4-AXIS 8-Button USB-Adapter Schematics

[Guest LeoL]

As promised, here is the schem + BOM for the one chip USB device.This will give you guys a chance to start putting it together. But first a few tips...1) Don't solder the PIC IC onto the board. You'll need to remove it and program it off the board so use a.28 pin DIP socket.2) You'll need the latest Beta IC-Prog to program this particular PIC. If the current version still doesn't support this PIC, give me a shout and I'll email you the beta version that does.3) Can't think of any item 3...I'll be programming the firmware in the coming days, but I'll need to know from one of you or from Mike (of course) that has built the MVDT decoder he designed, how much power it consumes...specifically how much current does it draw from the power source when in operation. Similarly, current drain from whatever osc you're using (555 timer or other device).What I may be able to do is specify in the code what the total current drain will be for 4 MVDTs + decoders & osc and you will then be able to use the USB Bus power instead of an off board power supply. FYI - Limit is 400 ma (+100 ma for the USB chip, that

[Guest MikePowell]

Leo,Thanks. That was quick.The prototype here pulls 13 ma from the +12 and 17 ma from the -12. The will vary based on the inductance of the LVDT primary. A lower indictance will result in more drive current.Mikewww.mikesflightdeck.com

[Guest LeoL]

Hmmm, +/-12V is a no go. I'm afraid you guys will have to use an external power source for this unless you think you can redesign the decoder using only a +5 V source.If not, then care will have to be taken when connecting these external decoders to the USB module. Its paramount that you avoid any floating gnd conditions and other nasties since the USB SIE engine is very finicky and easy to trip up. Some kind of Opto may be called for perhaps for the input.You can try it without any protection and connect the common grounds. If your USB device suddenly disappears form the windows control panel, then there is definitely a problem somewhere and isolation circuitry may be required.I've not needed one to date so I'm not all certain how the module will respond in this case.Let the games begin...:-beerchug -Leo

[Guest MikePowell]

Was afraid that might be the case. The quick and dirty fix is to install an inverter. A packaged inverter will cost more than everything else together, not a desirable solution, I think. A single voltage design is possible, though not something I can pursue right now.Let's see what performance we get with separate power supplies and carry on from there.Mikewww.mikesflightdeck.com

[Guest LeoL]

Sorry, but had I spent more time looking at your decoder, I might have noticed that.Indeed, lets keep it simple and cheap. I'm pretty sure it'll work. I've done some pretty stupid things to these USB chips in the process of development...most have somehow survived. ;)-Leo

[Guest MikePowell]

I've plugged 16F628's into the programmer backwards to no lasting ill effects. Didn't program particularly well. But once turned around they seem to work okay. Microchip apparently designs their products with us in mind.Mikewww.mikesflightdeck.com

[Guest LeoL]

Here is a link to a page I've set up with the complete project files including the pre-compiled HEX file for programming those USB PICs. Please discard the previous schematic since it had 2 errors.I've included some images of the prototype. Roughly 2 hours work to build and find that damned unconnected pin.http://www.vif.com/users/leolacava/usb/baby_plasma.htmlI'll probably add more features to the code (i.e. better filtering) in the future but this should do for now for testing those home made LVDTs.-Leo

[Guest Ramsberg]

Good work Leo!Question about the schematics:If used for L(M)VDTs, should the R2-R5 RPot be connected in serial with the lvdts?Should the LVDTs be connected "between" the VDD and for example RA0 of the 16c745? Common GND for the LVDT encoder and the USB circuit?Thanks!Olle

[Guest LeoL]

Sorry for the confusion in the schematic. The pots in there are for those who don't wish to use LVDTs. It's either pots or LVDTs but not both. I'll add a note in the schematics to clarify this.Let me know how it works with your decoder circuit. You'll need to connect the gnd of your decoder circuit to the gnd of the USB module. Connect the output of the decoder to any one of the analog inputs on the USB module. I hope it works.-Leo

[Guest MikePowell]

A possible issue is input protection for the 16C745. If the LVDT electronics is powered on while the PIC is not, the PIC inputs will have a voltage on them higher than the PIC's Vcc rail. While I don't offhand know what the specs are for the PIC in particular, this often is bad news for the powered down chip. Perhaps we should include a 1k current limiting resistor in series with the output from the LVDT electronics, and a pair of diodes for each PIC input to clamp the input terminal voltage between the PIC's GND and Vcc terminals.Leo, how robust are the PIC's input buffers?Mikewww.mikesflightdeck.com

[Guest LeoL]

They already have two clamping diodes on each port pin. The inputs feed a sample and hold cap for the conversion through a schmitt trigger buffer, but no specs on that.As far as the powered off situation is concerned, I honestly don't know. Clamping diodes should take care of it. I actually didn't think of this situation since I always use the USB bus power for everything and never needed an external power source. Perhaps some opto's with transistor outputs would eleminate any doubts in this case. MAXs are of course too expensive. Well...time to fry some PICs and experiment I guess.-L

[Guest MikePowell]

I'll slog through the spec's tomorrow and see if I can find any specs. Mikewww.mikesflightdeck.com

[Guest LeoL]

If you're looking for max Vin on IO pins, its Vdd + 0.3V and max sink / source current is 25 mA. Vdd MAX is 7.5V. If you're looking for info about the device in off state, I didn't see anything related to that.You might want to dig through the "PICmicro MID-RANGE MCU FAMILY" docs for more info on IO pins. It seems to have much more in-depth info.I've scribbled on paper an osc and signal conditioner circuit based on a single 5 volt source. I had to refresh my knowledge on analog design principles, but after a few hours of reading, I think I got it straight. Been spending way too much time in the digital world me thinks. ;)I'm going to pick up some components later this week and build myself an LVDT to test the circuit this weekend. I post results/schem if it works.-Leo

[Guest Ramsberg]

Here are a few "beta-layouts"Will they work? Did them from the attached schematic, only configured for LVDTs and 8 buttonshttp://forums.avsim.net/user_files/41497.jpgPostscript layouts:Component placinghttp://forums.avsim.net/user_files/41498.txt (rename to .ps)Mirrored bottom layer:http://forums.avsim.net/user_files/41499.txt (rename to .ps)Thinking of adding an ICSP plug so you dont need to remove the 16c745 to reprogram itIs it done right?/ Olle

[Guest LeoL]

"Thinking of adding an ICSP plug so you dont need to remove the 16c745 to reprogram it"It all looks fine to me. The ICSP is up to you but I put it on there since I was debugging the code and making changes often. Unless you plan to do some USB development, it may not be used very often.If you were thinking of creating a PCB then I would urge you to add headers to all the free pins on the PIC. I may add more functionality to the code when I finish work on my current project and have free time. Stuff like POV HAT support or an additional axis or buttons and maybe some extra options using jumpers.-Leo