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PCM2706: Sloppy MCLK signal

EJ Holleman
Prodigy 160 points
Part Number: PCM2706
Other Parts Discussed in Thread: TAS5713, , TAS5754M

Hi Steve, I was trying to make an I2S demonstrator using a PCM2706 evaluation module from AliExpress and a TAS5713 soldered to a breakout-board.

The I2S signal generated by the PCM2706 is recognized correctly by a Maxim chip that does not need the MCLK signal.  The TAS5713 however, which does need the MCLK signal, does not recognize the I2S signal that it gets from the PCM2706. The TAS5713 requires a rise-time of 5ns for MCLK, I couldn't find the specified MCLK risetime for PCM2706.

See attached scope-image of the MCLK output from my PCM2706, the image hardly changes when the pin is open or loaded by the TAS5713 with a 10k resistor in parallel.

What is your opinion on this situation? Is my PCM2706 chip behaving as it should or is it not?

Finally, can you confirm that MCLK is always 256*Fs as specified in the PCM2706 datasheet? For Fs=32, 44.1 or 48kHz?

Best regards, EJ

  • 0
    TI__Guru 57800 points

    HI, EJ,

    I don't think  the issue is related to the MCLK rise time or an incorrect timing spec. MCLK should be 256×Fs at all times as this is a fixed parameter. the I²S clocks from the PCM2706C should be compatible with the amplifier, I don't see a problem with the clocks. It may be possible that the configuration for the TAS5713 is not complete as this device requires a specific start sequence to operate, unlike some of our other amps which can operate as soon as valid clocks are provided. 

    A possible workaround to check the clock compatibility with the amplifier (44.1KHz and 48KHz only) is to tie the BCLK signal to the MCLK pin of the amplifier, as when these sampling rates are used, a 64×Fs MCLK is accepted by the amp.

    Best Regards,

      -Diego Meléndez López
       Audio Applications Engineer

  • 0 in reply to Diego Melendez
    Prodigy 160 points

    Hi Diego,

    Thanks for your answer!

    Could you provide data on the MCLK risetime of PCM2706?
    And could you compare the signal-shape in my photo to a test-setup in your lab? That would be very helpful to decide between trying another PCM2706 chip (maybe mine is broken) or removing it from my design.

    For a long time I have been thinking that indeed tying the BCLK to the MCLK of the amplifier should work, however the resulting MCLK frequency is out of range for the TAS5713, as pointed out by your colleague Adam:
    https://e2e.ti.com/support/amplifiers/audio_amplifiers/f/6/p/694786/2567247#2567247

    Indeed TAS5713 has a startup sequence, and I am quite sure that I provide it correctly, however I appreciate a suggestion for a similarly powerfull amp that just works right away. (And, ideally, works right away without MCLK.:) )

    Best regards, EJ

  • 0 in reply to EJ Holleman
    TI__Guru 57800 points

    Hi, EJ,

    Unfortunately, the PCM2706C is a quite old device and the specific timing data from the MCLK or other I²S clocks apart from the datasheet information is not available.

    I made a test with my EVM and the clocks from the PCM2706C seem really good. Rise time of the MCLK is around 4ns and have a frequency of 256×Fs at all times ( I tried with different sampling rates). In a similar test, BCLK has a frequency of 64×Fs at all times (with all the available sampling rates). From the post you mention, it seems that it was understood that BCLK has a frequency of 16×2×Fs, which actually is not correct as BCLK is fixed to 64×Fs, so you might want to try using BCLK tied with MCLK on the TAS5713. There are some amps which should work with 3-wire I²S mode, like TAS5754M.

    Best Regards,

      -Diego Meléndez López
       Audio Applications Engineer

  • 0 in reply to Diego Melendez
    Prodigy 160 points
    Than you Diego, your research and answer were really helpful.
    Best, EJ