> > First of all, I would like to use the internal oscillator to 
> > save money and 2 I/O ports.

What is the reason for wanting to save money ? For manufacture
you can shop around and probably get a reasonably low price
for common-value crystals in quantity. If it's a one-off then you
can likely find something suitable, either a crystal or can, on an
old PCB (fax, PC, printer etc)

You need to consider these three things for every project

Effort - cost - performance

Generally -

No effort, low cost, probably average performance
Lot of effort, high cost, state-of-the-art performance

And everything in between

Speaking personally, for something like a frequency meter I'd stump
up for a crystal to get at least decent and repeatable performance.
And so not have to worry too much (= less effort) about timing. For
a little more effort I could add temperature compensation. Improved
performance, but of course costs a little more

What range of requency do you need to measure and over
what period ?

> P.S.: Please, do you know what is the link or the name of oscillators 
> makers?

Google is your friend. Depends where you are, and how you intend
to get your components (and how many)


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