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Thread: Flexiboard another approach

  1. #1
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    Flexiboard another approach

    Hi all,

    Having looked at Frank's Flexiboard design, it spurred me to moving forward on an idea for a general purpose motherboard for the new 3.5 / 3.6 Teensies.

    Most of my projects are one offs, and creating a specific PCB for each is a pain, specially as I reuse favourite components for instance a rotary encoder + TFT or touch TFT for the UI...

    The specialist bits usually have I2C or SPI interfaces or simple digital / analogue I/O. Most need +5 or +3.3V.

    Having a quick look around at the marketplace for specialist modules, there seem to be a bunch of SPI/I2C modules for "Arduino" + Uno style shields + mikroBus modules.

    The mikroBus standard has a nice bunch of pins for most things:
    • SPI
    • I2C
    • 1 x Analog
    • 1 x Digital (PWM)
    • 1 x Interrupt
    • 5V, 3.3V power & GND

    My initial thoughts on a spec for such a board are:

    • 3 x mikroBus - each with separate CS, etc. Possibly separate SPIs, as the Teensy 3.5/6 have multiple SPIs available.
    • 3 x 16 pin headers (underneath the mikroBus ports) and connected to the mBus pins, to use for IDC cable extenders.
    • 1 x 14 pin touch TFT header (2.8"/3.5"). The MCUfriend type SPI interface seems to be quite common now. So, a TFT header on the back of the PCB, as in the Flexiboard design.
    • 2 or 3 ordinary 2 x 4 pin SPI headers - maybe in parallel with the mikroBus headers.
    • 5V -> 3.3V LDO regulator + power jack
    • All fitting as close to possible into the Eagle (free) board size and the shadow of a 3.5" TFT. (2.8" is too small, as the mikroBus footprint is reasonably chunky).


    I have something like this in mind as a layout:
    Click image for larger version. 

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    What would you add, delete or counsel me to change in this?

    Richard

  2. #2
    Senior Member+ defragster's Avatar
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    Rotate the T_3.6 90 so the USB and SD card are both accessible? SPI2 (base 0) access takes bottom pins - Mount the T_3.6 upside down? Place a Program and Reset button? Power or status LEDs?
    Are unique Serial# pins wired to each mikro BUS location, sharing i2C/SPI might be good, but not Serial.

  3. #3
    Senior Member+ defragster's Avatar
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    Can you link a 3.5" display that matches the 2.8" ILI9341 display?
    It looks like the mikroBus is wide enough to straddle a Teensy if they stacked nicely and didn't have the middle jumper rows.

  4. #4
    Senior Member+ Frank B's Avatar
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    I'd add a simple power-on LED. I'm missing it on my board :-)
    Which PCB Service do you use ? Boards at this size are $$$ from OSH..

  5. #5
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    Thanks for the suggestions.

    Absolutely: different Serial#s for each mikrobus connector.

    I'll probably use SPI0 and SPI1 as SPI2 is accessible only via the pads on the back. I could use the "bent header" method to connect to them. I've got a T3.2 using this method and it's safely come out of the header socket 3 times so far!

    Good idea to rotate the Teensy to a) fit under the MikroBus, b) allow access to the SD card. I tried it in Eagle and seem to have 0.05" clearance between each mB header and the edge of the Teensy. I'll grab my real Teensy 3.6 later and measure it to make sure!

    BTW: what do you think of the 2 x 8 IDC extenders? Good idea or not? If I rotate the Teensy under 2 of the mikroBuses, I could still include one 2x8.

    Click image for larger version. 

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    The downside of doing it this way is that the footprint no longer matches the 2.8" TFT so well (red polygon), but the 3.5" TFT (blue) seems a nice fit. There would be different connectors for the 2.8"/3.5" as I've only been able to get 3.5 TFTs with the RPi 13x2 connector so far.

    As for the TFT controller, the 320x480 TFTs come with either a HX8357 or ILI9488. I'll probe mine when it arrives. Hopefully it will be an Ilitek controller... the modification of the ILI9341_t3 library won't be as drastic as writing a whole new one for the HX controller.

    Any other improvements?

    Richard
    Last edited by palmerr; 10-14-2016 at 07:15 AM.

  6. #6
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    Yep.

    I'll add a power LED.

    Still thinking about the board service - perhaps PCBcart - $88AU for 10 boards. Probably 20-30% cheaper than OSHPark, but you have to wait for them to come from China!

  7. #7
    Senior Member+ defragster's Avatar
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    FlexiBoard was 4 copies for $26 from PCBS.io [historically they make a spare and ship it if it is good - I got 5 copies]

    A socketed T_3.6 is scary enough to pull out of Female headers, too long to grab nicely and all those pins do hold. Another 10 or 24 pins under there would be extreme.
    Some variations of the cutout bottom solder ConnectorBoard are needed to bring pins out for those - then flip the Teensy over, or under mount it?.
    Frank may publish a FlexiBoard v0.2 soon that shows ideas for what he changed or where he is headed. Frank had mount holes from TFT align with the PCB.
    Yikes - Rotated T_3.6 is very long - leaves a whole inch outside they 14 pin display header.

    I shifted p#5 to this - perhaps enough room to pass USB past the TFT connector:
    Click image for larger version. 

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    Undermounting Teensy - covers button:
    would clear the USB connector and allow bring those extra pins from a connector board back up.
    0.1" inset of the SD end might allow card removal without protruding
    add ~7mm to height if soldered

    <edit>: don't forget pull-up spaces for i2c, and SPI CS pins.
    Last edited by defragster; 10-14-2016 at 08:20 AM.

  8. #8
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    Having had a look at the price of mikroBus boards online (only MikroElectronika seem to make them, despite it being an open standard), I'm thinking I might spread the risk and bring it back to one mBus header, perhaps add a UEXT socket and increase the number of standard I2C, SPI, etc.

    Anyway, I'm going to wait for the 3.5" TFT board to arrive before finalising the design.

    Thanks for all the input.

    Richard

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