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### Topic: Crystal load capacitance (Read 749 times)previous topic - next topic

#### positivelydoped23

##### Jan 04, 2013, 07:43 pm
Hi!
I am confused regarding the load capacitance of the crystal to be used.
Some forums say 18 pf would be a good choice, but I do not understand how they arrive at that conlusion.
I do understand that the caps we use along with the crystal are 22 pf each in series, so they form a capacitance of 11 pf effectively.
So if I am using two 22 pf caps, which amounts to 11 pf capacitance, then what would be the right load capacitance of the crystal.
This seems really critical because I am using I2C to communicate to my sensor.

#### retrolefty

#1
##### Jan 04, 2013, 08:02 pm

Hi!
I am confused regarding the load capacitance of the crystal to be used.
Some forums say 18 pf would be a good choice, but I do not understand how they arrive at that conlusion.
I do understand that the caps we use along with the crystal are 22 pf each in series, so they form a capacitance of 11 pf effectively.
So if I am using two 22 pf caps, which amounts to 11 pf capacitance, then what would be the right load capacitance of the crystal.
This seems really critical because I am using I2C to communicate to my sensor.

Internal pin capacitance of the clock pins of the AVR chip is also added to form the total capacitance loading amount for the crystal. The 'exact' proper total load capacitance is determined by the crystal manufacture and is listed in a datasheet for the crystal. Being off some will only effect the frequency generated a very small amount. So don't sweat it, it won't effect your ability to use I2C modules.

Lefty

#### positivelydoped23

#2
##### Jan 06, 2013, 08:24 am
So suppose if I want to use a crystal with a load capacitance of 6pf, what should be the capacitance of the external capacitors i use?

#### retrolefty

#3
##### Jan 06, 2013, 09:50 am

So suppose if I want to use a crystal with a load capacitance requirement of 6pf, what should be the capacitance of the external capacitors i use?

Most likely none as the Arduino X1 and X2 pins probably have that much internal pin capacitance.

Lefty

#### cmiyc

#4
##### Jan 06, 2013, 10:47 am

Some forums say 18 pf would be a good choice

You would rather trust Internet forums because random people on the Internet know better than the engineers who designed the chip?
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

#### positivelydoped23

#5
##### Jan 07, 2013, 07:18 am

[/quote]
You would rather trust Internet forums because random people on the Internet know better than the engineers who designed the chip?
[/quote]

Exactly the reason why I came to this forum, because I did not trust those random people.
The datasheet mentions a load capacitance of 12-22 pf for the crystal. However, I have had problems with selecting the capacitors.
The standard we use with arduino is 18 pf crystal with 2 22 pf caps in series. So overall the capacitance becomes 18 + 11 = 29 pf.
Now my question is this, If I use a 6 pf crystal, do I need to put caps which amount to an equivalent capacitance of 29-6 = 23 pf?

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