[sdiy] STM32 vs WM8731

[Eric Brombaugh]

Richie,

Yes - on MCLK. The MCLK input to the WM8731 is going into the crystal 
oscillator input pin (XI) and works fine with a nearly sinusoidal 
waveform. RC filtering the higher harmonics allows the critical 
fundamental to get through to the oscillator input while reducing the 
high-frequency aggressor that bothers the I2C SCL input pin.

I used that approach on this circuit - worked great. See R103/C117:

http://ebrombaugh.studionebula.com/synth/dsPIC_sp/dsPIC_sp_pg1-2.pdf

Note that this usually isn't necessary with a good layout. It did help 
while prototyping on a solderless breadboard and I carried it over to 
the PCB design just because.

Eric

On 02/16/2016 08:53 AM, Richie Burnett wrote:
> Eric wrote:
>
>> If you have a scope with high impedance probes, look at the SCL input
>> pin - if you see MCLK there at a low level, it's a good bet your
>> WM8731 can't hear the I2C bus due to the crosstalk. Reroute the
>> signals or put a simple RC lowpass filter on the MCLK to mitigate that.
>
> On MCLK?  Or lowpass filter the I2C SCL input to remove the MCLK clock
> feedthrough?
>
> -Richie,
>