> I wonder how many people actually even consider RC as
> an option and needlessly spend money on crystals or
> resonators. If they were more aware of RC and had an
> accurate way (on paper) of determining values perhaps
> RC would be first choice. I have to admit I myself tend to
> reach for the crystal box rather easily

In hobbyist projects the cost of a crystal (around $NZ1 = $US0.50 or so) is
probably not a major factor.
The main reason to use RC is probably to get low power consumption due to
low clock rate or in volume production where every cent counts and clock
stability and accuracy are not critical.

I have used RC in one only PIC application (a small keyfob sized disability
aid with a production volume of only a handful of units so far).
I wanted a lower clock rate to get lower power but not as low as 32 KHz, and
getting crystals below 1 MHz is problematical and expensive.
I ran RC at about 200 to 400 KHz.
The clock frequency varies not only with temperature but also with supply
voltage. The PIC is run on an unregulated 3 cell NiMH battery until such
stage as it refuses to run so clock frequency variation is marked..
I'm not using the LP part but tests indicate that the standard 16F84 parts
work well in this application including EEROM writing and reading).

I also wanted very very low power in shutdown mode and was surprised to find
that the shut down current was approx Vcc/2/Rosc.
A search of the data sheet confirmed that this should have been expected.
This is a VERY undesirable feature.
I solved it in this application by returning the top of the Oscillator
resistor to the reset line rather than to Vcc.
When the reset pin is low the oscillator draws no power.

The PIC current in reset is still unacceptably high compared to some other
available processors, even with everything possible shut down - it's
measured in microamps so I ended up using a power turn off circuit using a
high side transistor which is operated on user key press and then held on by
the PIC as long as desired. Supply current in off mode is << 0.1 uA.


regards


            Russell McMahon

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