internal oscillator VS external XTAL

[cykrus]

What is the advantage of using an external XTAL with a PIC ? I see a lot of projects that use for example the 16f628 or 16f84 use an external XTAL. I have done a few projects and I always use the internal one and I save 2 pins and some space.

I do understand the purpose of using an external clock signal (to sync with other parts of the circuit) but the external XTAL ?

Thanks,
Vlad

[Ttelmah]

Accuracy.
The internal clock on many PIC's is 'barely adequate' for things like serial. Usually OK at room temperature, but for anything requiring better timing is just not good enough.
So (for instance) CAN bus is just useable at lower data rates, but for anything faster needs either a resonator or crystal.
USB at fast speed requires a crystal.
Anything with similar timing requirements, needs a more accurate timing source.

Best Wishes

[John P]

The accuracy may be important, and if you use a crystal you can go a lot faster than using the internal oscillator. But a serial link ought to work if at least one of the processors has a crystal-controlled clock.

The 16F84 is an obsolete part (as has been said here many times!) and one reason for avoiding it is that it doesn't have the internal oscillator feature.

[cykrus]

very clear now. thank you