Embedded Systems

Microcontrollers

A microcontroller is a compact integrated circuit containing a processor, memory and peripheral hardware in a single chip. Microcontrollers are used to control electronic equipment, read sensors, communicate with other devices, control motors and displays, and perform many other dedicated control functions.

What Is a Microcontroller?

A microcontroller, often abbreviated as MCU, is a programmable integrated circuit designed to control electronic systems.

Unlike a simple logic IC, a microcontroller contains a processor that executes instructions stored in memory.

A typical microcontroller contains:

CPU core
Program memory
RAM
GPIO pins
Timers and counters
Interrupt controller
Communication peripherals
Analog-to-digital converter
Clock circuitry
Power-management functions

Basic Microcontroller Structure

             ┌──────────────────────┐
             │   MICROCONTROLLER    │
             │                      │
             │      CPU CORE        │
             │         │            │
             │   ┌─────┴─────┐      │
             │   │           │      │
             │  RAM       Flash     │
             │                 │     │
             │   ┌─────────────┐    │
             │   │ Peripherals │    │
             │   └─────────────┘    │
             │      │  │  │  │      │
             └──────┼──┼──┼──┼──────┘
                    │  │  │  │
                   GPIO ADC UART
                         │
                       SPI/I2C

The CPU executes the program while the peripherals interact with the outside world.

Microcontroller vs Microprocessor

Feature	Microcontroller	Microprocessor
CPU	Integrated	Integrated
RAM	Usually integrated	Usually external
Program memory	Usually integrated	Often external or system-dependent
Peripherals	Many integrated	Often external
Power consumption	Generally low	Often higher
Typical use	Dedicated control	General-purpose computing

CPU Core

The CPU is the part of the microcontroller that executes the program instructions.

It performs operations such as:

Arithmetic calculations
Logical operations
Data movement
Comparisons
Branching
Peripheral control

Microcontroller Architectures

Different microcontrollers use different processor architectures.

Common architectures include:

ARM Cortex-M
AVR
8051
PIC
RISC-V
Xtensa

The architecture determines the instruction set, registers, memory organization and many other characteristics.

Flash Memory

Flash memory is commonly used to store the microcontroller's program.

The contents remain stored when power is removed.

The amount of Flash available determines how large the firmware can be.

RAM

RAM is temporary working memory used while the program is running.

It can contain:

Variables
Buffers
Temporary calculations
Stack data
Communication data

RAM contents are normally lost when power is removed.

EEPROM

Some microcontrollers contain EEPROM or provide another form of non-volatile data storage.

It can be used for information such as:

Configuration settings
Calibration values
User preferences
Device identification

GPIO

GPIO means General-Purpose Input/Output.

GPIO pins allow the microcontroller to interact directly with external electronics.

A GPIO pin can commonly be configured as:

Digital input
Digital output
Pull-up input
Pull-down input
Alternate-function peripheral pin

Digital Input

A digital input allows the microcontroller to detect a HIGH or LOW signal.

Examples include:

Push buttons
Switches
Digital sensors
Logic signals
Alarm outputs

Digital Output

A digital output allows the microcontroller to control external devices.

Examples include:

LEDs
Transistors
Relays through driver circuits
Logic ICs
Enable signals

The maximum current of a GPIO pin must always be respected.

Pull-Up Resistor

A pull-up resistor connects an input to the positive supply so that the input assumes a defined HIGH state when no external device is driving it LOW.

VCC
 │
 R
 │
 ├──── GPIO
 │
Switch
 │
GND

When the switch is open, the input is HIGH. When the switch is closed, the input becomes LOW.

Pull-Down Resistor

A pull-down resistor connects an input to ground so that it normally remains LOW.

VCC
 │
Switch
 │
 ├──── GPIO
 │
 R
 │
GND

When the switch closes, the input becomes HIGH.

Timers and Counters

Timers are hardware peripherals used for measuring time and generating precise timing events.

They can be used for:

Delays
Periodic interrupts
Pulse generation
PWM
Frequency measurement
Event counting

PWM

PWM stands for Pulse Width Modulation.

A PWM signal rapidly switches between HIGH and LOW while changing the percentage of time spent HIGH.

