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Thread: YATB - Yet Another Teensy Board :D

  1. #121

    Re: YATB - Yet Another Teensy Board :D

    Quote Originally Posted by jwatte View Post
    Try putting a capacitor (100-470 uF) after the BEC, before the power switch?

    For other integrated power converters, I like the Pololu ones. They are slightly more expensive than OEM parts from DigiKey, but sometimes tuned better for hobbyist needs:

    I've found that, for more specific needs, I do better by going with dedicated ICs on my own. I've built a number of buck and boost converters, and so far, none of them have disintegrated by self-oscillating to death :-)
    Was revisiting some of this as per some threads up on PJRC. On my most recent boards I have used the one from Pololu:

    Was wondering if you have used the PG pin to try to make sure you have valid power before you allow the Teensy (or other processors) to boot?

    I believe from looking at the docs, like it requires a PU resistor and then hook up to the reset pin of the Teensy.

    Have you done this with your boards?


  2. #122

    Re: YATB - Yet Another Teensy Board :D

    Also to those who are good at EE stuff..

    On a few of the boards, I have circuits to convert 3.3v signals to 5v signals. I have done it a few different ways, but the current board I am playing with has 4 BSS138CT transistors along with supporting resistors...

    For example one used to drive Neopixels, looks like:
    Click image for larger version. 

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    I probably don't need both current limiting resistors, my guess is I could remove the one on the Teensy side.

    But the real question I have, is this circuit defaults to the +5v signal being HIGH if the Teensy IO pin is floating. I would prefer instead that it be low... My gut tells me it would probably work to switch the PU resistor on the Teensy side to be a PD resistor (ie connect to GND?) Am I correct? Suggestion for alternative circuits for this? Note: I am not the greatest hand solder person, so I don't want to go to smaller/tigher pin spacing parts.


  3. #123

    Re: YATB - Yet Another Teensy Board :D

    I'm assuming R25 goes to +3V3 and R26 goes to +5V?

    If you're talking about R25, then no, just tying that low won't work, because it's needed to create the voltage drop that brings the gate above the source and opens the MOSFET.

    Also note that R26 is the only thing pulling the pin high when the output is high. 10 kOhm drive strength is not a lot, so you shouldn't expect very fast switching speeds on the other end. In this case, it looks like you're driving a SPI LED, which typically you'll want to drive faster.

    I find that, for high-performance 3V to 5V conversion, I want to use some kind of buffer. 74HCT125 is of course a good stand-by (the "T" is important to make 3V count as high with 5V VCC!) but there are faster and more convenient options.

    On the last board where I needed translation, I used two chips for two different needs:
    NTS0102 -- -- dual channel lift from 3V3 to 5V at up to 50 MHz (for driving SPI LEDs with clock.)
    TC7WT241 -- -- alternative to 74HCT125 for two-way switchable translation (DXL servo bus)

    I think the NTS0102 could also drive the Dynamixel bus; I forget why I chose the Toshiba chip for that. Maybe saved a few cents? :-)

    Hmm, no the Toshiba is more expensive. So, no clue why!
    Last edited by jwatte; 12-06-2017 at 12:02 AM.

  4. #124

    Re: YATB - Yet Another Teensy Board :D

    Thanks Jwatte,

    I have used some components like you mentioned before, but went with discreet as I think the one I used may have been a lot larger, like 2 channels in each direction... Will take a look again. Sometimes I think having specific chips like you mention make it easier to make it work correctly!

    I think you may have gone with the TC7W as with the Teensy, you could setup the Serial port to run both TX and RX to it and have it automatically handle changing the pins direction. You can of course do it with external software from the Hardware Serial code. But was here playing around with does it make sense to only use one ttl level converter and use the built in half duplex support.

    The NTS0102 looks like a nice option for driving XYZ servos...

  5. #125

    Re: YATB - Yet Another Teensy Board :D

    Forgot to ask,

    With these two chips do you typically add decoupling Caps on them. One or two? i.e. one on the 3.3v side and one on the 5v side? Also do you typically add inline resistors on them to limit current?

    Thanks again

  6. #126

    Re: YATB - Yet Another Teensy Board :D

    I've gone both ways on current limiting resistors. A 47 or 70 Ohm resistor on a UART bus running a 1 Mbps will not significantly degrade the signal, but will add a modicum of protection against brief shorts on the bus.
    I add de-coupling capacitors if they will be far away from some other capacitor and/or will share rails with some other noisy or sensitive chip.

    I also remembered why I used the Toshiba for a serial bus: The NXP chip goes in and out on the same pin, which is great for a bidirectional bus, but not great for a MCU that has one RX and one TX pin. The Toshiba is much more appropriate.
    For SPI output to LEDs, the NXP is great (I hard-wire it to go one way.)

  7. #127

    Re: YATB - Yet Another Teensy Board :D

    Thanks again!

    Looks like I no longer need to think about XYZ servos so not sure if I will make any updates to that board, but may get back to playing with some other boards...

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