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Thread: Audio shield pin 13 PC board jumper trace.

  1. #1
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    Audio shield pin 13 PC board jumper trace.

    I see that there's a copper jumper on the Audio Shield board on connector pin 13 and it looks like it ultimately connects to pin 25 of the SGTL5000 signal I2S_DOUT. The 3.6 schematic shows pin 13 connected to the 3.6's LED. It looks like the 3.6 is driving into an output on the SGTL5000. Perhaps I did something wrong tracing this out. Can someone explain the intent for the jumper on the Audio Shield PC board copper jumper trace connected between pin 13 of the audio shield and pin 25 of the SGTL5000? I'm having a problem with the headphone audio output disappearing after a few seconds after power up. The 3.6 LED starts out bright and starts to flicker and fade coincident with the loss of audio. I'm thinking there's something about that jumper that I'm not understanding that could be causing my issue. In the attached schematic snippet that pin is labeled 'RX' on the SGTL5000, but the link for the SGTL5000 data sheet seems to indicate that pin 25 of the 32 QFN package is I2S_DOUT. I can't read the markings on the package so I can't verify the part against the data sheet.
    Thanks.
    - AG6GU
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  2. #2
    Senior Member+ MichaelMeissner's Avatar
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    On Teensy boards (just like many Arduino boards), the pins are overloaded. Pin 13 does have a LED on it, but it is also used for:

    • If you are using SPI with the normal pins, it is the SCLK (clock) signal. In this case, you will see the LED flashing on/off very fast if you have a SPI display;
    • The Audio shield does use SPI, but it uses the alternate pin #14 for the clock signal. Instead it uses pin #13 for the I2S RX function. There is no duplicate pin for the I2S RX pin, so you have to use pin #13. Because pin #13 has a LED on it, it has a resistor for the LED. Presumably the read logic factors in that this resistor is present.
    • If you have the Prop Shield, pin #13 is one of the two pins that are usable for WS2812B (neopixel) and APA102 (dotstar) LED strings. The prop shield has 4 pins that you connect the LEDs to and the 2 data pins (pin #11 and #13) emit VIN voltage instead of 3.3v to drive the LED strings. The Prop Shield only transmits voltage if pin #7 is set high. With a little bit of manual switching of pin state, I am able to drive displays and use the prop shield for displaying LEDs.

  3. #3
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    Thanks for the reply.
    Makes sense that the 3.6 would be driving I2S RX on the Audio shield. My confusion is that when I look up pin 25 for the SGTL5000 using the data sheet link on this site, I see that it's labeled I2S_DOUT. Is that the same functionality as I2S RX? My interpretation was that pin 25 on the SGTL5000 is an output but perhaps I'm wrong about that and calling it 'I2S_DOUT' its referring to the external context for that signal name. The fact that the 3.6 is driving an LED on pin 13 and it's also connected to a signal on the Audio shield that appears to be labeled as an output had me hoping I'd found my issue with the disappearing audio. I was thinking that driving into an output might cause that behavior of the loss of audio a few seconds after power up due to heating or other electrical issue. Was hoping there may just be something I need to re-configure with that pin via the jumper. Sounds like the jumper is just there to allow alternate usage of pin 13. I think I need to concede that I have a second broken SGTL5000 device for some reason. The 3.6 board seems to work fine. I also have it connected to an ILI9341 display and that all works fine.

  4. #4
    Senior Member PaulStoffregen's Avatar
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    Yes. On the SGTL5000 it’s called DOUT, for data output. As far as the SGTL5000 is concerned, it’s an output.

    On Teensy it’s I2S RX, for receive. That is correct. The Teensy pin is receiving and the SGTL5000 pin is transmitting.

    Likewise with the hardware serial ports, you don’t connect the same names together like with SPI and I2C. You connect TX to RX, and RX to TX. It makes no sense to connect TX to TX (a conflict) or RX to RX.

  5. #5
    Senior Member PaulStoffregen's Avatar
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    Regarding the reason for those pads.... they exist so you can connect 2 audio shields, giving you quad channel audio. On the second audio shield, you would cut those pads and use wires to connect the data signals to other pins. A set of 3 pads also allows changing the I2C address, so you can control both shields.

  6. #6
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    Ok. I was assuming the 3.6 is driving the LED but appears it's actually the SGTL5000 driving the LED on the 3.6. I get 5.7ma for the LED assuming VDDIO 3.3V minus .6v drop through the LED and 570 ohm resistor. I couldn't find what the SGTL can source on that pin, but the LED load doesn't seem unreasonable and clearly seems to work for everyone else 8) Thanks for clearing that up.

  7. #7
    Senior Member PaulStoffregen's Avatar
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    Like all LEDs, the forward voltage drop is much more than the ~0.7V of ordinary silicon diodes. So the current is lower, approx 3mA.

    LEDs are made from other semiconductor materials, not silicon.

  8. #8
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    Careless mistake on my part. Thanks. I know you can light an LED on 1 volt or two without limiting the current so should have realized that.

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