This is partially my fault. I missed one connection when I wrote the app note, and in fact I am in the process of updating the app note to correct this omission. Please add one more wire from J9 pin 9 (HPH_REF) to GND at the jack. This should resolve the buzz when you are recording.
The next ‘issue’ is the headset mic needs a 1.8V Bias signal. Without a Bias voltage, the mic doesn’t work very well. The bias voltage (CDC_MIC_BIAS2) was supposed to be provided on the main board by R160 (2k Ohms), unfortunately all of the boards have been built without having R160 installed.
We have you have a 3 options to provide a bias voltage to the mic.
- The best solution is to install R160 in the board, unfortunately this requires soldering on an extremely tiny resistor and most of us don’t have the necessary dexterity to do this. When I update the app note I will show the location of R160 for the brave few who can do the rework.
I have not yet tested options 2 and 3 so I can’t be sure they work.
connect CDC_MIC_BIAS1 (J7 pin 12) through a 2k resistor to CDC_MIC2_P (J2 pin 6), I have not yet checked that CDC_MIC_BIAS1 is turned on, you can use a volt meter and confirm the presence of 1.8V on pin 12 (measure between J7 pin 4 and J7 pin 12). If the 1.8V is not present, then you can try option 3.
There is a 1.8V supply on the low-speed connector (J8 pin 35, LS_EXP_1V8), this supply may be a little noisy if you have a mezzanine card plugged into the low speed connector, but if there is no mezzanine board plugged in it should be quite quiet. Use a 2k resistor to connect LS_EXP_1V8 (J8 pin 35) to CDC_MIC2_P (J7 pin 6).
The last thing to check is the pin-out on your jack and the headset. Every jack seems to have different terminals for different rings make sure you read the correct manufacturer’s datasheet. The other thing to check is your headset, there are two common pinouts and you might have either one. Starting at the tip, the order of the rings can be either Left,Right,Ground,Mic, or Left,Right,Mic,Ground (Note: how Mic and Ground are swapped). if you have a swapped headset for your 3-ring headset it will not work until you swap the terminals.
Full Disclosure, I am an employee of Qualcomm Canada, any opinions expressed in this or any other post may not reflect the opinions of my employer.