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<title>ATmega328P at 3.3V and 5V</title>
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<h2 class="center" id="title">ATMEGA328P AT 3.3V AND 5V</h2>
<h6 class="center">10 JUNE 2025</h5>
<br>
<div class="twocol justify"><p>Quick reference for wiring ATmega328P ICs at 5V and 3.3V. 5V uses 16MHz
crystal, 3.3V uses 8MHz.</p>
<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>
<h2 id="5v-16mhz">5V-16MHz</h2>
<p>Standard setup. How Arduino Uno boards are wired.</p>
<p>Connections: Pin 1 → 5V via 10kΩ resistor. Pins 9/10 → 16MHz crystal via 22pF
capacitors to ground. Pins 7/20/21 → 5V supply. Pins 8/22 → ground. Add 0.1μF
decoupling caps between pins 7/20/21 and ground.</p>
<p>Sample Makefile: <a href="Makefile">Makefile</a></p>
<h2 id="33v-8mhz">3.3V-8MHz</h2>
<p>Electrical connections identical to 5V circuit. Replace 5V with 3.3V supply,
16MHz crystal with 8MHz.</p>
<p>Problem: ATmega328P ships configured for 5V. Must modify fuses (BOD level,
etc.) and replace bootloader. Standard Arduino bootloader expects 16MHz—needs
8MHz version.</p>
<p>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.</p>
<p>Arduino IDE: Select ‘ATmega328P (3.3V, 8MHz)’ processor, ‘Arduino as ISP’
programmer. Burn bootloader via tools menu.</p>
<p>Sample Makefile (8MHz): <a href="3v3.Makefile">Makefile.3v3</a></p>
<h2 id="remarks">Remarks</h2>
<p>Don’t connect AREF (pin 21) to V<sub>cc</sub> if using ADC with internal 1.1V
or AV<sub>cc</sub> reference. See datasheet section 23.5.2.</p>
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<p class="post-author right">by W. D. Sadeep Madurange</p>
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