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Topic: Difference Between ATMEGA2560 and Arduino Mega Schematic (Read 828 times) previous topic - next topic

mcsteeze

Hi,
As I prepare to fab my on PCB using a ATMEGA2560 I have a short question.  I am planning to base my PCB off the design for the mega (as I do not have much prior experience in design etc.) the only problem that I have is that when I compare the 2560 pin mapping to the Arduino Mega schematic, things look quite a bit different (the pins do not line up).  Firstly the Atmega itself is square, but on the schematic it looks rectangular.  Similarly, on the schematic, starting from the upper left corner and heading down, the pins go Reset, Xtal1, Xtal2 etc. but on the actual ATMEGA ICU, nowhere do the pins go in this order. The numbers align, the pin marked 1 is same on schematic and icu, put the actual physical layout is way different. Could someone please help me so that I can make sure my pins and components go where I need them? Thank you all.

hansibull

Remember, there is a big difference between the schematic and the actual PCB layout. The schematic is made as simple as possible.

codlink

They make the symbol for ICs this way because of the way the way they want the schematic lay out to be.  Some pins on the IC have a complex circuit design which need more room on the schematic page.  Some pins only need a single NET coming off.  Pins are moved closer together for organization and simplicity in the design. 

The pins on the IC correspond to the pin on the schematic.  So if you connect the reset pin in the schematic, it will be the correct pin on the IC in the board layout.  So just connect the components in the schematic and the board connections will be correct.
//LiNK

Graynomad

Schematics are supposed to be a document that clearly conveys the intent of the circuit, there are exceptions where it make sense (connector layout for example) but there is normally no physical similarity between a schematic and the real components. In fact when people do that it usually makes the schematic almost unreadable because the "wires" are all over the shop.

______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

mcsteeze

Thank you all. To clarify
"So if you connect the reset pin in the schematic, it will be the correct pin on the IC in the board layout.  So just connect the components in the schematic and the board connections will be correct."

The main thing that got me tripped up in the first place was that the schematic places the reset and TX pins (etc.) in a much different and seemingly arbitrary order. It seems obvious now, they are doing this for clarity sake.

So what you mean is connect the components in the schematic to the right pins (the corresponding number pins on the IC Pinout diagram) and everything will be aligned and work, correct?

Graynomad

Quote
So what you mean is connect the components in the schematic to the right pins (the corresponding number pins on the IC Pinout diagram) and everything will be aligned and work, correct?

Yep, that's the theory.

The CAD software doesn't care what the drawing looks like, it only sees a "net" say from U1 pin 5 to U29 pin 30. They don't even have to be connected with a "wire" on the schematic as long as they both have the same net name assigned. Sometimes it's clearer to connect things with wires, sometimes is it's clearer not to.

______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

john1993

personally i prefer to have the schematic resemble the pcb for a variety of reasons (hand layout, repair, etc). so much so that i go through the trouble of creating all new part bodies for virtually every ic used.

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