'PIC 16Cxx RC oscillator values'
John & Maria Perry
I have an application that has no need for accuracy (+-50% would be fine,
but isn't needed :-), so I'd like to use the RC oscillator. I don't have a
prototype to check empirically, and the manuals say "see the device
characterization sheets," which I can find neither in the manuals nor on the
How do I find the values without hardware to solder and test?
In the datasheets, look for "Typical RC Oscillator Frequency vs. Vdd"
Graphs. Each PIC should have 3 or so of them (at least they do in the 1995
databook and PIC16C8X datasheet I downloaded from the Web Site) plotting the
frequency for different Rs and Cs as well as operating voltages.
As you've stated in your note, you're comfortable +/- 50% which is about
what I found that they are capable in terms of accuracy (I have typically
found them to be about 30% accurate). This means that the RC oscillator
can't be used for anything that even requires moderate accuracy.
Do you ever feel like an XT Clone caught in the Pentium Pro Zone?
Karen L. Black
Take a look at section 12 in the databook. Figures 12-2 through 12-4
show oscillation frequencies for a given R, C, and Vdd. A bit of
interpolation will give you an answer that's in the ballpark.
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