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Thread: DNA Development Blog

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    Insider Curt's Avatar
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    DNA Development Blog

    The DNA board is meant to be an ultra-compact do-it-all project board for the hobbyist/enthusiast.

    I will be providing some wire harnesses for project I think this board is particularly well suited for.

    but I am getting ahead of myself. first let's design and program it! I am definitly interested in getting feedback on what features you want, to be sure I incorporate them, for feedback I will be using Doc's Tinker forum, since this is certainly a tinkerish toy

    some pics of the workspace:



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    Insider Curt's Avatar
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    Okay so I have done some research and consulting and I have decided to open up this design as a generic project board. In order to make it super-compact I am not going to have extra ground/V+ pins (which simplifies the header connections) and rather leave that up to the wiring harness.

    While this will run a paintball gun, It will definitely do a whole lot more, I am calling it the DNA board for now (working title, but I kinda like it).

    It will have 3 A/D inputs, two power-switch channels (can handle 20V at 9.4amps, with a 38amp pulse possible, I am confident it will snap any solenoid you can throw at it)

    Board voltage will be 3.5v, with a low-dropout regulator, so it will run from anything 4.5v and higher, on up to ... 18v so 3 AAA's will do the job if you really want, or a dual lipo.

    It will sense battery voltage, have an onboard LED and a separate USB connection terminal so you can talk to it to and from any smart device or windows box (I have the windows app already written, its very preliminary but it works)

    It is field-prorgammable, I have already written a boot loader for it so firmware upgrades (or hell just programming it) require only a windows box.

    All the code is written in avr-gcc, and I will be releasing substantial chunks of it.

    Now my favorite part.

    The board has a 3-prong connector coming out the bottom which will run either a single servo (it has a compatible pinout) OR use my "RNA" single-wire bus and communicate to an unlimited number of daughter-boards. I will have boards for servo breakout (as many channels as you want, I am thinking 8 to start), audio sampling, more power channels, a brushless speed control (and yes I will probably release the software used to drive it, which I don't think anyone else has ever done), and LCD display, a keyboard, hell anything we can think of.

    And... well anything you can think of. I will make the RNA bus an open standard and liekly publish my libraries in source form so you can do whatever you want and expand it, it will be kind of like an ethernet bus (look up THIN net) so as many devices on it as you want

    I like to publish snapshots so here is the current board layout. this is FAR from complete, it is very very VERY liekly going to change in size and dimention, but is currently 20mm x 9.25mm, including every feature I have just mentioned.


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    Insider Curt's Avatar
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    So I just got finished mocking up the power distribution for the USB port. USB standard states that a device shall never backfeed along the Vcc line. On the other hand I really loathe designs that tell you to either "have the power plugged into access the USB" or to "never have the power plugged in to access the USB" to me that is lazy, since if I'm right a single diode will allow the device to be powered by either bus without worry.

    So I wired the USB power line to the LDO regulator through a diode (which has a voltage drop on it of ... .7? 1.3? something like that. I'll have to go look it up but I seem to recall germanium has a lower drop than silicon. Anyways my voltage regulator specs 1.3v drop minimum to ensure proper regulation, USB voltage is 5 and I want 3.5 so.. as long as I can get it to 4.8v I'm okay. hmm.. the voltage drop across the diode is going to matter. I better nail that down.

    In any case when main power is applied, the diode will essentially shut down the USB voltage without backfeeding, so the final result will be it doesn't matter if you have the DNA plugged in or not, USB will always 'just work'

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    Insider Curt's Avatar
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    So I have a couple things to report, one is that the board actually SHRUNK when I was done with most of the layout, dimensions now stand at 9mmx19.6mm


    still not done of course, because of point 3.. seems the voltage regulator I chose,

    http://www.mouser.com/ProductDetail/...2b9cOayf09Q%3d while small, cheap, and sufficiantly low dropout had two faults, the first one being that it was adjustable (which I thought I had to do, I'll explain in a minute) and has a maximum rated input voltage of 18v.

    problem with 18 is it will likely not tolerate an e-mag which uses TWO 9v batteries, which (when fresh) outputs ~20v.

    See I thought I needed 3.5v to properly run the USB bus, and there was no suitable regulator that had that, nothing that was stocked and readily available, anyway. BUT then I did a load of reading and research and discovered that I can run the USB bus at 3.6v! Which means I could then use THIS little baby: http://www.mouser.com/ProductDetail/...fWIDUobZ4wU%3d . a full 1000ma output, low dropout, no resistor deivder necessary (saves board space) and input voltage of 24v. I'm in love. Will need a new layout, and have to order it so I can breadboard it up.

    Oh, why not 3.3v? The chip I am speccing does not operate at 12mhz reliably at 3.3v, needs a little more oomph.

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    Okay finished laying it out and routing it! I knew all those mazes and logic problems I did as a kid would pay off some day

    Now then comes the part where I check, recheck, then check again each layer, each trace and everything about this board. My Fab house asked that I please not have any tolerances closer than 8 thousands.. they know me... so I want to try and keep their lives simple.

    Sorry but this is just a damn small board. I also need to super-double-check the pins and labels and put together a bomb-proof BOM (Bill Of Materials) that they can build this from.

    Also need to check polarities and pinouts, I don't want 100 copies of the same wrong board I have to rework with a microscope and tweezers.

