---
title: Programming ATmega328P chips
date: 2025-04-10
author: Wickramage Don Sadeep Madurange
layout: post
---

This post is a step-by-step guide for wiring up ATmega328P ICs to run at 5 V
with a 16 MHz crystal and 3.3 V with an 8 MHz crystal. While the 5 V
configuration is common, the 3.3 V configuration can be advantageous in
low-power applications and when interfacing with parts that run at 3.3 V.

## 5 V - 16 MHz configuration 

The steps that follow refer to the following pinout.

<table style="border: none; width: 100%;">
  <tr style="border: none;">
    <td style="border: none; width: 50%; vertical-align: top;">
      <img src="pinout.png" alt="Pinout" style="width: 100%">
      <p style="text-align: center;">Pinout</p>
    </td>
    <td style="border: none; width: 50%; vertical-align: top;">
      <img src="breadboard.jpeg" alt="Circuit" style="width: 100%">
      <p style="text-align: center;">Breadboard</p>
    </td>
  </tr>
</table>

 1. Connect pin 1 to 5 V via a 10 kΩ resistor.
 2. Connect a 16 MHz crystal oscillator across pins 9 and 10.
 3. Connect each pin of the crystal to ground via 22 pF capacitors.
 4. Connect pins 7, 20, and 21 to 5 V.
 5. Connect pins 8 and 22 to ground.

In addition to the connections described above, it's a good idea to add 0.1 μF
decoupling capacitors between pins 7, 20, and 21 and ground.
[Here's](Makefile) a sample Makefile for avr-gcc and avrdude.

## 3.3 V - 8 MHz configuration 

The following steps use Arduino Uno as an ISP and Arduino utilities to program
ATmega328P's bootloader and the fuses (e.g., BOD level) for a 3.3 V supply.

 1. Upload the 'ArduinoISP' sketch to the Uno.
 2. Wire up the ATmega328P as described in the previous section. Replace the 5 V
    supply with a 3.3 V supply and use an 8 MHz crystal instead of the 16 MHz 
    crystal.
 3. Connect the SPI ports (SCK, MISO, and MOSI) of the two MCUs.
 4. Connect Uno's SS pin to the IC's pin 1 (RESET).
 5. The IC can be powered by the Arduino Uno's 5 V pin.
 6. Burn the bootloader to the ATmega328P:
     - Select 'ATmega328P (3.3 V, 8 MHz)' from Tools > Processor.
     - Select 'Arduino as ISP' from Tools > Programmer.
     - Select Tools > Burn Bootloader.

The ATmega328P is now ready to run at 8 MHz with a 3.3 V power supply. You can
upload programs to the ATmega328P as you usually would using avrdude.
[Here's](3v3.Makefile) a sample Makefile with adjusted parameters (e.g., baud
rate) for an 8 MHz clock.

In both configurations, if you intend to use the ATmega328P's analog-to-digital
converter with the internal 1.1 V or AV<sub>cc</sub> voltage as reference, do
not connect AREF (pin 21) to V<sub>cc</sub>. Refer to section 23.5.2 ADC
Voltage Reference in the datasheet for more information.