Standalone ATmega328

Learning Goals 5 min

Cluster A's capstone: move your sketch off the £20 UNO board onto a £2 bare chip on a breadboard. The minimum-viable Arduino. By the end of this lesson you will:

1. Wire a bare ATmega328P-PU chip on a breadboard with the "just enough" supporting components: 16 MHz crystal, 2 × 22 pF caps, reset pull-up, decoupling cap.
2. Burn the bootloader onto it via Arduino-as-ISP, then upload a Blink sketch directly.
3. Explain why this matters — a finished product based on Arduino code costs the price of one chip, not one whole development board.

Warm-Up 10 min

Take stock:

• ATmega328P-PU (DIP-28 package — the through-hole one). Bare chip, no PCB.
• 16 MHz crystal (HC-49 form factor, two leads).
• 2 × 22 pF ceramic capacitors (small disc / blob caps).
• 1 × 10 kΩ resistor (reset pull-up).
• 1 × 100 nF cap (decoupling).
• 1 × LED + 220 Ω resistor (Blink target).
• Breadboard.
• UNO loaded with ArduinoISP from L04-05.
• Jumper wires + 5 V supply (USB power bank, AA pack with regulator, or take 5 V from the UNO programmer).

The reveal

The UNO is essentially this chip + USB-serial + power LED + pin headers + a fancy PCB. Everything else is convenience. Bare chip on breadboard = same brain, ~£2 cost, no shield socket.

New Concept · Minimum Arduino circuit 20 min

ATmega328P pinout (DIP-28)

The minimum supporting circuit

That's it. 5 small components + the chip + power. Less than £3 in parts.

To upload to it

Wire the Arduino-as-ISP programmer from L04-05:

• Programmer D10 → chip pin 1 (RESET).
• Programmer D11 → chip pin 17 (MOSI).
• Programmer D12 → chip pin 18 (MISO).
• Programmer D13 → chip pin 19 (SCK).
• Programmer +5V → chip pins 7 + 20.
• Programmer GND → chip pins 8 + 22.

To run it standalone

Once uploaded, disconnect the programmer wires (keep power!). The chip runs its sketch from a 5 V supply (battery pack, USB power bank, etc.). No UNO board needed.

Worked Example · Standalone Blink 30 min

Step 1 — assemble the breadboard

Place the ATmega328P-PU across the central gap (chip straddles the middle, half on each side). Use the breadboard's rails for +5 V and GND. Add the crystal between pins 9 and 10, with one 22 pF cap from each crystal pin to GND. Add the 10 kΩ pull-up on pin 1. Add the 100 nF cap right next to pins 7/8.

Add the LED + 220 Ω resistor on pin 19 (PB5, which is "D13" in Arduino-speak — same pin as the UNO's on-board LED).

Step 2 — load the ArduinoISP sketch on the programmer UNO

From L04-05. Confirm the heartbeat LED on pin 9 of the programmer is blinking.

Step 3 — wire programmer to standalone chip

The 6 wires from §3.

Step 4 — burn the bootloader

1. IDE → Tools → Board → Arduino UNO.
2. Tools → Programmer → Arduino as ISP.
3. Tools → Burn Bootloader.
4. Wait ~10 s. Heartbeat keeps blinking; programming LED (pin 7) lights briefly. Error LED (pin 8) stays off. Done.

Step 5 — upload Blink directly

1. Open File → Examples → Basics → Blink.
2. Sketch → Upload Using Programmer (Ctrl+Shift+U).
3. Wait. The LED on pin 19 should start blinking at 1 Hz.

Step 6 — cut the cord

Disconnect the programmer wires (RESET, MOSI, MISO, SCK). Keep +5 V and GND. The standalone chip continues to blink the LED. You've replaced a £20 board with a £2 chip + £1 of components.

Step 7 — measure power draw

Bare chip with a slow-blinking LED draws ~5 mA average. A UNO with the same sketch draws ~50 mA (USB-serial chip + LED indicators + voltage regulator overhead). 10× saving in power, perfect for battery deployment.

Try It Yourself 15 min

Mini-Challenge · Build a finished standalone device 10 min

1. Pick a project from L1–L3 (mood lamp, doorbell, blink patterns).
2. Move it onto the standalone ATmega328 setup. Mount on a piece of perfboard or in a small enclosure.
3. Power from a battery pack.
4. Take a photo of the finished "product" — no USB cable, no UNO board, just the chip + supporting parts + the LEDs / sensors.

This is what shipping a product means: the brain is the chip; everything else is the brain's minimum needs. Adafruit/SparkFun's ATtiny / ATmega-based small products all do exactly this.

Recap 5 min

An Arduino UNO is an ATmega328P + USB-serial + voltage regulator + connectors. The chip itself runs Arduino sketches just fine on a breadboard with 5 small parts. Burning the bootloader requires an ISP (Arduino-as-ISP works); subsequent uploads either use the ISP or USB-serial via an FTDI adapter. The same approach scales to ATtiny chips (cheaper, smaller), bare AVR boards, and pro PCB designs. Cluster B starts tomorrow: Arduino IoT Cloud — the official cloud platform for "sketches in your browser, devices on your shelf".

ATmega328P-PU

The DIP-28 through-hole package of the chip used on UNO. -PU = pre-installed bootloader; -P = blank.

DIP package

Dual In-line Package — through-hole IC with two rows of legs, fits breadboards.

Crystal oscillator

Quartz resonator that gives the chip its clock. 16 MHz on UNO. Requires two load capacitors (typically 18–22 pF).

Load capacitor

Small cap between a crystal pin and ground that tunes the crystal to its specified frequency.

Pull-up resistor (on RESET)

10 kΩ between RESET and VCC. Holds RESET HIGH (inactive) when no external reset is applied.

Decoupling capacitor

100 nF directly between VCC and GND at the chip's power pins. Smooths supply noise.

Internal oscillator

The ATmega's built-in ~8 MHz RC clock. Less precise than a crystal but needs no external parts.

Standalone device

Microcontroller circuit with only the parts it actually needs — no USB chip, no extra LEDs, no shield headers.

Homework 5 min

1. Build the standalone Blink. Photo of the finished breadboard.
2. Save your "portable" sketch template (L04-03) — confirm it compiles for the bare ATmega328P-PU (it should, since it's an AVR target).
3. Read ahead to ARD-L04-07 (What Is IoT Cloud?). Optional: create a free Arduino account at create.arduino.cc and click around the IoT Cloud area to get the feel.