Home > General, Project Description > ZephyrEye Rev2: Call for Schematic Reviewers

ZephyrEye Rev2: Call for Schematic Reviewers

There are a few inevitable truths in this world.  Taxes will rise, Wookies shed all over the furniture, Luke and Leia are related, and there is no such thing as a perfect first draft schematic.

The Rev2 circuit is nearly complete.  It looks AMAZING if I do say so myself.  That’s the problem, though:  I’m inundated with excitement and therefore am unable to find things that are wrong because I don’t want to find any reason that might delay getting the circuit boards back as soon as possible.

I’m calling out for a few extra set of eyes to look over the schematic   If anyone could please go to the Google code, download the Eagle CAD files and take a look, I’d really appreciate it.  If you want to make changes, let me know so I can arrange for them to be merged back in properly.  Even if you’ve never looked at schematics before, take a look and as always, feel free to ask what’s going on in the comments.

This is also a great chance to give suggestions on functionality.  I should add, it may be your LAST chance!  Please give some comments if you think you might ever build one, if nothing else just to say you think it works for what you’d like to use it for.

If you’re not familiar with CAD schematics and circuit board layouts, it might be interesting to look at the history of the .sch and .brd files in the Google Code repository.  By looking at older revisions, you can see the steps taken along the way chronologically.  I commit changes at least at the end of almost every day I work on the project.

The current Bill of Materials can also be found at this Google Docs spreadsheet.  It includes estimated pricing – it currently comes in at just under $200.  A little bit higher than I was hoping, but about the same cost as Rev1 and Chuck Norris (adverbicized) packed with new extra features!

The schematic is hopefully organized well enough for someone not intimately familiar with this project to try and understand one section of the schematic at a time.  The capacitive touch schematic is separate, because it will be a separate board.  The way it works is you put copper pads on the board, glue it to the inside of your enclosure, and it senses you touching it on the outside of the enclosure.  Pretty nifty, and a great way to avoid milling the enclosure.

Please post comments below, or add to the Google Groups discussion page.

  1. 25 February 2010 at 5:22 AM

    There should be some bypass capacitors between L1 and U1:61.

    You should consider using AVCC to power your audio circuit (AVCC would come after L1) to reduce digital noise in the audio section.

    Read through Atmel’s recommendations for this microcontroller, especially regarding bypass capacitors – you are going pretty light on them right now (which may be fine most of the time…) http://www.atmel.com/dyn/resources/prod_documents/doc2521.pdf

    If you’re using a lithium chemistry battery, be aware that discharging it past a certain point will damage the battery, and possibly cause “venting, with flames” (They never call it “exploding”)…

    Why do you have a so many regulators? VR2 and VR_QT could both be supplied from the existing IC5. You may also be able to power the GPS with IC5 and use a mosfet to power the antenna, and send commands to the GPS to put it into ultra-low-power mode rather than switching it off altogether (putting it in low power mode will also allow it to keep the almanac, ephemeris, and last known location so startup can happen in under a second in many cases).

    I haven’t looked at the PCB layout (only have 15 minutes for this review) but the above are items you might want to either 1) explain your design decision or 2) investigate further.

    However, the design will certainly run. The lack of bypass caps is the only worrying thing that might cause hard to solve debugging chores down the line.

    • 25 February 2010 at 9:25 AM

      Thanks Adam! Great to get my memory jogged every now and again. Bypass caps! I knew I forgot something. The chips are hungry – in the spirit of Marie Antoinette: Let them eat bypass caps!

      I fixed the bypass capacitor location between L1 and AVCC of the XMega – it was definitely not right. The rest of the design follows Atmel’s design recommendations for XMegas, which can be found at http://www.atmel.com/dyn/resources/prod_documents/doc8278.pdf (or in the SVN reference folder). I’ve got a bypass cap for every VCC/GND pair on the chip – Should the caps be larger or more of them? I’ve also double checked and added bypass caps to most other chips on the board.

      Thanks for the tip on using the AVCC net for the audio circuit – Will do!

      There are a lot of regulators to following chip design requirements. The LCD has flickering issues if it does not get its own voltage regulator. SparkFun’s first LCD breakout board had this issue, and it was kind of annoying. It also needs +6.8V for the white LED backlight. So it gets two regulators, plus connection to the main Vcc regulator.

      The QT chip is an extremely sensitive analog chip, and the datasheet pretty much demands it have its own regulator.

      The batteries from SparkFun include built-in protection against over voltage, over current, and minimum voltage. I’ve personally left one of them dead as a doornail in my homebrew MP3 player for about a month (bad Brad!) without any safety issues arising. So I know they work as advertised (but don’t try this at home, kids ;). Of course, the battery never had the same umph it had before that, but definitely no “venting with flames”, lol…

      That should be a last resort, though – the XMega has a voltage sensing circuit and will shut down most parts of the system, at least to a low power state, if it detects the battery is getting low. The on/off switch removes power from everything except battery backups for the XMega and GPS.

      I’ve connected the VBAT pin of the Venus GPS chip directly to the battery. This way, I can shut it off without losing almanac, etc., and still hot start. If I’m happy with software low-power capabilities, then I will probably use that method. The datasheet doesn’t specify a low power mode or current draw for such a mode, however, so I’m playing it safe.

      Thanks for the pointers – you definitely made me go back through the datasheets and double check a lot! I probably still need to take another look at my intended low power scheme. BTW, if you check out or browse the repository, all of the datasheets I’ve used for the design are in the “reference” folder.

      I’ll get the changes implemented and update the above graphics. Let me know if I got your changes right!

  2. 26 February 2010 at 11:20 PM

    Schematic updated.

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