PDM microphone

The simplest possible sigma delta pulse counter for a microphone that outputs PDM pulses (like National Semi's discontinued LMV1024/6)

As an example, this code converts mic output to 8-bit unsigned PCM at about 22 kHz. This is then output as 16-bit signed PCM to XDK's audio DAC and resampled to 44.1 kHz.

The relevant code sequence is

// Snippet 1
case data :> unsigned z:
s += cnt1s[z];
n += 8;

and

// Snippet 2
case slave { dac <: x; dac <: x; }:
unsigned x = (n > 0) ? (256 * s / n) : 0;
n = 0;
s = 0;



.


Snippet 1 inputs 8 pulses (data port is configured for 1:8 deserialisation) and updates the sum (cnt1s counts ones in a character)

Snippet 2 computes 8-bit unsigned sample value from the sum (sample is set to zero if no pulses were available in order to avoid zero division).

See test.xc for full listing