What an embedded system is

An embedded system is a small computer hidden inside a product. It does one job for that product. A microwave has one. So does a TV remote, a washing machine and a car key fob. The owner never thinks "computer". They just press a button and the product reacts.

That hidden computer is usually a microcontroller, MCU for short. It is one chip that holds all the parts a tiny computer needs:

• The CPU, the central processing unit, follows the program's instructions one at a time.
• Flash memory stores the program. It keeps it even when the power is off, like a USB stick does.
• RAM, random-access memory, is scratch paper. The program writes numbers there while it runs. They vanish when the power goes.
• The peripherals are the chip's hands and ears. They switch pins on and off, keep time, and measure voltages.

Plug it in and it runs its one program straight away. The Uno R4 Minima, one of this course's two boards, runs at 48 MHz. That means 48 million ticks of its clock every second. It holds 256 kB of program and 32 kB of working memory. A kB is about a thousand bytes, and a byte holds one letter. Section 4 explains bytes properly.

A microprocessor is different. It is only the CPU. It needs other chips around it for memory, storage and power. Your laptop and your phone are built that way. Each is a full computer. It has a fast microprocessor and lots of RAM, often 16 GB. That is about half a million times the Uno's 32 kB. It also runs an operating system, such as Windows, macOS, Linux or Android. That is the big program that starts other programs and shares the machine between them.

So the difference is not just size. A microcontroller gives up speed and memory. In return it is one cheap chip. It starts almost instantly, can run for months on a battery, and connects straight to buttons, sensors and motors.

An embedded system is a computer built into a product to do one job, or a small set of jobs, as part of that product. The product's owner does not think of it as a computer: they press buttons on a microwave, and the computer inside reads them. Most of these computers are microcontrollers.

A microcontroller (often shortened to MCU, microcontroller unit) is a single chip that holds everything a small computer needs:

• a CPU (central processing unit), the part that follows instructions one at a time;
• flash memory, which stores the program and keeps it when the power is off;
• RAM (random-access memory), the working memory that holds values while the program runs and forgets them when power goes;
• peripherals, the hardware that connects the chip to the outside world: pins that can be switched on and off or read, timers, and circuits that measure a voltage.

Power the chip and it starts running the program in its flash at once. The course's 5 V board, the Arduino Uno R4 Minima, is built around a Renesas RA4M1 microcontroller running at 48 MHz (48 million clock ticks per second) with 256 kB of flash and 32 kB of RAM (Arduino docs). A kB is about a thousand bytes, and one byte holds a single letter of text. Section 4 builds bytes from scratch.

A microprocessor is a CPU chip on its own. To work it needs separate memory chips, storage, and power chips beside it on a circuit board. That is the design inside a laptop, a phone or a desktop computer: a fast microprocessor, gigabytes of RAM, and an operating system (Windows, macOS, Linux, Android) that loads and runs many programs and shares the hardware between them. A laptop's RAM is often 16 GB, about half a million times the Uno R4's 32 kB.

The line between the two is about design, not size. An MCU trades speed and memory for being one cheap chip that starts in milliseconds, runs for months on a battery, and talks directly to switches, sensors and motors.

Firmware is the program that lives inside a microcontroller. It sits in the chip's flash memory. It starts by itself the moment power arrives. No operating system runs underneath it, so it controls the pins and parts directly. It is written for one exact chip. It rarely changes, and an update means loading a whole new program.

An application, or app, is the kind of program you know from a laptop or phone. You start it yourself. The operating system loads it, lends it memory, and draws its windows for it. Lots of apps share one computer. You can add or delete each one.

One product often has both kinds. A smart thermostat has a microcontroller on the wall running firmware. The firmware reads the temperature and turns the heating on and off. The app on your phone sets the schedule. That app is an application. On the Arduino boards in this course, a firmware program is called a sketch. Chapter 11 has you write your first one.

Firmware is the program stored in a microcontroller's flash. It starts by itself when power arrives, runs without an operating system underneath it, and controls the hardware directly. It is written for one exact chip and board, and it changes rarely: a user updates it, if at all, by loading a whole new program.

An application (an app) is a program a user starts on a computer that already has an operating system. The operating system loads it, gives it memory, and handles the screen, files and network for it. Many applications share one computer, and each can be installed and removed on its own.

The same product often has both. A smart thermostat's microcontroller runs firmware that reads the temperature and switches the heating. The phone app that sets the schedule is an application. When either one is written for the Arduino boards in this course, the firmware is called a sketch, Arduino's word for a program; Chapter 11 writes the first one.

It helps to picture a connected product as four layers, stacked like floors in a building. Each floor rests on the one below it.

1. Hardware is the ground floor: the chip, the board, the sensors, the motors and the wires.
2. Firmware sits on top. It is the program in the microcontroller that runs that hardware.
3. Software comes next. These are apps on a phone or computer that talk to the device, by cable or by radio such as Bluetooth or Wi-Fi.
4. The cloud is the top floor. It is computers on the internet that gather data from many devices and send commands back.

Not every product has every floor. A microwave stops at firmware. A fitness band adds a phone app. A warehouse full of sensors feeding one web dashboard uses all four. This course builds the bottom two floors properly. It touches a little software for saving data on your computer. The cloud floor belongs to a later course called iot.

A connected product is easiest to think about as four layers, each built on the one below:

1. Hardware: the microcontroller, its board, sensors, motors, wires.
2. Firmware: the program on the microcontroller that runs the hardware.
3. Software: applications on phones and computers that talk to the device, for example over USB, Bluetooth or Wi-Fi.
4. Cloud: servers on the internet that collect data from many devices and pass commands back.

A product does not need all four. A microwave stops at firmware. A fitness band adds a phone app. A shop full of sensors reporting to one dashboard uses all four. This course covers hardware and firmware in depth, uses a little host software for logging, and leaves the cloud layer to a later course, iot.