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Thread: About the Teensy 4.0 3.3V limit.

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    About the Teensy 4.0 3.3V limit.

    The card which came with my teensy says "Teensy 4.0 signal pins are not 5V tolerant. Do not apply more than 3.3 volts to any pin, except VIN or VUSB".

    I was wondering how strict this rule is, and what would happen if I were to apply 5V to one of the digital pins, for example. Would it cause irreversible damage?

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    simple answer, probably.

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    Senior Member+ MichaelMeissner's Avatar
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    Quote Originally Posted by zacgarby View Post
    The card which came with my teensy says "Teensy 4.0 signal pins are not 5V tolerant. Do not apply more than 3.3 volts to any pin, except VIN or VUSB".

    I was wondering how strict this rule is, and what would happen if I were to apply 5V to one of the digital pins, for example. Would it cause irreversible damage?
    I've damaged previous Teensys that weren't 5v tolerant (Teensy 3.0, Teensy 3.6) with accidental 5v inputs. Usually the pin becomes unusable, but the rest of the Teensy limps along. I don't remember if I fried the teensy so it didn't work any more.

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    Quote Originally Posted by MichaelMeissner View Post
    I've damaged previous Teensys that weren't 5v tolerant (Teensy 3.0, Teensy 3.6) with accidental 5v inputs. Usually the pin becomes unusable, but the rest of the Teensy limps along. I don't remember if I fried the teensy so it didn't work any more.
    Ah, well that kind of sucks. Thanks though

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    Senior Member+ defragster's Avatar
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    Quote Originally Posted by zacgarby View Post
    Ah, well that kind of sucks. Thanks though
    Only as far as interfacing to 5V directly without making level shifting provisions.

    But going to 3.3V allows smaller faster more advanced design that can't tolerate 5V.

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    That's fair enough, I suppose they don't need to have voltage regulators in the circuit or whatever which is bound to save space.

    If it's easy to explain, do you know of any fairly straightforward methods to "shift" the 5V signal down to 3.3V?

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    At 600 MHz the T 4 MCU runs at 1.25 volts and 3.3 times the 180 MHz spec freq of the T_3.6 in a smaller package with a bigger feature set inside. So indeed guarding/supporting I/O pins at 5.5V would not be as Teensy.

    There are Forum notes on appropriate level shifters for the task as needed where some aren't fast enough for the required bus interface if not simple I/O. Not used any but those Forum notes and SparkFun or Adafruit offerings should get to a starting point.

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    Quote Originally Posted by zacgarby View Post
    That's fair enough, I suppose they don't need to have voltage regulators in the circuit or whatever which is bound to save space.

    If it's easy to explain, do you know of any fairly straightforward methods to "shift" the 5V signal down to 3.3V?
    It depends on how fast you need the conversion to. There are a bunch of level shifters that are made for i2c/spi/uart type conversions that will do the transfers in either direction. But they have limitations in terms of speed, etc. My favorite that I used back in the day when I used 5v i2c displays with the Teensy is made by pololu:


    Adafruit and Sparkfun also make some variants:


    Opto-couplers is one way to electrically isolate the Teensy from the sensors (an opto-coupler has a sealed package that has a LED and a light sensor in the package, so there is separation between the two events).

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    That pololu one does look pretty good, tempted to get a few.

    Found some much cheaper ones if anyone's interested:

    - Ali express
    - https://www.ebay.com/itm/221919114584

    No clue what the difference between all of them would be, I guess to do with speed.

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    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by MichaelMeissner View Post
    I've damaged previous Teensys that weren't 5v tolerant (Teensy 3.0, Teensy 3.6) with accidental 5v inputs. Usually the pin becomes unusable, but the rest of the Teensy limps along.
    Teensy LC, 3.2, 3.5, 3.6 have special ESD protection circuits that do not look like a diode between the pin and 3.3V. This is sort of a mix blessing. It lets you more easily drive the pin higher than ~3.9V, but if you do, there isn't a conductive path to the 3.3V power line, so just 1 pin takes to take the abuse.

    Teensy 4.0, and also Teensy 3.0, do have diodes as their ESD protection. Perhaps that protection is more robust? Difficult to say, since Freescale (now NXP) doesn't disclose much info about the ESD protection circuits. It's probably a closely guarded trade secret.

    The main consequence of a diode-based ESD protection is driving the pin very hard (low impedance) can put a lot of current through the diode. Most modern chips don't have latch up problems, but that could happen. You could also pour in enough current to possibly power the entire chip, raising the 3.3V power up higher.

    The other side effect of the diodes is the possibility of a phantom power effect if you drive the pin while the power is turned off. Defragster saw quite a lot of this during the beta test. It's not an issue we can solve in software. It's a matter that the pins do have diodes.

    So the behavior of Teensy 4.0 when abused will be somewhat different than most of us are used to with Teensy 3.1 to 3.6 and LC. We haven't had a 3.3V Teensy with diode-based ESD protection since the original Teensy 3.0.

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