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Multiple PDM converters together: ADAU7118

Is it feasible to use multiple ADAU7118 (etc) PDM-to-TDM/ converters with the same MCU? How would you manage the PDM clocks?

Each device has 2 clocks, and 8 PDM channels. Could I simply enable only a single clock on one of the devices, and pass it to all mics? Or does each IC need to be using its own clock?

Can you pass a mic clock from your MCU instead, and not use the ADAU7118's PDM clock? Thank you.

(I think I understand the output; will just send a TDM/I2S signal from each IC to a unique SAI block on my MCU, and send the clocks from the MCU to each IC. The confusion is due to the AD  ICs supplying the PDM clock instead of the MCU)

Clarified something
[edited by: The Alchemist at 3:21 PM (GMT -5) on 24 Dec 2021]
  • Best guess at this point: The PDM clocks should all be synced together, as long as they're provided with a common BCLK (aka SCK) line. My MCU can all the audio blocks with the same clock. If the ADAU7118 derives its PDM clocks from the BCLK line, all mics should be synced, right?

  • Hello Alchemist,

    You synchronize them by sending them all the same LRCLK ( Frame Clock) and the same Bit Clock. 

    Then you let each ADAU7118 develop its own two sets of clocks for the 8 mics attached to that part. It is very important to use the clock from the 7118 that the data is attached to. DO NOT use one clock for the entire design. In the ADAU7118 registers you actually have to set registers to say which of the two PDM clock outputs are being used for each of the four data inputs. This is because the timing of the edge is critical, so critical that when you set the register it does not use that PDM clock internally, it actually uses an input that is built into the pin so the clock signal to clock in the data can be affected by the stray board capacitance on the clock line. 

    So you can use only one of the two clock line on each chip and set the registers accordingly, but it is good that we have a built in extra clock out to make keeping the reflections on the signal a little easier to control.


    Dave T

  • Thank you very much; that answers my question completely. I've ordered a board with the ICs and mics, and have it, and the software set up that way: Ie both clock lines on each IC, and configured in the IC registers. With common Frame and Bit clocks

  • Thank you so much for this answer. 

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