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Thread: Designing a shield for Arduino Rev 3 boards that also work with non Rev 3 boards...

  1. #1

    Designing a shield for Arduino Rev 3 boards that also work with non Rev 3 boards...

    Just wondering if anyone has played around designing a sheild that made use of the new IOREF pin on the Rev 3 of the current Arduino boards. In particular what is the best way of handling this such that the shield can handle the newer boards, that may be 3.3v such as the Arduino Due that was released yesterday. But still work on older boards that do not have these additional IO pins...

    Suggestions?

    Kurt

    P.S. - Will you be stocking the new board?

  2. #2

    Re: Designing a shield for Arduino Rev 3 boards that also work with non Rev 3 boards...

    Level converters typically run on two power rails -- one for each end. The ioref pin is the supply for the side of the level converter that faces the Arduino. For example, if you use optocouplers, you'll use ioref to drive the VCC of the output sides that face the connector.
    Another option is to just use 3.3V parts that have 5V tolerant I/O. The AVR will treat anything that's above about 1.2V as a definite "high" on input, so driving it with 3.3V (or even 1.8V) is going to work fine, as long as you can stand being driven at 5V.

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    Re: Designing a shield for Arduino Rev 3 boards that also work with non Rev 3 boards...

    I'm not really sure how many shields do not currently accept a 3.3V input as a valid high signal, but if they try to use a 5V output to something like the new Due then it will be toast. The protection/level-shift circuit to make the Atmel SAM3X 5V tolerant is really simple/cheap (a zener diode and a resistor), but unfortunately the reference Due does not include it. I don't really see why one couldn't take advantage of the Vio pin, but it would be far, far easier and cheaper to make the Due 5V tolerant than to replace/upgrade all of the shields with level-shifters/opto-isolators to make their outputs high at Vio (be it 3.3V or even lower).

    For that reason, I'm really hoping Rugged Circuits creates their own version of the Due. I've been very happy with my Ruggeduino, and a well protected ARM for Arduino should definitely sell well.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
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  4. #4

    Re: Designing a shield for Arduino Rev 3 boards that also work with non Rev 3 boards...

    Thanks guys, I guess if I am building my own shield, that I wish to use on both Arduino Megas and the Due as well as maybe the Chipkit Max32 and some of these boards do and some don't support the IOREF. Example of a shield I built earlier.

    Click image for larger version. 

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    Was wondering how hard it would be to get it to run on the DUE as well. I could for example add the IOREF pin, but for example some of these boards don't have the IOREF, so should I then simply add a jumper to jumper to 5v? If I do that then it might be just as easy to have the jumper for +5 and +3.3 and not bother with IOREF?

    But regardless of this as mentioned, I need to make sure that for those boards who are 3.3v boards that I do not try to feed them +5v on the IO lines... My guess is servos will run fine with 3.3v on their IO line as that is what the chipkit does and they appear to be fine... Need to maybe update my jumpers to allow 3.3vs instead of +5...

    Yep I wish they made the board more rugged, like the Pic32 is tolerant of +5v on most of these IO pins...

    Kurt

  5. #5

    Re: Designing a shield for Arduino Rev 3 boards that also work with non Rev 3 boards...

    You could just make it always output 3.3V for "high," and always accept 3.0V and up for "high" on input, and take its power from 5V. If you do that, you'll work with whatever.

    Note that the Zener+resistor option isnt' particularly good, because Zener diodes need a certain current to actually provide their particular voltage gap. You can get away with a lower current by just using a resistor divider. Plus, if you put 3.0V into a circuit with a 3.0V Zener plus a resistor, you will get almost no current in that circuit, making for poor signal quality. A 3.3k:2.2k resistor divider will work much better, and will waste less than a milliamp per high line. (Make your lines low on idle to waste nothing.)

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