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Thread: T4.1 External Power Question

  1. #1

    T4.1 External Power Question

    I'm a software person trying to learn more about hardware design. I want to build an adapter to use T41 in place of another processor module. The adapter will have a socket for T41, and it will plug into a socket on an existing application board. The adapter will mostly map T41 pins onto pins of the larger socket, but it would be useful to also be able to add a CAN transceiver or other I/O on the adapter.

    My question is how to power the T4.1 via the 3.3V from the application board, which has a 3.3V supply. The module being replaced is 3.3V, and all of the I/O on the application board is 3.3V, including UART, I2C, SPI, digital I/O.

    We could take 3.3V from the application board, boost to 5V on the adapter to power T41, but some threads here say it's okay to use the 3.3V pin to power the T41. Is that okay? If we use the boost option, do we create issues by having T41 doing I/O with devices that are powered by a "different" 3.3V on the application board?

    This is one thread that seems to say it's okay to power T41 via the 3.3V pin.

    https://forum.pjrc.com/threads/58874...l=1#post226095

  2. #2
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    This post/thread is linked from the Teensy 4.1 product page:

    https://forum.pjrc.com/threads/64468...l=1#post259469

  3. #3
    Senior Member PaulStoffregen's Avatar
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    Usually this works, but if the 3.3V power supply starts up too slowly Teensy 4.1 can fail to boot up because power is seen on the 3.3V pins before SNVS reached about 1 volt.

  4. #4
    Quote Originally Posted by PaulStoffregen View Post
    Usually this works, but if the 3.3V power supply starts up too slowly Teensy 4.1 can fail to boot up because power is seen on the 3.3V pins before SNVS reached about 1 volt.
    Thanks very much. Is there any problem with connecting the T4.1 to a board that has a separate 3.3V supply? The supply on the board is what will be powering the I/O devices to which the T4.1 is connected.

  5. #5
    Senior Member PaulStoffregen's Avatar
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    Usually connecting 3.3V digital signals between separately powered boards is fine if their power supplies are within 0.3 volts of each other. While I don't know anything about this other circuitry you are using, the spec allowing input down to -0.3V and +0.3V above the power supply voltage is very common with modern 3.3V chips.

    Of course some parts, like Teensy 3.2 & 3.5 allow more because they are designed to be 5V tolerant. But Teensy 4.1 does not. It is 3.3V only, which really means it can accept between -0.3V to +3.6V (assuming its power supply is actually right at 3.3V).

    The difficult problems come in when you consider power up and power down sequencing. Most chips are not meant to have their pins driven while power is off. Teensy 4.1 definitely should not have any pins driven when it isn't powered by 3.3V. Your other circuitry is probably similar, though there are some types of chips which are designed to allow pins driven while power is off.

  6. #6
    The application board has a reset chip (DS1332A) that holds low the reset line of the original processor module until the 3.3V is above some value. We can't use that signal the same way with T41, but maybe there is some other way to make use of it.

    It must be a relatively common thing for an application board to require more power than the T4x can source via its 3.3V. Our plan for now is to provide 5V to the T41. Thanks for your help.

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