MC9S12DP512 crystal selection

[prog_ram]

hello,

I am designing a board for MC9S12DP512, and I have to select the crystal.

I chosen to go with the Pierce configuration. I understood that the two capacitors value will be equal to the load capacitor of the crystal (suppose to be 20pF so C1=C2=20pF) , and the Rb will be (according to Freescale notes AN2727) about 1M. Now in the Freescale notes, they say that there is a serial resistor to be added Rs to the crystal circuit and its value is to be obtained from the manufacturer. I am using the FOX 16Mhz crystal and I have searched the whole manuals for the Rs value and I couldn't find it..

Does anyone go through the Crystal design for the MC9S12DP512? and what are the value of the Pierce serial resistor?

I appriciate any help here..

thanks.

[Nabla69]

Hello,

The value of the series resistor is variable and depends on the crystal, as indicated.

This resistor is called damping resistor and is to limit the amplitude of the oscillations.
You can determine the value of this resistor by doing Safety Factor calculations.

The value is usually around the kilohm, but putting a wrong value can prevent your oscillator from starting.

Georges.

[prog_ram]

Thanks for the help,

I read in the AN2727 that the oscillator is a 2.5V module and if the crystal I am using is 5V (they didn't mention anything about the amplitude!), then I have to add some voltage divider. Now does the serial resistor has anything to do with this?

and , what is the Safety Factor calculations?

thanks again Nabla69.

[Nabla69]

Hey,

I looked at the Application Note, but I'm afraid you did not read properly.
The divider is only for external oscillators like "canned oscillators" and is not for crystals.
The crystal is powered by the MCU and will use the 2.5V.
Many canned oscillator is powered by an external 5V power supply to give a 5V signal.

Other posts on the Forum are talking about Negative Resistance and Safety Factor.
There is also a special Application Note, but I don't remember its number.

Jeremy.

[prog_ram]

hi,

thanks for the explanation for the Application note.

the problem is that I don't know what are the factors that affect the value of the serial resistor. in other words, how do I calculate the value it should be from the crystal information?

is it something related to the physical layout (or structure) of the crystal or it has something to do with the electrical characteristics?

thanks.

[bigmac]

Hello,

For the Pierce oscillator configuration, the series resistance is present to limit drive levels to the crystal (to prevent overload), and is usually only necessary for very low frequency crystals, e.g. 32kHz.  For a 16MHz crystal, the resistor value would typically be zero.

Regards,

Mac

[Nabla69]

This is with the electrical spec of your implementation:
- crystal ESR,
- PCB layout,
- MCU resistor.

As said previously, find and read the Application Note explaining how to do both measurements and calculations.

Fabrice.