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Thread: Custom teensy 3.5

  1. #1
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    Custom teensy 3.5

    I think this is the right part of the forum for this question.
    I have been perfecting my code with a stock Teensy 3.5 and now wish to try and take the plunge by moving the design to its own custom board. Before I get down to finalizing the design I have a few questions about component choice and what I could effectively remove from the stock design.

    Looking at the schematic posted on the site I would wish to remove the SD card connector, I cannot see an issue with this its just connected to some I/O but the other item I was wondering about was TPD3S014 connected to the USB1 port pins. Is this just protection if you use the device as HID connected to the aux pins? I don't plan to use the device as HID and would just look to program it through the standard USB0 port that the micro usb is wired to so can I safely omit this?

    I have read and and understand the requirement around the crystal ground but do you have any recommendations about which crystal to use? I was thinking digikey 490-18285-1-ND fits the requirement of 8pf load and 16mhz, should the crystal case ground be connected to the same ground island as it is sat on and connected to the ground pins nearest to the MK64 chip.

    Last question surrounds the bootloader, I know this is proprietary tech and I have to purchase from PJRC but is this correctly connected to an MK64FX512VLQ12 with associate power and caps all that is required to turn it into a functioning teensy.

    Any advice surrounding this or other things to avoid are welcome.

  2. #2
    Senior Member brtaylor's Avatar
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    Quote Originally Posted by stathej View Post
    Looking at the schematic posted on the site I would wish to remove the SD card connector, I cannot see an issue with this its just connected to some I/O but the other item I was wondering about was TPD3S014 connected to the USB1 port pins. Is this just protection if you use the device as HID connected to the aux pins? I don't plan to use the device as HID and would just look to program it through the standard USB0 port that the micro usb is wired to so can I safely omit this?
    Yes.

    Quote Originally Posted by stathej View Post
    I have read and and understand the requirement around the crystal ground but do you have any recommendations about which crystal to use? I was thinking digikey 490-18285-1-ND fits the requirement of 8pf load and 16mhz, should the crystal case ground be connected to the same ground island as it is sat on and connected to the ground pins nearest to the MK64 chip.
    I don't like the 490-18285-1-ND that you suggested, I haven't used it, but the ESR is really high and I had issues, likely with ESR, on a prior custom Teensy 3.6 that I did. I've successfully used this crystal in the past:
    https://www.digikey.com/en/products/...5dkDsATEAXQF8g

    I connect the mcu ground pin next to the crystal to the crystal grounds and draw a guard around the crystal. I put a ground plane on the layer directly beneath it and do not route traces under the crystal. I connect the ground pin to the ground plane with a via, this is a screenshot of a custom Teensy 3.6 using the LQFP package, the red traces are the top layer and the yellow is the ground plane immediately beneath it on a 4 layer board:
    Click image for larger version. 

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    Quote Originally Posted by stathej View Post
    Last question surrounds the bootloader, I know this is proprietary tech and I have to purchase from PJRC but is this correctly connected to an MK64FX512VLQ12 with associate power and caps all that is required to turn it into a functioning teensy.
    Yes.
    Last edited by brtaylor; 11-24-2020 at 06:21 PM.

  3. #3
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    Thanks for the advice and layout design. Not planning on a four layer board currently, will see how the design lays out in Proteus and take it from there. Good to know about Crystal ESR

    Jonathan

  4. #4
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by stathej View Post
    but the other item I was wondering about was TPD3S014 connected to the USB1 port pins.
    The TPD3S014 chip and 100uF capacitor are only used on Teensy 3.6 (which shares the same PCB as Teensy 3.5). The MK64FX512 chip used on Teensy 3.5 does not have a 2nd USB port, so those parts are not placed on Teensy 3.5.

    Click image for larger version. 

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  5. #5
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    Ah OK that makes it clear never looked at the top of the actual 3.5 I was just looking at the schematic to see what was used. Thanks Paul

  6. #6
    Senior Member PaulStoffregen's Avatar
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    Those parts not being placed on Teensy 3.5 is also documented in a footnote on the schematic.

    I know this may be stating the overly obvious, but usually prototyping all the stuff you're actually going to use with Teensy board using breadboards or other quick construction is a good idea before committing it to a custom PCB design. Learning what does & doesn't actually work and adjusting the circuit is much easier & quicker on a solderless breadboard.

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