I am a software engineer who is new(er) to electronics, I only started a couple of months ago. I've built a bunch of custom circuits, then some arduino nodes that are around my apartment that communicate with a C# server. So I've at least got the breadboarding part down.

So now I am working on a controller board that connects to my home automation system and determines the color on the LED light strips I installed into my bookshelf. So I want to transfer from working with pre-built Arduinos to building my own full node, there are some things I think I'm missing.

I've noticed that there are duplicate parts on the schematic for the Arduino Mega2560 schematic (such as there being 2 voltage regulators - one is replacement if the other is not available). I couldn't find any such description however for why there are 3 crystals located on the board. I see 3 different circuits for crystals (all of which are 16MHz).

Specifically, I am referencing the following Crystal 1

Crystal 2

  • Maybe different options. The top one may be a ceramic resonator rather than a crystal. What's actually fitted on the board? The other one with XT1 and XT2 may be for the FTDI serial chip.
    – jippie
    Sep 14 '15 at 19:07
  • Where did you get that schematic? That is not from the R3 schematic on arduino.cc. Sep 14 '15 at 19:07
  • link
    – clamport
    Sep 14 '15 at 19:08
  • @jippie That's a good point, I will trace back XT1 to see if it comes from the FTDI. This brings up an additional question, why are the FTDI chip and the ATMega on different clocks?
    – clamport
    Sep 14 '15 at 19:09
  • 2
    A shame this was migrated; it is actually a very good general EE question. Sep 14 '15 at 19:28

One of the crystals is for the ATMega8U2 chip which is being used for USB-Serial conversion.

The other two crystals are both for the ATMega2560. Why two? Basically they only populate one of the two, but they have two different footprints. One is for a surface mount resonator, the other for a through-hole crystal.

Having two footprints gives the option during assembly to go for the cheapest package at the time. It also means that the hobbyist can replaces the low accuracy resonator with a high accuracy crystal if their project requires.

As a side note, if you look at Y1, you will notice it is not the symbol for a crystal, it is of a resonator.

  • Awesome! I couldn't tell why the design was different. I am new to the hardware side of things, could you point me to where I could find some information on how resonators and crystals are different?
    – clamport
    Sep 14 '15 at 19:11
  • @clamport See thisEE.SE question and this PDF. Or just google "crystal vs ceramic resonator". Sep 14 '15 at 19:13
  • @NickAlexeev it does - look in the top right corner. Sep 14 '15 at 19:17
  • @TomCarpenter Also of note: the current version of the Eagle files for Arduino Mega2560 doesn't seem to have the spare footprints Q1 and Q2.
    – Nick Alexeev
    Sep 14 '15 at 19:19

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