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<h2 class="center" id="title">HOW TO CONFIGURE ATMEGA328P MICROCONTROLLERS TO RUN AT 3.3V AND 5V</h2>
<h6 class="center">10 APRIL 2025</h5>
<br>
<div class="twocol justify"><p>This is a quick reference for wiring up ATmega328P ICs to run at 5V and 3.3V.
While the 5V configuration is common, the 3.3V configuration can be useful in
low-power applications and when interfacing with parts that themselves run at
3.3V. In this guide, the 5V setup is configured with a 16MHz crystal
oscillator, while the 3.3V configuration makes use of an 8MHz crystal
oscillator.</p>
<p>The steps that follow refer to the following pinout.</p>
<table style="border: none; width: 100%;">
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<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>
<h2 id="5v-16mhz-configuration">5V-16MHz configuration</h2>
<ol>
<li>Connect pin 1 to 5V via a 10kΩ resistor.</li>
<li>Connect a 16MHz crystal oscillator across pins 9 and 10.</li>
<li>Connect each pin of the crystal to ground via 22pF capacitors.</li>
<li>Connect pins 7, 20, and 21 to 5V.</li>
<li>Connect pins 8 and 22 to ground.</li>
</ol>
<p>In addition to the the connections above, it’s a good idea to add 0.1μF
decoupling capacitors between pins 7, 20, and 21 and ground.
<a href="Makefile">Here’s</a> a sample Makefile for avr-gcc and avrdude.</p>
<h2 id="33v-8mhz-configuration">3.3V-8MHz configuration</h2>
<p>Standard ATmega328P chips are preconfigured to run at 5V. To run one at 3.3V,
we must first modify its fuses (e.g., BOD level). If the chip contains a
pre-installed bootloader that expects a 16MHz clock (such as the Arduino Uno
bootloader), it must be replaced with one that is more amenable to an 8MHz
clock.</p>
<p>In the following steps, we use an Arduino Uno as an in-system programmer to
replace the embedded bootloader and modify the appropriate fuses.</p>
<ol>
<li>Upload the ‘ArduinoISP’ sketch to the Arduino Uno.</li>
<li>Wire up the ATmega328P IC as described in the previous section, while
replacing the 5V supply with a 3.3V supply and 16MHz crystal with an 8MHz
crystal.</li>
<li>Connect the SPI ports (SCK, MISO, and MOSI) of the two MCUs.</li>
<li>Connect the Arduino Uno’s SS pin to the IC’s RESET pin (pin 1).</li>
<li>Connect the IC’s V<sub>CC</sub> to a 5V supply (e.g., the Arduino Uno’s 5V
pin).</li>
<li>Burn the bootloader to the ATmega328P:
<ul>
<li>Select ‘ATmega328P (3.3V, 8MHz)’ from Tools > Processor.</li>
<li>Select ‘Arduino as ISP’ from Tools > Programmer.</li>
<li>Select Tools > Burn Bootloader.</li>
</ul>
</li>
</ol>
<p>The ATmega328P is now ready to run at 8MHz with a 3.3V power supply. You can
upload programs to the ATmega328P as you normally would using avrdude.
<a href="3v3.Makefile">Here’s</a> a sample Makefile with adjusted parameters (e.g., baud
rate) for an 8MHz clock.</p>
<p>In both configurations, if you intend to use the ATmega328P’s analog-to-digital
converter with the internal 1.1V 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 in the
datasheet for more information.</p>
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<p class="post-author right">by Wickramage Don Sadeep Madurange</p>
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