BC127 I2S Mic

We are trying to connect Knowles SPH0645LM4H mounted on this eval board to BC127.

Here are our settings:
[size=85]GET AUDIO
AUDIO_DIGITAL=0 44100 64 120810

Here is the format of data coming from the Knowles mic:

Here is what the data from the mic looks like: (click to enlarge)

Here is what it looks like zoomed out: (click to enlarge)

We played a 1kHz tone into the mic, captured the data with a Tektronix scope with serial decode capability, and imported the 2’s complement data into a spreadsheet: (click to enlarge)

Note how the data is biased around -6000.

We repeated this blowing loudly into the mic instead of playing a tone: (click to enlarge)

Does that data look right? It looks perfect to me. Is it normal to get only about 500 units of swing with a tone played right into the mic? Is it okay to have a slight negative bias?

I’m not hearing anything at all when I use this mic. An analog mic works perfectly, and sounds great. I think we’re very close. There must be one setting we’re missing.


This is a tough one to answer through the forum, can you tell me who your distributor is and raise it with them so we can try to get some more in depth support?



Thanks. We are setting up a call with Future this Friday. Things I think could help Future and us with this:

  • List of settings used to test any digital mic with BC127 and which mic it was tested with
  • Procedure for Test #20 from the Blue Creations Melody firmware validation procedure.
  • Is there any way to make the Melody software dump I2s packets to serial to make sure I2S parameters are right?
  • Does a negative -6000 bias with +/- 250 units sound right for a digital mic?

Ron Vanhofwegen at Future is working with us on this. Maybe you could get him that information. Thanks for helping them help us.


I have pushed this to our US support team to try and aid future on this.



Hi Matt…

CJ and I work together… We have calls out to Future and getting help form Knowles as well…

To clarify we want to use 2 digital Mics in a vehicle to utilize hands free an cVc

What I’ve asked from Future is a wiring diagram of 2 ST Digital MEMS MIC connected to the inputs on the BC127
And the Firmware settings to make them work…

If you can help with the firmware commands ( were now using 6.1.5 standard and HD ) that would be great!!!


I have not seen any commands which make the micro phone rk individually or turn them on/off so I am fairly sure they should just work automatically.

With regards the circuit diagrams I presume you have the disc kit schematics, after that it should just be connecting the microphone.



Hi Matt:

We now have 2 Knowles digital Mics connected and working via I2S on a BC127 HD DISKIT…

Were using this command: SET AUDIO_DIGITAL=0 44100 64 184A90

We need more Mic volume ( as we are using the Mics in a vehicle for hands free ) Can you help us boost the gain of the Mics or point us in the right direction?

Thank you!