MEGA schematic question

hi everyone,
I'm trying to understand the (official) schematic for the MEGA 2560. I am fairly happy with my understanding of most of it but I'm just wondering one thing. Looking at the crystal oscillators I can see that the microcontrollers (2560 and 16u2) have their own seperated clock sources. one labelled Y1 (for the 2560) and the other labelled Y2 (for the 16u2) but then in the top right hand corner of the schematic there is another two crystals (labelled Q1 and Q2) which happen to be on the same nets as the two attached to the chips.

I'm wondering; are these the same crystals as the ones shown attached to the clock pins of the two chips? and if so why have they been shown a second time? or are they something completely different?

Thanks in advance

2560 crystal oscillator...

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16u2 crystal oscillator...

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the ones in question...

Can you post a link to the schematic you are referencing? I do not see the extra crystals on https://content.arduino.cc/assets/MEGA2560_Rev3e_sch.pdf

I don't know why the resonators are doubled. See the labels XT1/2 and XTAL1/2 occurring twice.

Yes of course. SOrry I should have done that before...

the one you have posted does not show the 16u2 serial controller chip.

The link I posted has the schematic over two pages, the 16u2 is on the 2nd page.
(the line comes from the Arduino Store page for the Mega)

yes it's strange. ONly reason I can think of is to allow for more room to display the values for both the capacitors and for the crystal frequencies. But i dont see why they couldn't have just made made a bit of room in the parts of the schematic next to the chips for these

ahh right... thanks david. I'll have another look

Actually the one one you have just posted david makes a lot more sense. I think I will just refer to this one from now on.
Thank you :slight_smile:

Even stranger:: I can find only one resonator on my Mega board!

The resonator for the Atmega2560 looks to be near the header 3.3v/5v/gnd pins - it is much, much smaller than the part used for the 16u2.

< edit >
UNO is the same way, looks like there is an actual crystal on the 16u2, and a resonator on the Atmega328/Atmega2560.

It is so you can use a resonator OR a crystal, this way, there is the option for either. It's called duel footpriting.

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Quite so.

I dont see the confusion myself, just because the schematic has components on it, that does not mean ALL the components are ALWAYS fitted.

Thank you very much missdrew... This makes sense. I'd never heard the term dual footprinting before. Every day is a school day :slight_smile: