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PCM1792 system clock frequency

Hernan Freschi
Prodigy 180 points
Other Parts Discussed in Thread: TPA6102A2, PCM1792, PCM5102A, PCM1792A, TPA6120A2, OPA4134

Hello,

I recently tried to use a USB-I2S bridge, CP2114 with the PCM1792. I couldn't make it work - no audio in any configuration. There is, however, a little high frequency pitch at the output, and under some conditions, i can hear the "rhythm". Though I don't know if i'm just hearing the "train of bits" making noise similar to music. (I'm using the circuit available in the TPA6102A2 datasheet)


Anyway, what I've noticed is that CP2114 has an internal 48MHz oscillator (to generate the 12MHz USB clock), and it generates SYSCLK of 250*LRCK. If I understand the PCM1792 datasheet correctly, this DAC needs an LRCK of 128, 256, 384, 512 or 768 * LRCK. Is this correct?

Sampling rate for this USB-I2S bridge is fixed at 48KHz, so LRCK=48KHz and SYSCLK= 12.000MHz (a frequency of 12.288MHz would be required for SYSCLCK = 256*48000)


I've also had no luck reading I2C registers from the DAC, but I'm not sure if this is a problem with my tool.

  • 0
    TI__Guru**** 189401 points

    Hi, Hernan,

    I think you've found the problem. I think you will find the PCM5102A works better under these conditions.

    -d2

  • 0 in reply to Don Dapkus
    Prodigy 180 points

    Thanks for your answer, Don. I'm just using the PCM1792A because I had one.

    I didn't have any 49.152MHz oscillators but I found a 46.something one. I know it won't work right, but it let me configure separate clocks for the USB-I2S chip. USB runs off the internal 12MHz oscillator, audio runs from the external clock. Should be 49.152MHz, but at least I can run SYSCLK as fs*256 instead of fs*250.


    Now, a very strange thing happens. I get audio (music) but VERY distorted and very low. BUT... I can't see it with my oscilloscope! All I see is around 100mVpp of noise at the headphone output. Here is a recording from the audio I get into my headphones: https://www.youtube.com/watch?v=JzVQpkVMClM

    So what I did was disconnect the opamp I/V stage and add a resistor from PCM1792A to GND. I assume this should let me measure the outputs with an oscilloscope. But, still, I get nothing.

    More interesting: the opamp I/V stage runs HOT. Must be about 45-50ºC. It's an OPA4134. Is this normal? The circuit I use is the one available in the TPA6120A2 datasheet. When the opamp is soldered, I read about 5.7V in the opamp's outputs going into the TPA6120. Of course, since the TPA is configured as a differential input, the signal cancels out and I get barely a few mV DC at the headphone output.

    I soldered the opamp back in, and I have music again. But I can't see it. Which makes me wonder, could this be a problem with ground bouncing? Or is my DAC IC just broken? I'm thinking the output stage might have been damaged. Otherwise I can't explain the low signal + high noise.

    I triple checked the circuit and component values and they all seem OK. I also get about 2.2V in VCOM which, I assume, is a reference for running the device in single-supply. The capacitors are in place and they're the right value. All GND and POWER pins have the correct voltages.

    I2C control now works, and I can read and set PCM1792A registers. But I'm not touching any of the registers since, according to the datasheet, the device "just works" in its default state (no MUTE, DAC is on, format is 24-bit I2S, etc).

  • 0 in reply to Hernan Freschi
    Prodigy 180 points

    This is embarassing.


    I've found the problem. I swapped BIT CLOCK and SYSTEM CLOCK. Now I have audio. This happened because one of the chips calls it SERIAL CLOCK and the other calls it SYSTEM CLOCK. So I mixed up the "SCLK" connection.

    The problem now is a very annoying, loud "chopping" noise, which i assume is because of the oscillator mismatch (46MHz vs 49MHz). The sound is probably the sample dropping. I'll try with a 49.152MHz oscillator as soon as i have one.