Re: 32kHz crystal issues


"RST Engineering \(jw\)" <jim@xxxxxxxxxxxxxxxxxx> writes:
All the bushwa about "added capacitance" making the sucker pull off
frequency neatly forgets one small item ... added capacitance will
LOWER the frequency, and yours is too high.

Some reminders:

* The load capacitance is what the spec says it should be, mostly.
Not changing that.

* I asked about using a *different* crystal that needs a higher load
cap, but still what *its* spec requires.

* The scope clearly shows digital glitches on the sine wave, which I'm
pretty sure is the cause of the 8% speedup - some glitches cause
extra clocks.

I'm not worried about the glitches on the protoboard, I don't think
there's anything I can do about those - it's a *protoboard*. The
final circuit will be on a 4 layer PCB with proper EMI etc.

I'm mostly wanting to make sure the design will work on the PCB. That
boils down to three questions:

1. Am I calculating the load caps right? If the spec says "8pF" and
the crystal is 1pF, I need 7pF*2 = 14pF capacitors each?

2. Is the 330k resistor the right value? How do I calculate the right
value for that specific crystal, given the OSC2 sine wave and 1uW
power?

3. Are there other design issues I haven't mentioned, that I'd need to
be aware of when laying out the PCB? I know about the guard ring
and the keep outs, anything else?
.

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