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Thread: PropShield LC

  1. #1

    PropShield LC

    The PropShield LC pin LED at pin 13 glows dimly without driving it HIGH - apparently when a sound file is played from Flash Mem or something is being read from Flash Mem. Is this possible default behavior?

    I'm thinking there could be a short on my connections somewhere. I need this LED pin for other event signals that occur at other conditions, and this constantly glowing LED condition is a bit irksome. Anybody have any ideas why this is occurring and how it could be suppressed?

  2. #2
    Senior Member+ MichaelMeissner's Avatar
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    Quote Originally Posted by vspear View Post
    The PropShield LC pin LED at pin 13 glows dimly without driving it HIGH - apparently when a sound file is played from Flash Mem or something is being read from Flash Mem. Is this possible default behavior?

    I'm thinking there could be a short on my connections somewhere. I need this LED pin for other event signals that occur at other conditions, and this constantly glowing LED condition is a bit irksome. Anybody have any ideas why this is occurring and how it could be suppressed?
    This is expected behavior, due to the fact that pin 13 is overloaded between the built-in LED and the SPI SCK signal. When you are reading flash memory, the Teensy uses SPI to talk to the flash memory controller on the prop shield. So pin 13 is rapidly set on/off (it is a clock pin, which means when the pin is one state, the data pin can be used to send/recieve data - the clock pin is flipped and then flipped back for the next round):


    This overloading was originally done by the Ardunio boards like the Uno, and Paul copied many of the pin layouts for the Teensy (i.e. pins 0/1 are the default serial connection, pins 18/19 are the i2c device, pins 11, 12, and 13 are the default SPI pins, etc.).

    So if you want to use a LED to signal a condition, you need to connect a LED + resistor to another pin, and not use pin 13.

    The Teensy does have an option to move the SPI SCK pin from pin 13 to pin 14, and this is used with the audio shield, because a third overloading of pin 13 is for the audio stream I2S receiver. However, the prop shield is set up to use pin 14 for the SPI clock.

    Besides SPI for flash memory, another use for pin 13 and 11 is being translated to VIN for ws2812/apa102 LEDs. There are two pins (6 for flash memory and 7 for LEDs). The flash memory controller will only act if pin 6 is high, and the LED support will only act if pin 7 is high. You don't want to set both at the same time.
    Last edited by MichaelMeissner; 11-30-2017 at 12:44 PM.

  3. #3
    Talk about digital recycling for pins! Many thanks for this info.

  4. #4
    Senior Member+ MichaelMeissner's Avatar
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    Quote Originally Posted by vspear View Post
    Talk about digital recycling for pins! Many thanks for this info.
    I have a google doc spreadsheet I maintain that lists most of these multiple uses across the Teensy LC, 3.0, 3.1/3.2, 3.5, and 3.6:

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