On Tue, 17 Jun 2008 22:33:19 +0100, "Tom=E1s =D3 h=C9ilidhe" said: > I know about RC circuits, about the time constant, about exponential = > decay and all that. What I don't know is how the RC values actually = > yield a specific processor speed. I just need an equation, something > like: > = > Processor Speed =3D 7.56 * Time Constant > = > And then my next question would be what capacitor value I should use, = > given that I can have a really small one with a really big resistance, = > or a really big one with a really small resistance. Below 1 MHz you might be able to do something that simple. But higher than that the internal(non-linear) resistance of the PIC's RC oscillator plays too big a part. And stray capacitance becomes a problem when you get down below 100pF. = I think Microchip used to put charts in the datasheet but not for parts that have internal oscillators. There's no point. If you are interested look at a datasheet for an old part like the 16F84 you might find more RC oscillator data. But it is a different chip so the exact numbers will be different on a 16F887. Cheerful regards, Bob -- = http://www.fastmail.fm - A no graphics, no pop-ups email service -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist