Unit A - Foundations

02. Microcontrollers & the Arduino Uno

Meet the computer on the Uno: MCU vs PC, power pins, I/O groups, R3 vs R4, and optional bare-chip essentials.

Estimated time 2-3 hours

Learning outcomes

Distinguish a microcontroller from a microprocessor system
Identify Uno R3 and R4 Minima power pins, I/O and on-board LEDs
Choose a safe power connection (USB, VIN / barrel)
Explain digital, analogue, UART, I2C and SPI pin roles
Compare key R3 and R4 Minima specs used in this course
Describe the minimum parts for an ATmega328P on a breadboard

Parts and preparation

Arduino Uno R3 (course default) or Uno R4 Minima, and a USB data cable. Optional extension: ATmega328P (DIP), breadboard, 16 MHz crystal, 2 x 22 pF capacitors, 10 k ohm resistor, 10 uF capacitor and a regulated 5 V supply.

Before power: inspect wiring, confirm supply voltage and ensure all connected circuits share GND.

Microcontrollers & the Arduino Uno instructional connection diagram

Microcontroller versus microprocessor

A microcontroller (MCU) combines a CPU, Flash program memory, SRAM, often EEPROM, and peripherals (timers, ADC, UART, and more) on one chip for a dedicated control job.

A microprocessor system (a typical PC) uses a CPU chip plus separate RAM, storage and chipsets - suited to general-purpose computing with an operating system and many programs.

An appliance controller, robot brain or Uno project usually wants an MCU: fewer chips, lower power, fixed task.

MCU all-in-one chip versus PC with separate CPU RAM and storage — MCU integrates the control computer. A PC splits those jobs across many chips.

Arduino Uno R3 board guide

This course defaults to the classic Arduino Uno Rev3: ATmega328P, 5 V logic, USB-B, and the standard Uno shield layout.

Arduino Uno Rev3 board showing USB-B, power jack, digital and analogue headers, and ATmega328P — Arduino Uno Rev3 - the default board for this course. Trace 5 V, 3.3 V, GND, VIN, AREF and the digital/analogue headers on your own board.Photo: Arduino® official product photo (Uno Rev3).Arduino® is a trademark of Arduino S.r.l.

Item	Uno R3
Microcontroller	ATmega328P @ 16 MHz
Flash / SRAM / EEPROM	32 KB (about 0.5 KB bootloader) / 2 KB / 1 KB
Logic / USB	5 V logic · USB-B
Digital I/O	D0-D13 · PWM on ~ pins 3, 5, 6, 9, 10, 11
Analogue inputs	A0-A5 · 10-bit ADC (0-1023) by default
VIN / barrel	About 7-12 V recommended
I/O current (design)	About 20 mA per pin (40 mA absolute max)
Built-in LED (L)	Pin 13 · LED_BUILTIN
TX / RX LEDs	Flash on USB Serial Monitor traffic

Arduino Uno R4 Minima board guide

Same Uno form factor and 5 V class as R3, but a faster Renesas RA4M1 MCU, more memory, USB-C, and extras. Select UNO R4 Minima in the IDE if you use this board.

Arduino Uno R4 Minima board showing USB-C, headers and Renesas microcontroller — Arduino Uno R4 Minima - same shield layout and 5 V logic class, stronger MCU and USB-C. Confirm IDE board selection before upload.Photo: Arduino® official product photo (UNO R4 Minima).Arduino® is a trademark of Arduino S.r.l.

Item	Uno R4 Minima
Microcontroller	RA4M1 (Arm Cortex-M4) @ 48 MHz
Flash / SRAM / EEPROM	256 KB / 32 KB / 8 KB
Logic / USB	5 V logic · USB-C
Digital I/O	14 pins · 6 PWM
Analogue inputs	6 inputs · up to 14-bit resolution available
VIN / barrel	About 6-24 V
I/O current (design)	About 8 mA per pin - do not assume R3 20 mA
Built-in LED (L)	Pin 13 · LED_BUILTIN
Extras	DAC, RTC, CAN (needs transceiver), HID, SWD
Sketches / libraries	Arduino API sketches often work; AVR-only code may need changes

Powering the board: USB vs VIN

For almost every early lesson, power the Uno from USB with a data cable. That provides regulated 5 V for the MCU and small indicator circuits.