Low duty cycle:

───┐   ┌───┐   ┌───
   └───┘   └───┘

High duty cycle:

───┐─────┐─────┐───
   └─────┘     └───

PWM is commonly used for:

LED brightness control
Motor speed control
Servo control
Power control
Audio generation

ADC

ADC stands for Analog-to-Digital Converter.

An ADC allows the microcontroller to measure analog voltages.

Analog sensor
      │
      ▼
     ADC
      │
      ▼
Digital value
      │
      ▼
     CPU

ADC resolution determines how many digital levels can represent the input voltage.

ADC Resolution

For an ideal N-bit ADC, the number of digital levels is:

2^N

For example, a 10-bit ADC provides:

2^10 = 1024 levels

A 12-bit ADC provides:

2^12 = 4096 levels

The actual accuracy depends on many factors beyond resolution.

DAC

Some microcontrollers contain a DAC, or Digital-to-Analog Converter.

A DAC converts digital values into an analog voltage or current.

Applications include:

Audio generation
Analog control
Waveform generation
Reference generation

UART

UART stands for Universal Asynchronous Receiver-Transmitter.

It provides serial communication between the microcontroller and another device.

Microcontroller
      │
      │ TX
      ├────────────► RX
      │
      │ RX
      ◄────────────┤
                   │
                Other Device

UART is commonly used for:

Serial debugging
GPS modules
Bluetooth modules
Serial displays
Other microcontrollers

I2C

I2C is a two-wire serial communication interface.

The two main signals are:

SDA — Serial Data
SCL — Serial Clock

Multiple compatible devices can share the same bus using device addresses.

SPI

SPI is a high-speed synchronous serial interface.

Typical signals include:

SCLK
MOSI
MISO
CS

SPI is commonly used with:

Displays
SD cards
Flash memory
ADCs
DACs
Sensors

CAN

CAN stands for Controller Area Network.

It is widely used in automotive and industrial applications.

CAN allows multiple controllers and devices to communicate over a shared bus using message-based communication.

USB

Some modern microcontrollers include USB hardware.

This can allow the microcontroller to communicate directly with a computer or other USB device.

Possible applications include:

USB serial devices
USB keyboards
USB controllers
USB audio
Data acquisition

Interrupts

An interrupt allows a peripheral or external event to temporarily interrupt the normal execution of the program.

Examples include:

Timer overflow
GPIO change
UART reception
ADC completion
External interrupt

Interrupts allow the microcontroller to respond quickly to events without continuously polling every peripheral.

Watchdog Timer

A watchdog timer can reset the microcontroller if the firmware stops operating correctly.

The software periodically services the watchdog. If it fails to do so within the configured time, the watchdog can reset the device.

This is particularly useful in unattended embedded systems.

Clock System

The microcontroller requires a clock to execute instructions and operate many of its peripherals.

Clock sources may include:

Internal oscillator
External crystal
External oscillator
PLL-generated clock

The clock frequency affects processing speed and peripheral timing.

Crystal Oscillator

An external crystal can provide an accurate frequency reference.

The crystal is normally connected to dedicated oscillator pins through the recommended supporting components.

The exact circuit depends on the microcontroller.

Reset

Reset places the microcontroller into a known startup state.

Reset can occur because of:

Power-on reset
External reset pin
Watchdog timeout
Brownout condition
Software reset

Brownout Detection

Brownout detection monitors the supply voltage.

If the supply falls below a specified threshold, the microcontroller can reset or enter a protected state.

This helps prevent unpredictable operation when the supply voltage is too low.

Microcontroller Programming

The behavior of a microcontroller is determined by firmware stored in its program memory.

The firmware is commonly developed using languages such as:

C
C++
Assembly
Rust
MicroPython on suitable devices
Other supported languages

The exact development environment depends on the microcontroller family.

Firmware

Firmware is the software stored inside an embedded device that controls its operation.

A typical firmware program may:

Initialize the hardware.
Configure GPIO.
Configure timers.
Initialize communication interfaces.
Read sensors.
Process data.
Control outputs.
Repeat continuously.

