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<h2 class="brand center" id="title">NOTES ON PROGRAMMING ATMEGA328P CHIPS</h2>
<h6 class="center">10 APRIL 2025</h5>
<br>
<div class="threecol justify"><p>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.</p>
<h2 id="5-v---16-mhz-configuration">5 V - 16 MHz configuration</h2>
<p>The steps that follow refer to the following pinout.</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>
<ol>
<li>Connect pin 1 to 5 V via a 10 kΩ resistor.</li>
<li>Connect a 16 MHz crystal oscillator across pins 9 and 10.</li>
<li>Connect each pin of the crystal to ground via 22 pF capacitors.</li>
<li>Connect pins 7, 20, and 21 to 5 V.</li>
<li>Connect pins 8 and 22 to ground.</li>
</ol>
<p>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.
<a href="Makefile">Here’s</a> a sample Makefile for avr-gcc and avrdude.</p>
<h2 id="33-v---8-mhz-configuration">3.3 V - 8 MHz configuration</h2>
<p>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.</p>
<ol>
<li>Upload the ‘ArduinoISP’ sketch to the Uno.</li>
<li>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.</li>
<li>Connect the SPI ports (SCK, MISO, and MOSI) of the two MCUs.</li>
<li>Connect Uno’s SS pin to the IC’s pin 1 (RESET).</li>
<li>The IC can be powered by the Arduino Uno’s 5 V pin.</li>
<li>Burn the bootloader to the ATmega328P:
<ul>
<li>Select ‘ATmega328P (3.3 V, 8 MHz)’ 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 8 MHz with a 3.3 V power supply. You can
upload programs to the ATmega328P as you usually would using avrdude.
<a href="3v3.Makefile">Here’s</a> a sample Makefile with adjusted parameters (e.g., baud
rate) for an 8 MHz clock.</p>
<p>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.</p>
</div>
<p class="post-author right italics">by W. D. Sadeep Madurange</p>
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