Clock Speed accuracy

Gadgetman

How accurate is the RC-clock, and is there any way to anticipate some of the deviance?

I'm asking because it states in the manual that the RC-Slow 20KHz can vary from 13 - 32KHz and the RC-Fast 12MHz can vary from 8 - 20MHz.
(The 20KHz is more important to me)

I assume the deviation is mostly caused by temperature changes?

If so, is it a gradual and predictable deviation, or is it more random?

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Don't visit my new website...

Beau Schwabe

Gadgetman,

Because of the variance in frequency from the RC clock, you might consider using an inductor
for an alternative solution to using the RC clock or a Crystal
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Just a few preliminary tests with some inductors I had lying around, the math seems to agree.
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_clkmode setting --> xtal1        xtal2        xtal3 

             .13uH - 73.7MHz 
             .16uH - 66.6MHz 
             .27uH - 50.9MHz 
             .33uH - 46.2MHz 

              50uH -  3.76MHz 
             100uH -  2.67MHz     3.13MHz      3.97MHz 
             200uH -  1.86MHz

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The Propeller Code simply defines or "sees"·the clock as if it were a crystal....
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···· _clkmode = {xtal1, xtal2, or xtal3}
_xinfrequency = {this is just for reference}
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Note:· xtal1 uses an internal capacitor value of 36pF
········· xtal2 uses an internal capacitor value of 26pF
········ ·xtal3 uses an internal capacitor value of 16pF

········· You can also use the PLL to synthesize a frequency in this mode.·
········· This could be useful to the Propeller so it could track it's own frequency.






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Beau Schwabe

IC Layout Engineer
Parallax, Inc.