AVR Schematic Review

Hello everyone.

I'm making my first electronics project that will (hopefully) end up as a PCB. It's a Serial to Ethernet Gateway that uses an Atmega328p (Arduino) and a WizNet WIZ811MJ (W5100) Ethernet board. I was hoping to get some review and critique of my circuit before I go and buy parts.

Here is some info on it...
I expect about 12VDC as input with regulators providing 5V and 3.3V on the board. The Atmega will have a dip switch for config settings and LED's for status. There is also a MAX232 for RS-232 level conversion.

I believe that I have everything wired and selected correctly, but since this is my first try at producing a PCB, I'd like a review.

I'm a little concerned that I might have selected the wrong cap values or types, so please let me know.

Also, for the WIZ811MJ board, there are 3 4.7k resistors pulling up unused inputs. Could I take the three unused inputs, tie them together, and then connect them to a single 10k resistor and then to 3.3V? Would that also work?

Finally, I wasn't sure, but do I need to add some kind of cap from the Atmega chip's reset pin to ground? I believe that this might be unneeded, but I wasn't sure.

Thanks for any help!

 

I don't really know why it must be Tantalum.

And why not use switcher ICs such as MC34063, both for 5v and 3.3v?
You'd need 470uF or something like this on the output, and 220uF on the input.

A voltage reference also could not be wrong, for instance a 78L05 + voltage divider (for 2.5V).

I never used MC34063 until a few days ago but it worked as intended, did not calculate anything. Keeps the heatsink away, and it's still reasonably small.

 

This is a doozy for a first project!
I hope you're planning to breadboard and test this before you build? Even an experienced designer and builder rarely goes directly from design to PCB. There are just so many things that can go wrong. I always breadboard and test all the individual functions, to see if they're working as I thought they would. It's rare that I don't make tweaks after seeing what's happening. DOn't build until you can specify every single component.

Speaking of "seeing what's happening", do you plan to use an oscilloscope? I mean, what happens when you hook it all up and it doesn't work? These things happen, and you need a plan for how you can diagnose and fix problems. This one would be a challenge.

 

I will be building this on a breadboard before making a PCB. I just wanted to make sure that my schematic looked ok to you guys with experience before I proceeded to spend money on parts. I was going to start with through hole parts for testing, and I was initially going to build the PCB with all though hole parts as well. But, I was considering moving to surface mount on the PCB.

The only way to test surface mount would be on the actual PCB correct?

Also, I do have a scope for testing if needed.

 

You are definitely ok with a single resistor, even without one it should work provided the inputs are 3.3v compatible.

This brings me to other issue, you have one chip powered by 5V and the other by 3.3V without any resistors on the data lines between them. When the atmega outputs a 5V on the data line, this voltage goes straight through the protection schottky diodes on the input of wiz chip to the 3.3V power rail. Since the 3.3V regulator doesn´t have any way of lowering its output voltage, the 3.3V rail will climb to about 4.3-4.8V which definitely is not good for either the wiz board because of overvoltage neither the atmega because of overcurrent on the output pin.

I suggest you make everything running on 3.3V if you can. Using max3232 and stripping the 5V supply should solve it alltogether, but if you have any inputs from outer world other than the rs232 you should use series resistors on them to accomodate 5V levels.

 

Are you planning on selling these or something?
Hope you've got plenty of orders lined up..
I guess you intend to make an Arduino board with Ethernet built into that board..
Are you making the pin headers compatible with other shields? You better...
Have you done a rough cost estimate based on estimated volumes? I'm hinting at your solution will be so much more expensive than just buying an arduino and ethernet shield so no one will buy it. (based on volume..)

You aren't reinventing the wheel.. There is already an Arduino and an ethernet shield that uses the same WIZ chip.. You just need to make the connections to the right pins that would normally go through the arduinos headers on the same board instead..

Here is the schematic already.. Just combine this with the arduino boards schematic and you are done...
http://arduino.cc/en/Main/ArduinoEthernetShield
http://arduino.cc/en/uploads/Main/arduino-ethernet-shield-06-schematic.pdf

 

The WizNet chip is supposed to be 5V tolerant on all i/o pins, so I think I'm ok there.

EDIT: I see what you are saying now... You mean run the Atmega328p at 3.3V. I could do that, but in order to keep it at 16MHz, it needs the 5V. Good point though.

 

I'm not planning to sell them. This is more of a learning project where I can have a nice device to show afterward. (I wasn't making an arduino shield.)

Currently, I have the arduino with ethernet shield already working and this project tested. I wanted to learn the process to circuit and PCB creation going from arduino to custom PCB.

Since you mentioned it, and just wondering... If I were going to sell these as a standalone device. Would what I'm doing be the wrong way to go?

Thanks.

 

No, SMD adapters can be used on breadboard. But not TQFP44 of course, or you need to mount it on bell wires.

using SMD is better because it's space saving in terms of inventory. But not cheaper if you use SMD adapters.

 

You can use this http://www.sparkfun.com/products/8879 or similar, but still half the pins would need wires on a breadboard, or you coud combine it with a piece of vector board and have all the pins in two rows.

 

Well, I meant that on a breadboard I would use, for example, a 10K through hold resistor even if I was doing a smt PCB. You wouldn't get adapters for all of the passive components would you?

 

I've updated my schematic based on your suggestions and suggestions from another forum.

Please let me know what you think.

Thanks.

 

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or this if you want cortex fun
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