How do I deal with different XTAL pin capacitance when selecting 32kHz XTAL

[miceuz]

Hi all,

I'm trying to select a 32kHz crystal for Atmega48A MCU and avoid using load capacitors to keep the part count down.

Atmega48A specifies different capacitances for Xtal1 and Xtal2 pins:

18pF for Xtal1
8pF for Xtal2

The datasheet says: "The capacitance (Ce+Ci) needed at each TOSC pin can be calculated by using:C = 2 * CL – Cs" Where CL is load capacitance specified for the crystal and Cs is stray capacitance. http://www.atmel.com/images/Atmel-8271-8-bit-AVR-Microcontroller-ATmega48A-48PA-88A-88PA-168A-168PA-328-328P_datasheet_Complete.pdf

Can I assume that each pin has (18+8)/2=13pF of capacitance? Can I select a 8pF crystal and use it without external load caps assuming ~3pF of stray capacitance?

[miceuz]

anyone? 

[LA7SJA]

Just try end see if it works. You can't kill any thing so just try, and you can change the startup delay to be shure it's stable before executing the program. Check with your scope (you do have a scope?) whit the caps and whithout and watch the startup and signal of the osc. I have nevere needed cap's on the type of 32kHz XTals I use.

[HackedFridgeMagnet]

Can I select a 8pF crystal and use it without external load caps assuming ~3pF of stray capacitance?

From the datasheet page 33
it says

Crystals specifying load capacitance (CL) higher than 6 pF, require external capacitors applied as described in
Figure 9-2 on page 29.

So I would say no to 8pF


My calcs, correct me if I am wrong.

C = Ci + Ce = 2 * CL – Cs

therefore Cexternal = 2 * Cload - Cstray - Cinternal
assuming 5.5pF xtal load cap. (just because this is where Xtal2 hits zero.)

Ce(Xtal1) = 2 * 5.5pf - 3pF - 18pF  which is going to be less than zero so it is zero.

Ce(Xtal2) = 2 * 5.5pf - 3pF - 8pF  which equals zero.

all you got to do is find a 5.5pF xtal and make sure your stray cap on Xtal2  is  >= 3pf.

If you are going into a large production run I would be very careful here. If not try it and see.

[LA7SJA]

Some info from Atmel AVR4100: Selecting and testing 32kHz crystal oscillators for Atmel AVR microcontrollers

Johan-Fredrik