---
title: ATmega328P at 3.3 V and 5 V 
date: 2025-06-10
layout: post
---

Quick reference for wiring ATmega328P ICs at 5 V and 3.3 V. 5 V uses 16 MHz
crystal, 3.3 V uses 8 MHz.

<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>
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  </tr>
</table>

## 5 V - 16 MHz

Standard setup. How Arduino Uno boards are wired.

Connections: Pin 1 → 5 V via 10 kΩ resistor. Pins 9/10 → 16 MHz crystal via 22
pF capacitors to ground. Pins 7/20/21 → 5 V supply. Pins 8/22 → ground. Add 0.1
μF decoupling caps between pins 7/20/21 and ground.

Sample Makefile: [Makefile](Makefile)

## 3.3 V - 8 MHz

Electrical connections identical to 5 V circuit. Replace 5 V with 3.3 V supply,
16 MHz crystal with 8 MHz.

Problem: ATmega328P ships configured for 5 V. Must modify fuses (BOD level,
etc.) and replace bootloader. Standard Arduino bootloader expects 16 MHz—needs
8 MHz version.

Solution: Use Arduino Uno as ISP. Upload 'ArduinoISP' sketch from Arduino IDE.
Connect SPI pins (ATmega328P ↔ Uno), Uno's SS pin → ATmega328P RESET pin. Power
ATmega328P from Uno's 5V pin during programming.

Arduino IDE: Select 'ATmega328P (3.3 V, 8 MHz)' processor, 'Arduino as ISP'
programmer. Burn bootloader via tools menu.

Sample Makefile (8 MHz): [Makefile.3v3](3v3.Makefile)

## Remarks

Don't connect AREF (pin 21) to V<sub>cc</sub> if using ADC with internal 1.1 V
or AV<sub>cc</sub> reference. See datasheet section 23.5.2.