on Chip is the most crucial element in the IoT and embedded devices built nowadays. It is not common to add peripheral elements external to the microcontroller, so the choice of this element influences almost all hardware parameters and the set of peripherals of the board. Because many versions of Arduino boards are available, only their selection based on the AVR family of microcontrollers is presented in the following chapters.

The initial, still very popular version of the Arduino board - Arduino Uno, is based on the ATmega328P microcontroller. The same chip is used in, e.g. Arduino Nano and Pro Mini. Arduino Leonardo or Micro is based on ATmega32u4, which has a built-in USB interface. The Arduino Mega board is created with an extended microcontroller ATmega2560, which has many more interface pins.

There are three different types of memory on the Arduino board: flash memory, SRAM and EEPROM. They are usually built into the main microcontroller, so their type determines the amount of memory available. A list of memory sizes regarding the microcontroller type is presented in table 1.

The flash memory stores the Arduino code, a non-volatile type of memory. That means the information in the memory is not deleted when the power is turned off.

The SRAM (static random access memory) is used for storing variables' values when the Arduino program is running. This volatile memory keeps information only until the power is turned off or the board is reset.

The EEPROM (electrically erasable programmable read-only memory) is a non-volatile type of memory that can be used as long-term memory storage.

Peripherals are all functional units which play the roles of external elements of the CPU. Arduino boards are mainly implemented internally in the microcontroller, so the number and type of peripherals depend on the microcontroller version. Peripherals include Timers, Communication and networking interfaces, GPIOs, Analog comparators and converters, and supervisory units.

The basic Arduino boards do not implement any networking connectivity. This capability to use Ethernet, WiFi, Bluetooth, ZigBee, and other wireless protocols can be added with an external module or shield. Example shields are Arduino Ethernet Shield, WiFly Shield, Arduino WiFi Shield, Electric Imp Shield, XBee Shield, Cellular Shield SM5100B and GPS Shield. In the simplest version, the WiFi module like Espressif ESP01S can be connected to Arduino's serial port and programmed with AT commands.

Communication interfaces for Arduino are used to send and receive information to and from other external devices. Standard interfaces for Arduino are UART, I2C (also called TWI - Two-Wire Interface), SPI, and USB.

Timers are implemented as the essential elements of almost every microcontroller. These units can operate in timer mode or counter mode. In the first mode, they count pulses generated internally in the microcontroller. This makes it possible to generate square signals of specified frequency, signal periodic interrupts, or generate pulse width modulated signals at PWM outputs. In counter mode, counting the number of external pulses is possible. In selected Arduino boards, there are 8-bit and 16-bit timers, an additional real-time clock with a separate generator, and a watchdog timer that can work as a supervisory unit which resets the microcontroller in case of software hang-up. The list of interfaces and timers is presented in table 2.

Arduino boards do not contain specialised video chips. Their memory size does not allow them to generate, capture, or even store complex high-resolution images. The most common approach to display images is connecting the LCD, OLED or TFT display with an SPI port. Connecting the camera is even more complicated. None of the microcontrollers used in basic Arduino boards have an adequate camera port to convey high-speed video signals. An answer to this challenge is the Arducam, which implements the camera and the hardware to capture the image to the RAM. It can be connected to an Arduino board with an SPI interface, allowing it to read and process the image data at the main processor speed.

Digital Input/Output Pins
Digital input/output (I/O) pins are contacts on the Arduino board that can receive or transmit a digital signal. The status of the pin can be set either to 0, which represents LOW signal or to 1 – HIGH signal. The maximum current of the pin output is 40 mA.

Pulse Width Modulation
Pulse Width Modulation (PWM) is a function of a pin to generate a square wave signal with a variable length of the HIGH level of the output signal. The PWM is used for digital pins to simulate the analogue output.

Analog Pins
Analog pins convert the analogue input value to a 10-bit number using Analog Digital Converter (ADC). This function maps the input voltage between 0 and the reference voltage to numbers between 0 and 1023. By default, the reference voltage is set to a microcontroller operating voltage. Usually, it is 5 V or 3.3 V. Also, other internal or external reference sources, for example, AREF pin, can be used.

A list of pins and hardware interfaces for popular Arduino boards is present in table 3.

Power and Other Pins

• Power pins on the Arduino board connect the power source to the microcontroller and/or voltage regulators. They can also be a power source for external components and devices.
• The VIN pin connects the external power source to the internal regulator to provide the regulated 5 V output. The input voltage of the board must be within the specific range, mainly between 7 V and 12 V.
• The 5V pin is used to supply a microcontroller with the regulated 5 V from the external source or is used as a power source for the external components in the case when the board is already powered using the USB interface or the VIN pin.
• The 3V3 pin provides the regulated 3.3 V output for the board components and external devices. The GND (ground pin) is where the negative terminal of the power supply is applied.
• The reset pin and button reset the Arduino board and the program. Resetting using the reset pin is done by connecting it to the GND.