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'[EE]: Oscillator calculations'
2004\04\03@042417
by
Luis Moreira

Hi Guys
I am looking into the Motorola 68HC908GP32 for a project and I got stuck on
a problem that I never needed to look into, oscillator circuit calculations.
on the Motorola uC you have to add to the normal oscillator circuit used for
the Pic, Rs(series resistor with the oscillator) and Rb (resistor in
parallel). I know what they are for I just do not know how to calculate it.
I googled already and every info I found is a bit inconclusive.
The crystal manufacturer gives you CL ( load capacitance), drive level which
Rs controls and static capacitance of the crystal.
So what I want to know is:
From this parameters given by the manufacturer, how do I calculate
Rs and Rb.
I know this may seem to be a bit to much detail, and for what I am doing at
the moment it is but if I start working with higher speed oscillators I
would like to know how to calculate the components.
P.S. the oscillator circuit is a pierce configuration.
regards
Luis

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2004\04\05@051121
by
Peter L. Peres
The Pierce with symmetrical in and out can be assumed to dissipate no
energy so you can consider the drive level into the input impedance of the
osc. in pin of the chip to be your target. This will make you undershoot a
little so reduce the Rs you calculate like this while testing. Rp is given
by Moto (read between the lines wrt input leakage current worst case
assuming output is dc at the opposite polarity). E.g. Voh=5V leakage=100nA
you want to achieve 2/3Voh on the input so Rp=(Voh2/3Voh)/1e7~=3*10^6
ohms. You can choose 1Meg probably.
Peter

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2004\04\05@054913
by
Luis Moreira
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?
Is it possible for you to explain your formula for RP calculation ?
what do you mean with Voh and 1e7.
thanks for your answer
regards
Luis
{Original Message removed}
2004\04\06@055945
by
Peter L. Peres

>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 1e7.
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(VolVil)/Iin, abs(VohVih)/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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