PIC Oscillator Crystal

hugoender said:

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Does that mean that instructions can be carried out faster with a higher oscillator frequency?

For example:
4MHz Oscillator: Period (clock cycle) = 1/4MHz = 250ns; 1 instruction = 4 clock cycles = 1us

20MHz Oscillator: Period (clock cycle) = 1/20MHz= 50ns; 1 instruction = 4 clock cycles = 200ns

Is this correct? I would greatly appreciate any examples where certain frequencies are required or additional information on oscillators and their function in PIC circuits.

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hugoender said:

No one has answered if my calculations were correct... also, in layman's terms, what is PLL and is it possible to use the internal and external oscillators? If so, how do you calculate the overall frequency (with both the internal and external oscillators in use). I still have not received my PICkit2 nor my PICs so I have not yet been able to get some hands on experience so excuse me if my questions seem a bit dumb.

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The frequency of the crystal osc is divided by 4, this means a 4MHz xtal would give a PIC instruction
execution time of 4MHz/4 = 1uSec.
The usual range of xtal frequencies are 4, 6,8,10MHz.
If you were going to decide to use the PIC as a RTC or Timer, a 3.2768MHz xtal would be a good choice
as its divides nicely down to 1 sec.

The PIC that you use will determine the maximum xtal frequency that can be used.

Some PIC's have an internal PLL which can increase the frequency of the external xtal,
so for example a 10MHz clock could use the PLL to give an internal frequency of say 40MHz.

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hugoender said:

No one has answered if my calculations were correct...

You guys still have not told me if my original calculations make sense

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