Bootloader

A bootloader is a small program that runs before or alongside the main application firmware.

It can provide functions such as:

Firmware programming
Firmware updates
Recovery
Communication with a programming tool

Programming Interfaces

Microcontrollers can be programmed through different interfaces depending on the device.

Examples include:

USB
UART bootloader
SWD
JTAG
ISP
ICSP

Debugging

Debugging allows the developer to inspect the program while it is running.

Depending on the microcontroller, debugging may provide:

Breakpoints
Single stepping
Register inspection
Memory inspection
Variable inspection
Watchpoints

Microcontroller Development Board

A development board combines a microcontroller with supporting hardware that makes experimentation easier.

A board may include:

USB connector
Voltage regulator
Programming interface
LEDs
Buttons
Pin headers
Crystal or oscillator

Development boards are useful for learning and prototyping before designing a custom PCB.

Common Microcontroller Families

Many different microcontroller families are available.

Arduino-compatible AVR devices
STM32
ESP32
ESP8266
PIC
RP2040
MSP430
AVR
8051-family devices
RISC-V microcontrollers

ESP32

ESP32 is a family of microcontrollers widely used in connected embedded applications.

Depending on the particular ESP32 variant, features can include:

Wi-Fi
Bluetooth
GPIO
Timers
ADC
UART
SPI
I2C
PWM

The exact features differ between ESP32 variants.

STM32

STM32 is a large family of ARM-based microcontrollers.

Different STM32 families target applications ranging from simple control systems to high-performance embedded processing.

Features vary significantly between models.

AVR

AVR microcontrollers are widely known from the Arduino ecosystem and other embedded applications.

They are available in many different configurations with varying amounts of memory and peripheral hardware.

PIC Microcontrollers

PIC is a family of microcontrollers developed by Microchip.

PIC devices range from relatively simple controllers to high-performance microcontrollers with advanced peripherals.

Microcontroller Power Consumption

Power consumption depends on:

Clock frequency
Supply voltage
CPU activity
Peripheral activity
GPIO loading
Wireless operation
Sleep mode

Low-power applications often use sleep modes and wake the microcontroller only when necessary.

Sleep Modes

Many microcontrollers provide low-power operating modes.

Depending on the device, these may disable some or most internal circuitry while preserving selected functions.

The microcontroller can then wake in response to an event such as a timer, GPIO signal or communication event.

Microcontroller Applications

Home appliances
Audio equipment
Internet radios
Remote controls
Industrial controllers
Motor controllers
Robotics
Security systems
Displays
Measurement instruments
Automotive electronics
IoT devices
Battery-powered equipment
Consumer electronics

Microcontroller and Sensors

A microcontroller can read information from sensors and use that information to make decisions.

Sensor
  │
  ▼
ADC / GPIO
  │
  ▼
Microcontroller
  │
  ▼
Decision
  │
  ▼
Output

For example, a temperature sensor can be monitored and a fan switched on when a selected temperature is reached.

Microcontroller and Displays

Microcontrollers can control many types of displays.

LED indicators
7-segment displays
Character LCDs
OLED displays
TFT displays
Graphic displays

The communication interface may use GPIO, SPI, I2C, parallel interfaces or another protocol depending on the display.

Microcontroller and Motors

A microcontroller normally does not drive a large motor directly.

Instead, it generates control signals for a driver circuit.

Microcontroller
      │
      ▼
Motor Driver
      │
      ▼
Motor

The driver may use transistors, MOSFETs, H-bridges or dedicated motor-driver ICs.

Microcontroller and Relays

A GPIO pin normally should not directly drive a relay coil.

A transistor or dedicated driver is normally used between the MCU and relay.

GPIO
 │
 ▼
Transistor
 │
 ▼
Relay Coil
 │
 ▼
Load

A suitable flyback protection arrangement is required for a conventional DC relay coil.

Microcontroller Testing

When a microcontroller circuit does not work, start with the hardware before assuming the firmware is defective.

Check the supply voltage.
Check ground connections.
Check reset circuitry.
Check the clock source.
Check programming connections.
Check firmware programming.
Check GPIO configuration.
Check peripheral configuration.
Check external components.
Use an oscilloscope or logic analyzer when appropriate.

