Why the crystal oscillator circuit of the Duemilanove reference design uses 22pf capacitors?

[Dob]

Hi guys, I'm trying to understand why the crystal oscillator circuit of the Duemilanove reference design uses 22pf capacitors. As far as I know, the crystal is designed for a 20pf load capacitance.

The formula for this is:
Cl = (((C1 * C2)/(C1 + C2)) + Cs

Among them, C1 is the load capacitance, C1 and C2 are the capacitance values, and Cs is the stray capacitance of the circuit board.

Suppose we want to use two identical capacitor values, the equation becomes:

Cl = ((C ^ 2)/(2 * C)) + Cs

Solving for C gives:

C = 2 (chlorine-cesium)

Assuming that the stray capacitance is 5pf to 8pf, the range of C values we obtain is 30pf to 24pf. So, why use 22pf capacitors instead of larger capacitors in the design? I can think of 4 possible reasons, but I don’t know which one is correct:

1. My math is wrong.
2. My estimate of stray capacitance is wrong.
3. I have not considered other sources of stray capacitance.

This value is not important for accuracy, and the easiest capacitor to find is the 22pf crystal oscillator.
One of the reasons for the difference? If it is #4, can you estimate how much error a 30pf capacitor will introduce when compared to a 16pf crystal with a 22pf capacitor?

To make matters more confusing, Table 8-3 on page 29 of the ATmega328 data sheet lists the recommended capacitor value of 12pf-22pf. It seems that the only way to use these values is to choose a lower load capacitance for the crystal itself. The lowest load capacitance I can find is 18pf. There are some 10pf crystals, but it seems that the C value is lower than the 12pf limit in the data sheet, and for RoHS reasons, they are no longer manufactured.

Can anyone help unravel my thinking process? Why use a 22pf crystal oscillator capacitor?

 

[AdamG]

Hi guys, I'm trying to understand why the crystal oscillator circuit of the Duemilanove reference design uses 22pf capacitors. As far as I know, the crystal is designed for a 20pf load capacitance.
View attachment 128983
The formula for this is:
Cl = (((C1 * C2)/(C1 + C2)) + Cs

Among them, C1 is the load capacitance, C1 and C2 are the capacitance values, and Cs is the stray capacitance of the circuit board.

Suppose we want to use two identical capacitor values, the equation becomes:

Cl = ((C ^ 2)/(2 * C)) + Cs

Solving for C gives:

C = 2 (chlorine-cesium)

Assuming that the stray capacitance is 5pf to 8pf, the range of C values we obtain is 30pf to 24pf. So, why use 22pf capacitors instead of larger capacitors in the design? I can think of 4 possible reasons, but I don’t know which one is correct:

1. My math is wrong.
2. My estimate of stray capacitance is wrong.
3. I have not considered other sources of stray capacitance.

This value is not important for accuracy, and the easiest capacitor to find is the 22pf crystal oscillator.
One of the reasons for the difference? If it is #4, can you estimate how much error a 30pf capacitor will introduce when compared to a 16pf crystal with a 22pf capacitor?

To make matters more confusing, Table 8-3 on page 29 of the ATmega328 data sheet lists the recommended capacitor value of 12pf-22pf. It seems that the only way to use these values is to choose a lower load capacitance for the crystal itself. The lowest load capacitance I can find is 18pf. There are some 10pf crystals, but it seems that the C value is lower than the 12pf limit in the data sheet, and for RoHS reasons, they are no longer manufactured.

Can anyone help unravel my thinking process? Why use a 22pf crystal oscillator capacitor?

Welcome Aboard @Dob.

 

[PaulD]

Hi guys, I'm trying to understand why the crystal oscillator circuit of the Duemilanove reference design uses 22pf capacitors. As far as I know, the crystal is designed for a 20pf load capacitance.
View attachment 128983
The formula for this is:
Cl = (((C1 * C2)/(C1 + C2)) + Cs

Among them, C1 is the load capacitance, C1 and C2 are the capacitance values, and Cs is the stray capacitance of the circuit board.

Suppose we want to use two identical capacitor values, the equation becomes:

Cl = ((C ^ 2)/(2 * C)) + Cs

Solving for C gives:

C = 2 (chlorine-cesium)

Assuming that the stray capacitance is 5pf to 8pf, the range of C values we obtain is 30pf to 24pf. So, why use 22pf capacitors instead of larger capacitors in the design? I can think of 4 possible reasons, but I don’t know which one is correct:

1. My math is wrong.
2. My estimate of stray capacitance is wrong.
3. I have not considered other sources of stray capacitance.

This value is not important for accuracy, and the easiest capacitor to find is the 22pf crystal oscillator.
One of the reasons for the difference? If it is #4, can you estimate how much error a 30pf capacitor will introduce when compared to a 16pf crystal with a 22pf capacitor?

To make matters more confusing, Table 8-3 on page 29 of the ATmega328 data sheet lists the recommended capacitor value of 12pf-22pf. It seems that the only way to use these values is to choose a lower load capacitance for the crystal itself. The lowest load capacitance I can find is 18pf. There are some 10pf crystals, but it seems that the C value is lower than the 12pf limit in the data sheet, and for RoHS reasons, they are no longer manufactured.

Can anyone help unravel my thinking process? Why use a 22pf crystal oscillator capacitor?

22pF is a standard value, and 20pF is not. These capacitors are typically +/- 10%, so that puts it in range. It is likely that the value is not super critical. Also, you are right that there may easily be a few pF of stray capacitance, depending on the layout details...

 

[mohragk]

Wrong subforum? I believe there is an electronics subforum: https://www.audiosciencereview.com/forum/index.php?forums/audio-electronics-and-hardware.26/

 

[Dob]

Wrong subforum? I believe there is an electronics subforum: https://www.audiosciencereview.com/forum/index.php?forums/audio-electronics-and-hardware.26/

thank you!