PIC internal vs external mhz

Download the data sheet and refer to it any time you wonder about this chip:

Section 3 gives you all the clock choices available or usable. External clock source, 3 different frequency ranges of crystals or ceramic resonators, External RC fed oscillator, internal RC oscillator.

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You're new at this aren't you ?

It helps if you use the right terms. The data sheet will assist. I don't know the PIC but I do know some of flavours of the 8051 have an internal RC oscillator. If you use that, you don't need an external crystal or clock oscillator. That's the whole point.

RC oscillators aren't very accurate so no good for accurate timing. Nice and cheap though.

I tend to use resonators rather than crystals. Still quite accurate but a bit lower cost.

Graham

Pro: no additional parts required. Con: a less accurate system clock that's relatively sensitive to temperature changes as compared to other possible clock choices.