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Thread: Teensy LC: Bug in documentation

  1. #1
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    Teensy LC: Bug in documentation

    On this page
    https://www.pjrc.com/store/teensylc.html
    is written:
    "The AREF pin is used to set the analog input range. By default, AREF is 3.3V due to a resistor. External shunt-type reference chips may be connected for lower reference voltage."

    This is wrong. There is no resistor connected to the AREF pin, neither on the board nor on the chip. A shunt reference won't work without an external resistor.

    Michael

  2. #2
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    Quote Originally Posted by mkoch View Post
    On this page
    https://www.pjrc.com/store/teensylc.html
    is written:
    "The AREF pin is used to set the analog input range. By default, AREF is 3.3V due to a resistor. External shunt-type reference chips may be connected for lower reference voltage."

    This is wrong. There is no resistor connected to the AREF pin, neither on the board nor on the chip. A shunt reference won't work without an external resistor.

    Michael
    According to the MKL26Z64 datasheet, in Paragraph 3.6.11, Note 3, Table 25 states: "For packages without dedicated VREFH and VREFL pins, VREFH is internally tied to VDDA, and VREFL is internally tied to VSSA." which seems to imply that AREF will be at 3.3VDC as stated. Likewise, Paragraph 3.6.11, Note 4, Table 25 states (in reference to Ras, the Analog source resistance (external)): "This resistance is external to MCU. To achieve the best results, the analog source resistance must be kept as low as possible." and the associated diagram shows Ras as an external resistor (5kohm or less) used with the shunt reference. Maybe I'm missing something obvious, but I don't see anything indicating that an external shunt reference can/should be used without an external resistor.

    Mark J Culross
    KD5RXT

  3. #3
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    The sentence in the documentation says that the AREF pin (not the internal reference voltage) is 3.3V by default. That's clearly wrong. Without an external resistor, the AREF pin is 0V by default.

  4. #4
    Senior Member+ KurtE's Avatar
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    I created an issue on github... https://github.com/PaulStoffregen/We...ates/issues/32

    This points to the two recent threads on the Teensy LC documentation page.

  5. #5
    Senior Member PaulStoffregen's Avatar
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    Opps, indeed this info was wrong. It was copied from other Teensy 3.x models, but indeed Teensy LC is somewhat different.

    I've updated the Teensy LC page to say "By default Teensy LC uses 3.3V power an its analog reference. An external reference voltage between 1.13V to 3.3V may be applied to AREF and used by analogReference(EXTERNAL)."

  6. #6
    Senior Member BriComp's Avatar
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    an
    should be as.

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