In some Proteus examples I found circuits based on PIC micro but they don't use any crystal or RC Circuit.
Is it possible to use PIC without crystal?
I wounder if it's used as an RC circuit with R=infinity but I didn't find information in the data sheets of pic about this point.
I have another question, What's the frequency of operation in these cases where there's no crystal is connected.
That’s because they have on-chip RC oscillator thus you don’t need external oscillator if frequency range provided by internal oscillator is suitable for your application.
Some PIC models have the internal RC oscillator but the frequency and accuracy are variable depending on the model.
As for example: 16F628 has 2 frequencies: 4MHz and 37kHz (5% precision not adjustable)
16F88 works with 8, 4, 2, 1 MHz, 500, 250, 125 and 31 kHz (adjustable).
Where external oscillator circuitry is present on an ISIS schematic it is there purely for aesthetic reasons and is excluded from simulation (by adding the property PRIMITIVE=NULL in the edit component dialogue form).
Processor clock frequency is specified as a property of the microcontroller (edit the component and type in the clock frequency). It would *seriously* affect simulation performance (a 20Mhz analog oscillation takes a whack of computation for example) for no benefit.
This is undoubtedly the most common reason for slow simulation performance.