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PCM1865: Common LRCK

mike clery
Prodigy 10 points
Part Number: PCM1865

I am using the PCM1865 in a 24 channel recorder for air traffic control. For this I will be using 6 PCM1865 devices. I would like to connect them in 3 pairs, 3 masters and 3 slaves. I intend to distribute a single crystal output to the master SCK1 inputs and then use the BCK and LRCK outputs to drive the slave devices. All devices will be driven at 48KHz sample rate with a 24 bit resolution. The outputs of each pair is TDM 10 channels (4 A/D outputs with their associated secondary A/D channels). 

Is their a method to connect these 6 device to use a common LRCK? As the TDM inputs are to fed into an FPGA for processing it would save a lot of hassle if they shared a common LRCK.

Thanks

  • 0
    TI__Genius 13610 points

    Hello Mike,

    Why is it you are doing 3 pairs of master and slave, why not just make one of them master and the other 5 slaves as to have convenience to supply 1 MCLK to the master which will generate LRCLK and BCLK from internal PLL which then can be outputted to the other 5 devices. This is given the lines connecting the device clocks wouldn't contain too much capacitance that would cause phase shift or be susceptible to noise

    So 1 device would be in master mode and the other 5 could be in either 2 or 3rd mode.

  • 0
    Prodigy 10 points

    Thanks for the reply

    I looked at this option but then could not see how I could output the data without a lot of lines.

    I am designing the the analogue board seperate to the FPGA board. The pcm1865's will be on the analogue board with a 40 pin interconnection socket.

    I am trying to keep the number of interconnection to minimum. I have calculated that I could tolerate a 24MHz Bck. If I use two devices in TDM mode, then I can at 96khz sample rate at 24bit resolution the bit rate would be 24MHz.

    In the connection you suggested can I pair up the devices to obtain three 24MHz TDM feeds down to the FPGA board?

    Regards

    Mike Clery 

  • 0 in reply to mike clery
    TI__Genius 13610 points

    Hello Mike,

    When you say pair up I guess you mean share same DOUT then yes you could since you said each pair would output 10 channels and the TDM can only handle up to ten channels.

    When you say 24MHz BCK, the ratio of BCK to LRCK has to be exactly 256 to it would need to be a 24.576 MHz as seen in table below