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ATMEGA 328P Crystal pf

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ianscott:
Hi I'm ordering parts from digikey and I'm not sure what crystal I need,  I'm Running a ATMEGA 328p with 22pf capacitors (399-8034-1-ND). I Have found two types of crystals a 16mhz 20pf (X1103-ND) and  16mhz 18pf (CTX1085-ND). Which one should I go for. The 18pf's are cheaper.

 Also Would anyone know the exact crystals the arduino uno boards use something similar to those would work for me.

Thanks Ian

zapta:

--- Quote from: ianscott on May 07, 2014, 02:46:18 am ---Hi I'm ordering parts from digikey and I'm not sure what crystal I need,  I'm Running a ATMEGA 328p with 22pf capacitors (399-8034-1-ND). I Have found two types of crystals a 16mhz 20pf (X1103-ND) and  16mhz 18pf (CTX1085-ND). Which one should I go for. The 18pf's are cheaper.

 Also Would anyone know the exact crystals the arduino uno boards use something similar to those would work for me.

Thanks Ian

--- End quote ---

I had good success with 328P and resonators with internal capacitors. For example
http://www.digikey.com/product-detail/en/CSTCE16M0V53-R0/490-1198-1-ND/584635 for smd (there are also through hole versions).  Accuracy ~1% which was good for my application.

As for your crystal, i think both should be fine.

retrolefty:

--- Quote from: ianscott on May 07, 2014, 02:46:18 am ---Hi I'm ordering parts from digikey and I'm not sure what crystal I need,  I'm Running a ATMEGA 328p with 22pf capacitors (399-8034-1-ND). I Have found two types of crystals a 16mhz 20pf (X1103-ND) and  16mhz 18pf (CTX1085-ND). Which one should I go for. The 18pf's are cheaper.

 Also Would anyone know the exact crystals the arduino uno boards use something similar to those would work for me.

Thanks Ian

--- End quote ---

Actually the present generation of Arduino Uno and mega boards (rev3) don't utilize crystal resonators but rather use 3 terminal ceramic resonators that have internal padding caps.

sleemanj:

--- Quote from: ianscott on May 07, 2014, 02:46:18 am ---Hi I'm ordering parts from digikey and I'm not sure what crystal I need,  I'm Running a ATMEGA 328p with 22pf capacitors (399-8034-1-ND). I Have found two types of crystals a 16mhz 20pf (X1103-ND) and  16mhz 18pf (CTX1085-ND). Which one should I go for. The 18pf's are cheaper.

 Also Would anyone know the exact crystals the arduino uno boards use something similar to those would work for me.

Thanks Ian

--- End quote ---

It's probably not going to matter a lot realistically, unless you really want to really hit the marks as it were.  Either will be fine.

If you want to get into specifics though, there is a formula for calculating the capacitor values, given the load capacitance of the crystal from a datasheet.

  CX = 2(CL - Cstray)

CX is the capacitor value you use (assuming you use the same value for both caps),
CL is the load capacitance specified by the crystal
Cstray is stray capacitance inherent in the construction

Let's say you call the stray capacitance 5pF, that means for
  Crystal 1 which has 20pF load, each capacitor should be 2(20-5) = 30pF
  Crystal 2 which has 18pF load, 2(18-5) = 26pF

I'd probably use the 18pF one, because it's cheaper.

Bored@Work:
If you want to know more about it, and *shock* *horror* actually get an idea how to calculate this, get Atmel's application note 042.

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