Hi both,
aarg:
Do you have the Mega2560 data sheet in hand? If you connect a 15Mhz signal to an input pin, you are expecting some kind of response every 66 ns. Is that realistic considering that it is almost the period of one processor instruction cycle?
Paul_KD7HB:
Let start over. Exactly how often do you need an external interrupt for your project?
Answering both: As what I want to do is a project that can output VGA signals in different formats, more important for me is not the frequency of the particular clock I am using now but actually how to make use of it.
For the beginning I chose the 15 MHz to try to generate a signal that remotely could be like the one from the Amiga. Later on, I plan to use a 32 MHz one to be able to produce a "standard" 640x480 or even 800x600 @ 70 Hz. Only concern for the 32 MHz one is that Mega 2560 is half that speed, so I guess that the ISR would take (most probably...) more time than the interval until the next pulse. In that case I have an M0 waiting to be used.
So basically, code should be the same, getting triggered by an external clock's pulse via ISR. Only things that would differ are the number of pixels per segments of both signals (V and H).
Hope that answered the questions. Anyway, if sticking to 15 Mhz or 32 Mhz is a problem, I can adjust... I have absolutely no knowledge on crystals / oscillators so whatever I can take from this is a very good learning I will make use of, know or in future.
But as said, in case a crystal or an oscillator is not the solution for what I am trying to achieve, I will be happy to know.
Thank you!!