The barrel jack / VIN path feeds the onboard regulator from a higher DC voltage (R3 typically about 7-12 V recommended). Never put that higher voltage on a digital or analogue I/O pin. Treat the 5V header as an output for small loads - do not casually feed an external supply into it.

USB regulated 5 V versus VIN barrel higher DC into the regulator — USB for labs. VIN/barrel when you need a separate supply - I/O still 5 V max.

Power pins: 5V, 3.3V, VIN, AREF and GND

Both boards use the same power-header ideas. Treat these pins carefully.

Pin	Use
5V	Regulated 5 V for the MCU and many sensors. Power via USB or VIN/barrel.
3.3V / 3V3	3.3 V for suitable modules (limited current). Never put 5 V into a 3.3 V-only device.
GND	Common 0 V. Every external circuit must share GND with the Uno.
VIN	Raw board input (barrel). Not a sensor logic output.
AREF	ADC reference. Default analogue range about 0-5 V. Leave open until a lesson uses analogReference().
IOREF	Tells shields the I/O voltage (5 V here).
RESET	Pull low to restart the sketch (reset button).

Digital pins, analogue pins and buses

Header roles match on R3 and R4 Minima for typical shields. Memorise UART, I2C and SPI early - they return often.

Uno pin groups for digital PWM analogue and power — Find digital, PWM (~), analogue and power groups on your board.

Group	Pins / notes
Digital I/O	D0-D13 · pins marked ~ support PWM (analogWrite)
UART / USB Serial	D0 RX, D1 TX - avoid for buttons/LEDs while using Serial Monitor
Analogue in	A0-A5 · R3 default 10-bit (0-1023)
I2C	A4 SDA, A5 SCL
SPI	D13 SCK, D12 MISO, D11 MOSI, D10 SS
External interrupts	Commonly D2 and D3 on Uno-class boards

Uno R3 versus R4 Minima

Same family layout and 5 V class, different microcontroller and limits. This course targets Uno R3 unless a note says otherwise - if you use R4 Minima, select that board in the IDE and watch pin-current limits.

Feature	Uno R3	Uno R4 Minima
Microcontroller	ATmega328P (8-bit AVR)	Renesas RA4M1 (Arm Cortex-M4)
Clock	16 MHz	48 MHz
Flash / SRAM / EEPROM	32 KB / 2 KB / 1 KB	256 KB / 32 KB / 8 KB
Logic voltage	5 V	5 V
USB	USB-B	USB-C
I/O pin current (design)	About 20 mA (40 mA abs. max)	About 8 mA
Extras	ICSP, shield ecosystem	DAC, RTC, CAN, HID, SWD

Electrical limits

Both boards use 5 V logic on I/O - never apply more than 5 V to a digital or analogue pin. On Uno R3 design for about 20 mA or less per pin (40 mA is an absolute maximum, not a target). On Uno R4 Minima assume a lower per-pin budget (about 8 mA) and use drivers for heavier loads. Total chip current limits still apply.

OK loads like LED with resistor versus motors that need a driver — Indicator LEDs: OK with resistors. Motors and relays: use a driver (Lesson 12).

ATmega328P on a breadboard

The Uno R3 board is mainly power regulation, USB-to-serial, a crystal clock, reset support and labelled headers around the ATmega328P chip. You can run that chip alone on a breadboard if you add the same essentials: a stable 5 V supply, ground, a 16 MHz crystal with load capacitors, a reset pull-up and decoupling.

Orient the DIP package so the notch (or pin-1 dot) is at the top. Pin 1 is RESET at top-left; pins then count down the left side (1-14) and up the right side (15-28).

