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Thread: Quick beginner question - manual wiring of Teensy 4 & Audio Shield Rev D

  1. #1

    Quick beginner question - manual wiring of Teensy 4 & Audio Shield Rev D

    Hey there,
    i am currently designing a simple adapter PCB for the audio shield, stacking as originally intended is not an option because of enclosure limitations.

    Can someone confirm/correct which pins i actually need to connect? i will not be using the SD card on this project.

    According to the audio shield page i would assume it is the following:
    20, 21, 23 (I2S control)
    7, 8 (audio data)
    18, 19 (I2C control)

    plus i guess the audio shield needs 3.3V and GND?

    i can still use any of the remaining serial RX/TX pairs to connect a midi interface, correct? (ie. 14-17)

    thanks!!
    Last edited by weasel; 01-19-2022 at 03:24 PM.

  2. #2
    Senior Member+ MichaelMeissner's Avatar
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    Cool

    Quote Originally Posted by weasel View Post
    Hey there,
    i am currently designing a simple adapter PCB for the audio shield, stacking as originally intended is not an option because of enclosure limitations.

    Can someone confirm/correct which pins i actually need to connect? i will not be using the SD card on this project.

    According to the audio shield page i would assume it is the following:
    20, 21, 23 (I2S control)
    7, 8 (audio data)
    18, 19 (I2C control)

    plus i guess the audio shield needs 3.3V and GND?

    i can still use any of the remaining serial RX/TX pairs to connect a midi interface, correct? (ie. 14-17)

    thanks!!
    You will also need these pins:
    • 3.3v
    • Ground pin between 3.3v and VIN pins
    • I don't think you need VIN, but it probably doesn't hurt to add it also.


    If you want to use either the flash memory that can be soldered to the audio shield or with the micro-SD card on the audio shield, you will also need to attach:
    • Pin 6 -- CS pin for flash memory
    • Pin 10 -- CS pin for the SD card reader
    • Pin 11 -- SPI MOSI pin
    • Pin 12 -- SPI MISO pin
    • Pin 13 -- SPI SCLK pin
    • The ground pin next to pin #1.


    The reason for the two separate ground pin connections is for the Teensy 3.x microprocessors, the pin between 3.3v and VIN is the analog ground pin, which is meant to be the pin used for analog devices. I think it has some extra hardware (capacitors, or such). The ground next to pin #0 is the normal digital ground pin. The Teensy 4.x and LC microprocessors do not differentiate between analog and digital grounds, but the Audio revision D shield is a modification of the A-C shields, and it kept the two different grounds (i.e. analog ground for the audio/I2C stuff and digital ground for the SPI devices).

    Other pins that aren't mentioned should be safe to use.

  3. #3
    Thanks a lot!!! Kind of messes me up because i just finished routing the traces, but a seperate AGND connection makes total sense!!! Maybe one day i'll look into why 4.0/Rev D doesn't use these anymore i assume the Rev D Audio Shield has that same GND filtering onboard.

    fwiw here's my OLED/AUDIO/MIDI PCB - really messy and poorly designed as i'm just getting my feet wet in hacking up some PCBs and actually know jack shit about electronics... this PCB is supposed to stack onto a SSD1309 2.42" OLED.
    Click image for larger version. 

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    Last edited by weasel; 01-19-2022 at 05:46 PM.

  4. #4
    Senior Member+ MichaelMeissner's Avatar
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    Quote Originally Posted by weasel View Post
    Thanks a lot!!! Kind of messes me up because i just finished routing the traces, but a seperate AGND connection makes total sense!!! Maybe one day i'll look into why 4.0/Rev D doesn't use these anymore i assume the Rev D Audio Shield has that same GND filtering onboard.
    It is the Teensy 4.0/4.1/micromod microprocessor that doesn't support AGND. I assume it was a cost/benefit calculation when the NXP designers laid out the chip. Just like why the chip doesn't have on-board DAC's. Paul can't wave a magic want to make the chip deliver more than the chip designers put into it.

    Given both grounds are the same on the Teensy 4, I imagine you could just interconnect the two ground pins where the audio shield goes to the single ground pin that you connect to the Teensy. There was a previous post where somebody discovered that the SPI devices need the ground next to pin #0 (not pin #1 as I mentioned earlier), but the audio stuff used the ground pin between VIN and 3.3v.

  5. #5
    Ok thanks for the explanation! Makes sense!! I already connected the hypothetical AGND with an extra trace now, but yeah, in that case i could have just left the design as it was, with the AGND pin of the audio shield just going to my "general" GND. Maybe i'll even change it back before manufacturing, because then the audio shield will have a shorter & thicker GND connection, less resistance.

    But while you are probably right on all the technicalities, at the same time i am 100% convinced that Paul indeed has a pretty powerful magic wand.

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