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Thread: Does the input have already a Schmitt-Trigger?

  1. #1
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    Does the input have already a Schmitt-Trigger?

    Does the input of a teensy 2.0 board, specialy the XCK input, have a Schmitt-Trigger integrated?

  2. #2
    Senior Member PaulStoffregen's Avatar
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    Figure 10-5 in the ATMEGA32U4 datasheet seems to say all the digital inputs have schmitt trigger.

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  3. #3
    Senior Member PaulStoffregen's Avatar
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    But the only electrical spec about the amount of hysteresis is only in the part about I2C pins. So hard to know....

  4. #4
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    Quote Originally Posted by PaulStoffregen View Post
    Figure 10-5 in the ATMEGA32U4 datasheet seems to say all the digital inputs have schmitt trigger.

    Click image for larger version. 

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    What do you thing, where is the threshold where it switch back to 0?

  5. #5
    Why do you ask? The reason I ask is because what you are asking may not be important to what you need to do.

    Can you explain what you are trying to do?

  6. #6
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    Quote Originally Posted by Loren42 View Post
    Why do you ask? The reason I ask is because what you are asking may not be important to what you need to do.

    Can you explain what you are trying to do?
    I feed a teensy 2.0 with a external clock of an old device. Some of the devices have a dirty clock signal, so I have to set the low pass filter of the clock lower. This helps on most devices, but on some not.
    I notice that, if I change the filter only, get an DC offset of 0.5V and maybe this is too short on the threshold and in some cases it does not work.
    I have no idea where the input switch to high and back to low.

  7. #7
    If you are looking for an exact value, I think it is somewhat arbitrary by nature.

    The problem is that the Schmitt-Trigger is defined internally to the chip, not some dedicated manufactured part.

    If you need to know, and it's not in the chip's data sheet, then I would put in a request with technical support at NXP. Even then, they may not have a definitive answer, but that is the best you can do.

    Otherwise, you can look up a number of different Schmitt-Trigger chips and take the average, but that is only an educated guess.

  8. #8
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    Seems like you should be able to test for the hysteresis points fairly easily.

  9. #9
    Of course, but a sample size of one isn't very telling, statistically speaking.

    There may be other factors that could cause that empirical value to change under different conditions, too. That might be a bigger problem if the operation becomes intermittent.

    I am guessing it may be better to condition the signal externally than to rely on unpublished functionality. That, and to the best of my knowledge a Schmitt-Trigger only deals with level hysteresis, and that may not help with ringing of a signal.

  10. #10
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    Quote Originally Posted by Loren42 View Post
    Of course, but a sample size of one isn't very telling, statistically speaking.

    There may be other factors that could cause that empirical value to change under different conditions, too. That might be a bigger problem if the operation becomes intermittent.

    I am guessing it may be better to condition the signal externally than to rely on unpublished functionality. That, and to the best of my knowledge a Schmitt-Trigger only deals with level hysteresis, and that may not help with ringing of a signal.
    Sure, but then the OP was talking about a single T2.0 and asking if it was Schmidt Triggered.

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