ATmega48 and crystal 32.768 KHz

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#1
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Hi there,

 

I would like to ask a simple question.

 

While reading the  AVR134 apnote (Real Time Clock (RTC) Using the Asynchronous Timer), I see that ATMEL suggests to connect the 32.768 KHz crystal directly at TOSC1 and TOSC2 pins without using any caps (nor resistors).  Is it OK to do this or is it critical design ?  Do I have to connect any caps there ?

http://www.atmel.com/Images/Atme...

 

Your suggestion please ?

 

Thank you.

Michael.

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You don't need any caps.

 

David.

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Thank you David.

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

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You don't need a cap IF (big IF) you purchase the crystals for the correct parallel capacitance that matches what the M48 provides. In the spec sheet, there is somewhere that describes the built-in net parallel capacitance (or maybe the values of the two individual caps). If it shows the values of the two caps, figure the series combination and add a few PF for board capacitance. That should  come very close to one of the standard parallel capacitance values. Purchase THAT crystal. If you don't, the frequency WILL be off.

 

Jim

 

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