Why do I see a stereo signal in a codec when I connect it to a single PDM microphone? With one PDM microphone connected, I see identical signals in both the left and right channels.
This question was first posted in another thread on this forum here: https://ez.analog.com/message/44559#44559
This may not be intuitive, but when you have a single PDM microphone connected to a codec's PDM input pin, you will see identical signals in both the right and left channels in the codec. This happens because the codec clocks the PDM data stream on both the falling edge, for the left channel, and the rising edge, for the right channel, of the clock signal. The data output from a single microphone will transition just once during each clock period, so the digital level of the data will be the same on both the rising and falling edge of the clock. Even though the microphone's data driver is tristated during the half-frame during which another microphone could drive the signal, the leakage on the signal line is typically not enough to pull the signal from its current state. The codec is still reading the data on both of these edges, so it's seeing identical data in the right and left channels.
To illustrate this, below are diagrams showing the PDM DATA and CLK signals for both a mono and stereo microphone configuration. These are taken from the ADMP521 data sheet.
Mono PDM Format
Stereo PDM Format
In the codec, you can ignore one of the audio channels, since the data is identical in both channels.