    WELL.. okay I have a problem.. what good is an A/D if there is no reference voltage? I need to get the onb oard 3.6v run out somehow.. problem is it's buried in pretty deep, and there is no spare pin. I'd have to go 10 (from 8) on the header and then what? what to use the extra pin for? That is assuming I can find a way to route it which right now I sure can't. I don't want to expand the board for one stinkin' trace, but there are just too many applications where access to the regulated voltage will be necessary.

    perhaps I could bring it out of one of the bottom pins? say "hey if you need a reference voltage, wire up another connector? no that's user abuse. damnit I don't know how to solve this.

    ...Okay after much soul-searching I expanded the header. its now a 10-pin instead of 8. this is still narrower than the mini-morlock's 12 so I guess I'm still at my goal of reducing size. So.. what to use the extra pin for? All the I/O on the MCU is taken, the only iffy one is the battery voltage A/D sensor.. can I heave that? How much will people care? Its something people won't miss if I never include I think.. hmm... dunno what to do.

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    Thanks to a very knowledgable forum member who goes by Scott Herman I have re-evaluated a couple of design choices. In particular I am a bit worried about the voltage regulator handling 20v, in my previous design I used a .. lessee.. one of these and I know it runs just dandy on the E-Mag, but it is dropping to 5v, not 3.6, and is not low-droput. Tell you the truth I don't know exactly what makes an LDO an LDO, so I'm only going to do what I know how to do.. get one in, hook it up, and see what cooks!

    I am alos rerouting some traces to keep the signal path a bit cleaner and away from the main current traces, as well as move vias off pads, which I understand makes assembly a real pain.
    I'll post another drop of the design here in a minute..

    got it, I think.. i added a whole bunch of copper to the VR, moved vias off solder pads, and provided a ground-path for U1 that hits the decoupling caps directly. Also providing a bit more useful snapshot suite.


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    Scott again pointed out to me (and I like to think I would have noticed eventually myself) that the schottkey anti-backlash diode is of course required, since it has to be across the drain and Batt+, duh.

    Luckily I found THIS little wonder http://www.mouser.com/ds/2/308/NSR20F30-118998.pdf which does the job in an 0603 package! way small! perfect for the anti-backfeed diode for Vin on the USB port as well. Landing pad for it is SUPER tight though, hopefully it won't give my fab guys much trouble. Didn't even need to increase the board dimentions to jam them on there.

    Still reviewing, reviewing more and then reviewing it all AGAIN to before I send this out. Also starting to think about what is going into the first daughter board.

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    Okay major rethink. The USB header has been bugging me, I don't like the idea that a custom cable is required.. who wants that? not me. What I want is a design that can be mounted somewhere and the USB plug be accessible from outside the case.

    So that means a 90-degree mount

    I also must keep the .1" 3-pin accessory plug for expansion.

    I looked into some USB plugs and found one that will do the job, SMT, and then puzzled and puzzled until I could figure out how I wanted to jam it in.. see there has to be a mounting screw involved, otherwise the force of insertion/removal of the usb plug will not be captured by anything, so I need a hole right behind the USB connector. a big one.

    Like big enough for a 4-40 cap screw.. so a quick jaunt over to McMaster-Carr and I got the specs for one, and I can JJJJUST fit it in there.

    Okay great. new poblem.. I have parts on both sides of this board, and headers poking out top and bottom. That means horibbly unbalanced. can't have that. all the tall parts need to be on one side for balance (and protection).

    hmmm...

    Okay I have it, the board is going to have to get a teensy bit longer, but the configuration will work. This is a complete re-work of what was a completed and reviewed lower section so give me a day or so to iron it out.

    Final analysis: yes there will be a standard USB socket, yes the servo-drive capability is retained, yes there will be a mechanical fastener to hold the board in and no, it is not going to be much larger than it is now, by my measure it will bee.... 22.3mm tall now (up from 19.6mm)

    Session 10

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    It was as much of a redesign as I was afraid itw as going to be, but it's done now! Well I still need a few review passes but mostly this is it:



    Current dimentions are 9.8x23 mm

    Note the mounting hole in the lower right, it is big enough for a 4-40 hex cap screw, the large red outline is the size of the cap (so the outline of the screw head) for design reference.

    NOTE the red is now the BOTTOM of the board and the blue the TOP. so the headers go into the page, the USB connector is on the blue side. I am going to try and do a layer-swap to keep red on the top where I am used to seeing it.

    It now uses a standard USB port, and the screw will anchor the board so a plug can be inserted/removed. At least I hope it will. I really wanted the screw centered in back of the connector but it had to be off by a few mills, I will be testing that when I get the part in to make sure its going to work.

    the LED remains on the red side, meaning the bottom of the board, this is so when the board is mounted against something, that something can be drilled and the LED will shine through. It also means I couldn't figure out how to put it on the blue side, and while trying to make lemonade, discovered that.. DUH.. I probably should have been doing that from the beginning.

    Oh and someone asked about a schematic? here is like a third draft, this is what I work from when I'm prototyping, then I erase/make notes and redraw it. This is like the 5th redraw, before another build-pass to make sure it works:


  10. #10
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    So excited! Got the USB port in and it fits perfectly, just as I had hoped it would. FedEx is supposed to have the OLED display in today too, which will be too much for me to bear NOT firing up asap.. which will be a load of wiring.. ah well... but it'll be worth it


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