>Hi Peter The formula I have to calculate driver level is: Px =
>2R1[piF(CL+CO)Vcc]sqr
>the manufacturer of the crystal oscillator gives me for example:
>F= 20MHz
>C0=7pF max.
>CL=10pF to 75pF
>drive level=1mW
>
>R1 is the resistance component of the Crystal when you look at it's
>model, which at lets say 20MHz is the only impedance you will see is this
>the value of RS I should be aiming?

Your formula is not computable as you show it. I only have access to a
formula for 32 kHz Rs calculation now. It assumes amplitude loss of 25% on
the crystal (roughly 3dB).

>Is it possible for you to explain your formula for RP calculation ?
>what do you mean with Voh and 1e-7.

Rp is simply calculated to ensure that with osc output voltage at one
extreme (f.ex. GND or Vcc), and current equal to the worst case input
leakage, the voltage on the input exceeds the threshold voltage.
Voh=output high voltage, Vol=same low, Vil=input l, Vih=input h. These
parameters and the leakage can de deduced from the datasheet. Then you
write (assuming the current is worst case and opposing the drive):

        Rp <= min( abs(Vol-Vil)/Iin, abs(Voh-Vih)/Iin )

Usually 1Meg works for Motorola parts, but the 68HC705 datasheet says
10Meg. This is a nightmare wrt. creep currents on the board. Rs (external)
can be zero for 20MHz acc. older Motorola datasheets.

Peter

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