what are capacitor values used with crystal oscillator 20Mhz

[intan]

modify crystal oscillator

hi! i want to knw what is the capacitor values used with crystal oscillator 20Mhz. how to calculate it? and why capacitor is needed?
thanks

 

[btbass]

crystal oscillator markings forum

If it is a Pic micro any value between 22pF and 33pF will work ok.
The input to the oscillator on board the micro goes to an amplifier which forms part of the clock circuit. You set the gain of the amp using the config registers depending on xtal speed.
The caps stabalise the amp input and filter out spurious oscillations to keep the clock at the correct frequency. The larger the values, the slower the start up time but more stable clock.
If you want to get technical, get a data sheet for the crystal you are going to use.
There you will find the information and formulas for calculating the values.

 

[FvM]

crystal oscillator load capacitance

Basically, the load capacitance required for stable operation of an oscillator circuit, e.g. of a processor and the capacitance specified by the crystal manufacturer to achieve the nominal frequency don't match necessarily.

In this case, you should follow the processor specification, but possibly have to accept a frequency deviation. If a specified frequency must be exactly met, you have to choose a crystal designed for the respective load capacitance.

All crystal manufacturers have an option to order crystals designed for particular load capacitance. It's not feasible for a small quantities production usually.

 

[ME]

modify crystal oscillator frequency

Read these Ateml AVR application notes, especially AVR042 section 5: http://www.atmel.com/dyn/products/app_notes.asp?family_id=607
AVR040: EMC Design Considerations
AVR042: AVR Hardware Design Considerations
AVR186: Best practices for the PCB layout of Oscillators

AVR042 Section 5 (Using crystal and ceramic resonators) explains how to calculate the crystal oscillator capacitor values.

Please not that crystal datahseets specify a load capcitance, but this is not the same as the capacitor values you should use in your design. The load capacitance specified in crystal datasheets is the total load capacitance and the PCB traces and MCU pins also add capacitance so the actual capacitors you should use is lower than the load capacitance specified in crystal datasheets. AVR042 explains how to calculate the actual capacitor values based on the load capacitance specified in the crystal datasheet.

The PCB layout for oscilltors is also very important. Keep tracks from MCU to capacitors and crystal as short as possible and make sure to have good PCB sheilding to reduce EMC.


Here's another application note to read:
Crystal Oscillator Circuit Design **broken link removed**

 

[XtremeTuningSolutions]

Re: crystal oscillator

Hello,
I am using a PIC16F877A (TQFP). I wanted to know which crystal should i use? I have the Orginal crystal, but i cannot seem to find it.

The reference location is: XTAL2.
The Markings on the crystal is: 20,0C71

I know its a 20Mhz.

Please, if anybody can assist me in getting this exact crystal by reading the markings off the crystal?
Thank You

 

[pranam77]

crystal oscillator

I guess you cannot use any other crystal other than previously used. Coz using other values will result in the processor executing the firmware fast or slow in regards to the variations done from the original value. In case you do not get any crystal, only way is to modify the code. Good luck.