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Topic: SMD crystals for Arduino? (Read 4697 times) previous topic - next topic

JMcGrath

#15
Aug 20, 2017, 09:53 am Last Edit: Aug 20, 2017, 10:01 am by JMcGrath
I realise this is an old post now, but because it still comes up as a top Google search result regarding Arduino's, Crystals and Capacitors I figured I'd add this in hopes to help people in the future...

First off all, I am not an EE or claim to be a pro, but I have worked with my fair share of "custom" arduino projects (as in an embedded "arduino" in my project / built into the PCB) opposed to connecting a real arduino to my PCB.

The Crystal is actually a very important aspect of many projects, especially if things like timing, Serial Communications, Ethernet, etc are involved.

Also, be sure to read the full datasheet for the Crystal you purchase. There are a number of specs you need to know.

As a general rule of thumb, I try to stick with Crystals that have 30ppm accuracy OR BETTER (lower is better). Also, keep traces from the Atmel chip to the crystal as short as possible, avoid using vias, and along the same lines through hole - your SMD Crystal is a perfect choice.

Each Crystal out there, even if the same manufacturer, speed, etc will have different requirements and you must also be aware of "Cstray"... traces, vias, through holes, etc will all add Stray Capacitance (Cstray) - even the PCB itself adds some Capacitance between layers.

Check out this page from Adafruit for more info, it's a great resource!

https://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/

Jiggy-Ninja

#16
Aug 20, 2017, 06:52 pm Last Edit: Aug 20, 2017, 06:53 pm by Jiggy-Ninja
Unless you need better than 0.5% frequency accuracy, you can use a 3-terminal ceramic resonator instead, such as http://uk.farnell.com/abracon/awscr-16-00mtd-t/ceramic-resonator-16mhz-smd/dp/2101362?in_merch=New%20Products. These have the caps built in.

If you do use a crystal, the 2 caps are in series as seen by the resonator. So 2x22pf gives 11pF, and when you add the stray capacitance and mcu pin capacitance you will have close to 18pF.
If you don't care that much about the oscillator's tolerance, the internal oscillator is an even better option because it doesn't need any external components. It's not very often that a microcontroller project actually needs the extra performance of 16 MHz over 8 MHz.
Hackaday: https://hackaday.io/MarkRD
Advanced C++ Techniques: https://forum.arduino.cc/index.php?topic=493075.0

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