user62169: I'm using an Arduino Uno Rev3 Board and need to send a trigger signal at a certain frequency (20-50Hz).
This trigger signal should be as accurate as possible (+/- 0.01 ms).
The Arduino site states that the arduino uno has a 16MHz Quartz Crystal, but I'm measuring a frequency accuracy of about 0.35% (which would be typical for a ceramic resonator) instead of what i could expect from a quartz (0.001%).
I'm using the micros() function to measure time.
Am I doing something wrong? How is my frequency so inaccurate?
Thanks for your help! :)

Edit: Indeed when I have a look at the schematics, I see the ceramic resonator (Blue circle). However, there seems to be another 16 MHz part (red circle). could that be a quartz oscillator?

user4143: The ATMega328P chip is, indeed, clocked by a ceramic resonator. The website is misleading in stating that it has a crystal (I have emailed the webmaster to try and get this corrected, though I don't hold out much hope).
The 16MHz crystal in the schematics is part of the USB interface circuitry and has nothing to do with the timing of your running code. It is purely used to clock the ATMega16U2 chip which provides USB connectivity (USB requires a higher accuracy for stable communication).
There are other boards available with better crystals - most often boards that have native USB connections, such as the Arduino Due or Leonardo, or any of the myriad of boards by other manufacturers, like Teensy or chipKIT.

62169: <p>I'm using an Arduino Uno Rev3 Board and need to send a trigger signal at a certain frequency (20-50Hz).
This trigger signal should be as accurate as possible (+/- 0.01 ms).
The Arduino site states that the arduino uno has a 16MHz Quartz Crystal, but I'm measuring a frequency accuracy of about 0.35% (which would be typical for a ceramic resonator) instead of what i could expect from a quartz (0.001%).
I'm using the micros() function to measure time.
Am I doing something wrong? How is my frequency so inaccurate?</p>

<p>Thanks for your help! :)</p>

<p><a href="https://i.stack.imgur.com/PlTKj.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/PlTKj.png" alt="enter image description here"></a></p>

<p>Edit: Indeed when I have a look at the schematics, I see the ceramic resonator (Blue circle). However, there seems to be another 16 MHz part (red circle). could that be a quartz oscillator?</p>
 4143: <p>The ATMega328P chip is, indeed, clocked by a ceramic resonator. The website is misleading in stating that it has a crystal (I have emailed the webmaster to try and get this corrected, though I don't hold out much hope).</p>

<p>The 16MHz crystal in the schematics is part of the USB interface circuitry and has nothing to do with the timing of your running code. It is purely used to clock the ATMega16U2 chip which provides USB connectivity (USB requires a higher accuracy for stable communication).</p>

<p>There are other boards available with better crystals - most often boards that have native USB connections, such as the Arduino Due or Leonardo, or any of the myriad of boards by other manufacturers, like Teensy or chipKIT.</p>
