What is a microcontroller?

A microcontroller is a small computer on a single chip. Alongside its processor it holds memory for a program, memory for variables, timers and a set of pins that read buttons and sensors and switch lights and motors. It runs one program, by itself, for as long as it has power.

What's inside the chip

• CPU: fetches the program's instructions and carries them out, one after another
• Flash memory: keeps the program, even with the power off
• RAM: holds the variables while the program runs
• Timers: counters fed by a steady clock, so the chip can measure time
• ADC: an analog-to-digital converter that turns a voltage into a number
• Input and output pins: each one set by the program to read a voltage or to drive one

The Arduino Uno's chip, the ATmega328P, has a 16 MHz clock, 32 KB of flash and just 2 KB of RAM. That's tiny next to a laptop, and plenty for reading a sensor and running a motor.

How the program runs

You write the program on a computer, then upload it: it's copied into the chip's flash, and the chip starts running it straight away. Arduino programs have two parts. setup() runs once at power-on, to get things ready. loop() then runs again and again, forever.

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);   // once
}

void loop() {
  digitalWrite(LED_BUILTIN, HIGH);
  delay(100);                     // on 100 ms
  digitalWrite(LED_BUILTIN, LOW);
  delay(900);                     // off 900 ms
}

One pass of that loop takes 100 + 900 = 1000 ms, so the LED flashes once a second: 1 Hz.

Live experiment

Watch a program run, line by line

A heartbeat program: setup() runs once and says Hello, then loop() flashes L for 100 ms and waits 900 ms, forever. Run it in real time or step a line at a time. Then move the Hello line into loop() and watch the serial monitor.

Try it yourself: free lesson

“What's a microcontroller?” is lesson 1 of 6 in The Core, Electronics World 8. Free with an account. Lessons unlock in order, so you’ll do the 49 before it first.

Microcontroller or microprocessor?

A microprocessor, like the one in a laptop or phone, is mostly CPU. It relies on separate chips for gigabytes of memory and storage, and runs an operating system that juggles many programs. A microcontroller keeps everything on one chip, can cost under a dollar, uses a few milliamps, and starts running its one program in a fraction of a second.

Where you'll find them

Almost everything with a button has one: microwaves, washing machines, keyboards, remote controls, thermostats and electric toothbrushes. A modern car has dozens, each running one job like the windows, the brakes or the engine.