What is a microcontroller?

Microcontrollers are often shortened to MCU, for microcontroller unit. A device with a microcontroller inside, doing one job, is called an embedded system. Almost anything with buttons and a battery has one, and many devices have several.

What is inside a microcontroller?

A microcontroller packs the main parts of a computer onto one piece of silicon.

The Uno's figures come from Arduino's tech specs. Compare that 2 KB of working memory with a laptop's 16 GB: the laptop has about 8 million times as much. Microcontrollers don't need more, because they run one small program and no operating system.

How does a microcontroller work?

When power arrives, the microcontroller starts running the program in its flash memory straight away. There is no boot screen, because there is no operating system to load. Most programs follow the same shape: set things up once, then repeat a loop forever.

1. Read inputs: check buttons, read sensors, receive messages.
2. Decide: compare the readings with what should happen. Is the water hot enough? Has the timer run out?
3. Set outputs: switch a heater, turn a motor, update a display.
4. Repeat, often thousands of times a second.

Here is the classic first program for an Arduino, called a sketch. It flashes the small LED on pin 13 once a second.

void setup() {
  pinMode(13, OUTPUT);      // pin 13 is an output
}

void loop() {
  digitalWrite(13, HIGH);   // LED on
  delay(500);               // wait 500 ms
  digitalWrite(13, LOW);    // LED off
  delay(500);
}

setup() runs once at power-up and loop() runs over and over until the power goes. A microcontroller can also respond to events instantly with interrupts: a button press can pause the loop, run a short piece of code, and then return to where it was.

Talking to the real world

The processor only understands numbers, so the pins translate. A digital pin reads a voltage as HIGH or LOW. An ADC measures a voltage in steps: the Uno's 10-bit ADC turns 0 to 5 V into a number from 0 to 1023, so a reading of 512 means about 512 × 5 ÷ 1023 = 2.50 V. Going the other way, PWM switches a pin on and off very fast. On for a quarter of the time, an LED looks a quarter as bright.

Microcontroller vs microprocessor

A microprocessor, like the processor in a laptop, is only the CPU. It needs separate memory chips, storage and other chips around it, and an operating system to run programs. A microcontroller has its memory and input and output built in.

Microcontroller vs Raspberry Pi

A Raspberry Pi 5 is not a microcontroller. It is a single-board computer: a microprocessor with gigabytes of memory that runs Linux, with a desktop and a web browser. Raspberry Pi also makes the Pico, which is a microcontroller board. As a rule of thumb, use a microcontroller to control hardware cheaply, reliably and on batteries, and a Raspberry Pi when you need a full computer, for example to run a web server or process camera images. Our Raspberry Pi course covers that side.

Popular microcontroller boards

A bare chip is fiddly to use, so beginners start with a development board, which adds a USB port, a power regulator and labelled pins.

Figures come from Espressif's ESP32 datasheet and Raspberry Pi's Pico documentation. Newer versions keep the same idea with faster chips, such as the Uno R4 and the Pico 2. The logic level matters: a 3.3 V board can be damaged by a 5 V signal on one of its pins. Our guide to Arduino compares the Uno and the ESP32 in more detail.

Try matching a project to the right board, then press Mark to check it in our Micro Controllers course.

What are microcontrollers used for?

• Home appliances: washing machines, microwaves, dishwashers, thermostats and smart plugs.
• Cars: engine control, airbags, anti-lock brakes, windows, mirrors and dashboard displays each use microcontrollers.
• Computer accessories: keyboards, mice, game controllers and printers.
• Health and fitness: fitness trackers, digital thermometers and medical devices such as insulin pumps.
• Toys and gadgets: remote controls, drones, electronic toys and greetings cards that play a tune.
• Industry and farming: sensors and controllers in factories, and soil moisture monitors in fields.
• Hobby projects: weather stations, robots, LED displays and plant waterers built on Arduino, ESP32 and Pico boards.

How microcontrollers are programmed

You write the program on a computer, then upload it to the board over USB, which stores it in the chip's flash memory. The common languages are:

• C and C++: the most common. The Arduino IDE uses C++ with simple built-in functions like digitalWrite().
• MicroPython: a small version of Python that runs on the Pico and the ESP32, good for beginners.
• Assembly: instructions for the processor itself, used when every byte or microsecond counts.

Worked examples

Example 1: reading a sensor

A temperature sensor on an Uno's analog pin gives a reading of 300. What voltage is that? The Uno maps 0 to 5 V onto 0 to 1023, as Arduino's reference describes, so the voltage is 300 × 5 ÷ 1023 = 1.47 V.

On a 12-bit, 3.3 V board like the ESP32 or the Pico, the readings run from 0 to 4095 instead, so a reading of 2048 is 2048 × 3.3 ÷ 4095 = 1.65 V, about half way.

Example 2: dimming an LED

On an Uno, analogWrite() takes a value from 0 (always off) to 255 (always on). What value makes an LED a quarter as bright? 0.25 × 255 = 63.75, so use 64.

Example 3: battery life with sleep

A sensor wakes once a minute, works for 1 second drawing 20 mA, then sleeps for 59 seconds drawing 0.01 mA. How long will a 2,000 mAh battery last?

1. Average current: (20 × 1 + 0.01 × 59) ÷ 60 = 0.343 mA.
2. Battery life: 2,000 ÷ 0.343 = about 5,830 hours.
3. In days: 5,830 ÷ 24 = about 243 days.

Left awake at 20 mA all the time, the same battery would last 2,000 ÷ 20 = 100 hours, about 4 days. Sleeping is why a microcontroller can run a doorbell or a sensor for months.

A short history

Early calculators needed a cheap single chip to run them. At Texas Instruments, Gary Boone and Michael Cochran's 1971 single-chip calculator design led to the TMS1000, a 4-bit microcontroller family that TI announced in 1974, according to the Computer History Museum. Chips like it went into calculators, toys, microwave ovens and games. Today's microcontrollers are mostly 32-bit and far faster, but the idea is the same: one cheap chip that runs one job.

Common mistakes

• Calling a Raspberry Pi 5 a microcontroller. It is a full computer running Linux. The Pico is Raspberry Pi's microcontroller board.
• Connecting 5 V signals to a 3.3 V board. It can damage the pin or the whole chip. Check the logic level first.
• Powering motors straight from a pin. A pin can only supply a small current. Motors need a driver or transistor.
• Thinking the program is lost when the power goes off. It is in flash memory and runs again at power-up. Only the variables in SRAM are lost.
• Using delay() for everything. While the board waits, it can't read buttons. Bigger projects check the time with millis() instead.

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