Hi Paul,

Yeah, I see the error of my ways <G>

I will admit to not having thought about it much before saying it. I was
going from what I thought I had learned by reading a paragraph in Mchip app
note AN588, the Oscillator Design Guide. Actually, the paragraph is
correct, it is just worded strangely (IMHO) and when I read it quickly, it
sounded to me as if the crystal actually expected TWICE its rated
capacitance, when in fact, it was saying exactly what you are saying,when I
went back and read it more carefully. The paragraph is the second one under
the section "Load Capacitors".

What you say becomes clear when you draw only one ground symbol for the two
caps,and tie their ends together, and then to GND. Then realizing that
around the whole loop from GND thru C1,including the internal OSC1 cap,thru
the xtal, down thru C2 including internal OSC2 cap,to GND again,the total
reactance must be 0.

Thanks for the correction and explanation,

Sean

At 03:16 PM 7/2/99 +1000, you wrote:
>Sean Breheny wrote:
>
>> The value specified by the manufacturer is actually HALF of what the
>> crystal should really see, but since you use two caps, it IS the
>> actual value of the caps themselves.  For example, if you have a
>> crystal that specifies a 20pF load cap,then you use TWO 20 pF caps,
>> one on each lead,which actually gives a total of 40pF.
>
>  Eh Sean, care to have another try at that?
>
>  The manufacturer usually specifies the parallel loading capacitance
>to the crystal.  Since this is applied in practice by a capacitance to
>each end, being two capacitances in series, each of these capacitances
>must be *twice* the nominal load capacitance.
>
>  *Each* actual capacitance however is composed of your external
>component in *parallel* with the circuitry (inside the chip), so that
>component must add to the internal (chip) capacitance to make up the
>twice-load figure.  If it *happens* that the internal chip capacitance
>(each of two terminals) approximates the desired capacitance, then each
>of the loading capacitors will *happen* to be the same value as the
>crystal load spec.
>
>  I'd draw this, but am in a rush ;-)
>--
>  Cheers,
>        Paul B.
>
|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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