Ok, so we’re going with a 16Mhz Crystal Oscillator. Ultimately, it could have been a case of “hey, we got a really good deal on this 16Mhz crystals.” too. Alternatively, it might have been forward planning, allowing greater use of the 16U2 in the future. We are using the internal USB voltage regulator, meaning our VCC has a minimum of 4.0V, and use of a 16Mhz crystal is allowed at 4.5V VCC, so it may have been a case of “well, we’re going with 5V VCC across the board, so we may as well give ourselves the extra speed”. The technical reason why 16Mhz was chosen as the operating speed is unclear to me, as the only thing the 16U2 micro does is USB/Serial conversion, which could easily be fulfilled at 8Mhz. We do have a minimum speed imposed on us by a very application specific reason: because we are doing USB signalling, we have to use an 8Mhz crystal at a minimum. Now, since power isn’t really an issue for us (ok, it could be if you’re running off a battery), we could just say, “Hell, let’s go all the way to 16Mhz! WOOT!”. However, the faster your oscillator runs, the more power you’re likely to consume. How fast does it need to be? The datasheet (table 8-3 page 30) says any oscillator from 400kHz to 16Mhz is valid. But which one? A search for 16Mhz Crystals on parts.io netted 104,000 search results! Looking around, I couldn’t find a resonator that had the accuracy I wanted (30ppm or less hopefully), so a crystal oscillator (xtal) was the right solution. It’s also impossible to figure this out without having the Atmel ATMEGA16U2 Datasheet to reference.įirst off, the datasheet says we can use either a crystal oscillator or a ceramic resonator. I leaned heavily on the Crystal Oscillator Basics Appnote from Microchip in explaining many of the following specs to me.
ARDUINO NANO SCHEMATIC CRYSTAL OR RESONATOR HOW TO
So we have to do some digging to understand how to select this part correctly. Unfortunately, the official schematic doesn’t list any manufacturer information for it! Only “16Mhz”. This little piece of quartz bangs away 16 million times a second, and does it with pretty good accuracy. The xtal acts as the ridiculously fast metronome for your microcontroller. It’s a mess! The only thing missing is a coffee ring stain across the middle of it. Ok, I said I wasn’t going to harp on the quality of the schematic any more, but just look at that thing.
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