Microcontroller Not Starting

Possible causes include:

Missing power
Incorrect supply voltage
Reset held active
Incorrect clock configuration
Incorrect firmware
Corrupted program memory
Incorrect boot configuration
Hardware damage

GPIO Not Working

If a GPIO pin does not behave correctly, check:

Pin configuration
Input/output direction
Pull-up or pull-down configuration
Alternate-function selection
Output drive mode
External circuitry
Firmware logic

Communication Problems

When UART, SPI or I2C communication fails, check:

Supply voltage
Ground connection
Pin assignment
Baud rate
Clock frequency
Protocol configuration
Pull-up resistors where required
Signal integrity

Microcontroller Replacement

Replacing a microcontroller is more complicated than replacing a simple logic IC.

The replacement must be compatible in both hardware and firmware.

Important factors include:

Package
Pinout
CPU architecture
Flash memory
RAM
GPIO count
Peripheral availability
ADC characteristics
Communication interfaces
Supply voltage
Clock requirements
Firmware compatibility

How to Select a Microcontroller

Determine the required number of GPIO pins.
Determine the required supply voltage.
Determine processing requirements.
Determine Flash requirements.
Determine RAM requirements.
Determine ADC requirements.
Determine communication interfaces.
Determine timer and PWM requirements.
Determine package requirements.
Determine power-consumption requirements.
Check development-tool availability.
Check component availability and cost.

Microcontroller Datasheet

The datasheet is the most important technical reference when designing with a particular microcontroller.

It contains information such as:

Pin configuration
Electrical characteristics
Absolute maximum ratings
Supply voltage
Clock requirements
Memory size
Peripheral specifications
GPIO characteristics
ADC specifications
Package information

Absolute Maximum Ratings

Absolute maximum ratings specify limits that should not be exceeded.

They are not necessarily recommended operating conditions.

Examples include:

Maximum supply voltage
Maximum input voltage
Maximum GPIO current
Maximum junction temperature

Microcontroller Development Tools

Development normally requires a combination of software and hardware tools.

Common tools include:

Compiler
IDE
Programmer
Debugger
USB interface
Serial terminal
Oscilloscope
Logic analyzer

Microcontroller Debugging Tools

A multimeter is useful for checking power and DC voltages.

An oscilloscope can reveal:

Clock signals
PWM signals
UART data
SPI activity
Reset pulses
Analog signals

A logic analyzer is particularly useful for examining digital communication protocols.

Advantages of Microcontrollers

Programmable
Compact
Low component count
Low power consumption
Many integrated peripherals
Flexible control
Easy to update through firmware
Available in many performance levels

Limitations of Microcontrollers

Firmware is required
Development tools are required
Programming errors can cause system failures
GPIO current is limited
Memory is limited by the device
Peripheral capabilities vary considerably
Firmware compatibility must be considered during replacement

Common Microcontroller Design Mistakes

Incorrect supply voltage
Exceeding GPIO current limits
Leaving required inputs floating
Poor power-supply decoupling
Incorrect reset design
Incorrect clock configuration
Ignoring ADC reference requirements
Incorrect communication configuration
Insufficient PCB grounding
Ignoring thermal requirements

Key Points

A microcontroller is a programmable control IC.
It normally contains a CPU, memory and peripherals.
GPIO allows the MCU to interact with external hardware.
Timers provide accurate hardware timing.
PWM is useful for motor and LED control.
ADC allows the MCU to measure analog signals.
UART, I2C and SPI are common communication interfaces.
Interrupts allow rapid response to hardware events.
Watchdogs can recover systems when firmware stops operating correctly.
Flash stores firmware while RAM stores temporary working data.
Microcontrollers are used extensively in embedded systems.
The datasheet should always be consulted before designing with a device.
A replacement MCU must be compatible with both the hardware and firmware.

Continue Learning About Microcontrollers

The next pages can cover microcontroller types, architecture, GPIO, memory, timers, interrupts, ADC, PWM, UART, SPI, I2C, programming, debugging, development boards, testing, troubleshooting, replacement and selection.