novice oscillator questions
John Payson email (remove spam text)
> 1. I measured the actual rate of the RC oscillator, and found it to be
> 909091 Hz. If I put the scope probe on pin OSC1, I see a sawtooth wave. If
> I look on OSC2, I see a more-or-less square wave with the same period as
> the sawtooth. If I read my spec sheet, it says the frequency on OSC2
> should be 1/4 times what I see on OSC1. What am I doing wrong here?
The act of putting the probe on the active part of the oscillator will
add some capacitive loading and, in the case of an RC oscillator, slow
it down. While it would seem strange that the OSC2 would be running at
about 1MHz for an RC spec'ed at about 2MHz, it could be that the part is
simply not very accurate or that you didn't measure the OSC2 output freq.
very well (did you measure it, or just eyeball it as being similar to OSC1)?
> 2. I bought a 4 MHz crystal and some 22pF capacitors. When I insert
> these into the holes provided for their use on the board, I can only find
> a (very weak--fractions of a volt peak-to-peak) signal that gives me
> around 200 KHz. The signal is much weaker than it was for the RC
> oscillator, and doesn't look to me like a very healthy one either. It sort
> of has regular spikes, but I wouldn't call it a triangle wave or a
> sawtooth. (What shape does an oscillating crystal make?) I gather from
> some recent discussion on the subject that not all crystals are created
> alike, but shouldn't I at least be able to get close?
Putting the probe on a crystal oscillator, especially on OSC1, will cause
it to run poorly. Note as well, you must reprogram the PIC's oscillator
configuration fuse when you change oscillator types. I'd suggest checking
the oscillator type fuse, and also checking oscillator frequency without
loading the crystal [best way to do that is to write a loop to wiggle a port
pin and see how fast the pin wiggles.]
In reply to: <m0u4hvC-000BEiC@mailbox.mcs.com> from "Tom Sgouros" at Apr 3, 96 04:56:40 pm
See also: www.piclist.com/techref/timers.htm?key=oscillator
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