Tim Shoppa
Guest
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 Posted:
Mon Feb 14, 2005 7:09 pm Post subject:
Re: Recommend 32.768KHz crystal for use with 16LF88 Timer1? |
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| Quote: | Can someone recommend a thru-hole crystal for use with
the 16LF88 (one of the newer NanoWatt PICs with the
low power optimized Timer1 oscillator). (Recommend a digikey part #?)
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It doesn't matter much. All common 32.768kHz crystals are pretty much
the same. They all have the same cut so they've
all got the same temperature curve. Series resistance is somewhat
variable, and the better units have lower series resistance, but when
you put a multi-hundred K or M resistor in series (which you have to do
to limit the crystal drive level) then the ESR becomes irrelevant in
comparison.
If you'll be mass-producing this gadget, then choosing the
drive-level-limiting series resistance can become somewhat difficult
given the variables of crystal parameters vs PIC oscillator gain
parameters as a function of voltage temperature etc. But on the bench
it'll always work :-)
You will want to put a variable capacitor in there if you want to trim
the frequency. You can also do this trimming in the firmware and just
use fixed capacitors on either side of the crystal. Others will tell
you that the capacitors are determined by crystal parameters but really
the oscillator parameters are more relevant because you don't really
need to match the load capacitance to get the thing to oscillate. And
if it's not oscillating it doesn't matter that you've matched the
crystal's load capacitance!
| Quote: | Will be prototyping circuit on solderless breadboard - any problems
with doing this?
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Not a problem that will stop oscillation at 32.768kHz, but the trimmer
setting will be a bit different due to the breadboard capacitance (many
pF).
Tim. |
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