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PCM2704C: External ROM for HID strings on PCM2704C

Martin Andersen
Prodigy 10 points
Part Number: PCM2704C


Tool/software:

Hi!

We are designing a product for cellphones. Its a USB C audio output.

We made some prototypes and work just fine, but the audio source shows as "USB AUDIO DAC". We would like to show our brand/product name there.

Ive seen the datasheet that an external ROM can be used, but we are not sure on how to implement this:

"D+ pull-up must not be activated before completion of programming the descriptor data"

Is this really needed? Do I have to implement some D+ pullup control? Or I can just plug any generic I2C EEPROM into the DT/CK signals and it will work out of the box?

Thanks for your time!

  • 0
    TI__Mastermind 49160 points

    Hi Martin,

    I believe the requirement to disconnect the D+ pull up comes from the fact that descriptor modification only works in bus-powered mode. Therefore you don't want any action on the USB host side to conflict with the external ROM trying to be read by the device since they will share the connection to VDD. If you can ensure that the D+ line won't have any activity to conflict with the I2C reads then I don't think controlling the pull up would be necessary.

    Best regards,
    Jeff McPherson

  • 0 in reply to Jeff McPherson
    Prodigy 10 points

    Thanks Jeff!

    We will try and see. Do you know if standard pullups are needed for the I2C lines?

    R9 is shown in the schematic, which is 1K5 ohms but no other mention to this is made in the datasheet as far as I can tell.

    Should we remove R9 in case we use an external EEPROM? Keep it? Add pullups?

    Thanks

  • 0 in reply to Martin Andersen
    TI__Mastermind 49160 points

    Hi Martin,

    That will depend on exactly which EEPROM you use and its own requirements. That pull up isn't a requirement of the codec itself. As far as I know EEPROMs need the external pull up but it wouldn't surprise me if models existed where it wasn't needed.

    Best regards,
    Jeff McPherson