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---
title: ATmega328P 3.3 V and 5 V configurations
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>
</td>
</tr>
</table>
## 5 V - 16 MHz
Standard setup. How Arduino Uno boards are wired:
<pre class="pre-no-style">
Pin 1 --> [ 10k Resistor ] --> 5V
Pin 7 --> 5V (w/ 0.1uF Cap to GND)
Pin 8 --> GND
Pin 9 --> [ 16MHz Crystal ] --> [ 22pF Cap ] --> GND
Pin 10 --> [ 16MHz Crystal ] --> [ 22pF Cap ] --> GND
Pin 20 --> 5V (w/ 0.1uF Cap to GND)
Pin 21 --> 5V (w/ 0.1uF Cap to GND)
Pin 22 --> GND
</pre>
Sample Makefile: [Makefile](Makefile)
## 3.3 V - 8 MHz
Electrical connections are identical to 5 V circuit. Replace 5 V with 3.3 V supply,
16 MHz crystal with 8 MHz.
ATmega328P ships with its fuses(BOD level, etc.) configured for 5 V. Need to
modify them. The standard Arduino bootloader requires a 16 MHz clock. Replace
the default bootloader with one that works witha 8 MHz clock.
Upload the 'ArduinoISP' sketch from the Arduino IDE to an Arduino Uno to use it
as an ISP.
<pre class="pre-no-style">
ARDUINO UNO (ISP) ATMEGA328P
+-------------------+ +-------\/-------+
| D13 |-------------->| 19 PB5 (SCK) |
| D12 |<--------------| 18 PB4 (MISO) |
| D11 |-------------->| 17 PB3 (MOSI) |
| D10 |-------------->| 1 RESET |
| | | |
| 5V |-------------->| 7 VCC |
| GND |-------------->| 8 GND |
+-------------------+ +----------------+
</pre>
From the Arduino IDE, select 'ATmega328P (3.3 V, 8 MHz)' processor, 'Arduino as
ISP' programmer. Upload the bootloader from the tools menu.
Sample Makefile (8 MHz): [Makefile.3v3](3v3.Makefile)
Note: 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.
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