Minimum wiring matching the diagram: 10 k ohm from RESET (pin 1) to 5 V; VCC (pin 7) and AVCC (pin 20) to 5 V; GND (pins 8 and 22) to GND; 16 MHz crystal between XTAL1 (pin 9) and XTAL2 (pin 10); 22 pF from each crystal pin to GND; 10 uF across the 5 V and GND rails close to the chip.

This section is for understanding and optional hardware builds. Day-to-day course work still uses a ready-made Uno. The R4 Minima does not use a DIP ATmega328P - you cannot breadboard it the same way.

ATmega328P breadboard Arduino pinout showing 5 V, GND, 10k reset pull-up, 16 MHz crystal with 22 pF capacitors and 10 uF decoupling — Minimum ATmega328P breadboard circuit (Uno R3 chip family). Trace every rail before applying 5 V.

Wiring and safe build sequence

USB supplies regulated 5 V for the board and small test circuits
Uno R3: use about 7-12 V only at the barrel jack / VIN as recommended
Uno R4 Minima: VIN may accept about 6-24 V - still never put that voltage on an I/O pin
Never apply voltage above 5 V to a digital or analogue I/O pin
All connected external circuits need a common GND
Optional bare-chip build (R3 / ATmega328P only): regulated 5 V; fit crystal, 22 pF caps, 10 k ohm RESET pull-up and 10 uF rail decoupling before powering

Power rule: switch off before moving wires. Arduino I/O pins are control signals; high-current loads require a driver and suitable external supply.

Worked sketch

Download .ino sketch

void setup() {
  Serial.begin(9600);
  Serial.println("Lesson 02: Arduino Uno ready");
}

void loop() {
  Serial.print("Uptime (s): ");
  Serial.println(millis() / 1000);
  delay(1000);
}

How the code works

Serial uses the USB connection through the board USB-to-serial interface.
millis returns time since reset, not calendar date/time.
Watch the TX/RX LEDs while the Serial Monitor is open - they show USB serial activity.
Match Monitor baud to Serial.begin(9600).

Test and record evidence

Expected result: The Serial Monitor displays an increasing uptime value once each second. TX/RX activity LEDs flicker with the traffic.

Practical evidence checklist

Labelled wiring or block diagram Uploaded sketch and observed result At least one measured/tested value Fault or improvement recorded in logbook

Common faults and checks

Use a USB data cable, not a charging-only cable (R3 USB-B or R4 USB-C as fitted).
Match the Serial Monitor baud rate to 9600.
Wrong board selected in the IDE: choose Arduino Uno for R3, or Arduino UNO R4 Minima for R4.
Bare chip dead: check 5 V on VCC and AVCC, both GNDs, crystal orientation and the 10 k ohm RESET pull-up to 5 V.

Extension challenge: On your board photo or the pin map, point to 5V, 3.3V, GND, VIN, AREF, LED L, and the TX/RX LEDs. Then list three differences between Uno R3 and Uno R4 Minima.

Check your understanding

Q1. Why is a microcontroller suitable for an appliance controller?

Show answer

It integrates processing, memory and I/O for a dedicated embedded task.

Q2. What do the TX and RX LEDs usually indicate on an Uno?

Show answer

USB serial communication activity with the computer (for example Serial Monitor traffic).

Q3. Name one memory or clock difference between Uno R3 and Uno R4 Minima.

Show answer

Examples: R3 has 32 KB Flash / 2 KB SRAM at 16 MHz; R4 Minima has 256 KB Flash / 32 KB SRAM at 48 MHz.

Q4. Why prefer USB power for early labs?

Show answer

It provides a convenient regulated 5 V supply and a programming/serial link without a separate barrel supply.

Q5. Name four parts needed for an ATmega328P to run on a breadboard besides the chip and 5 V/GND.

Show answer

16 MHz crystal, two 22 pF capacitors, 10 k ohm RESET pull-up and a decoupling capacitor (for example 10 uF) across the supply rails.