PDM MEMS microphone to 16 bit at 16 kHz PCM using a 2 MHz crystal oscillator and a logic analyzer

pdm.c

/*
2 MHz clock goes to the PDM MEMS microphone and channel 0 of the logic analyzer.
The logic analyzer needs at least a 6 MHz sample rate or three times the clock rate.
Data signal from the microphone goes to channel 4 of the logic analyzer.
Tested using a MP34DT01-M with select connected to ground.

compile:
gcc -Ofast -Wall -o pdm pdm.c

stream to speakers:
sigrok-cli -d fx2lafw --continuous --config samplerate=8MHz -O binary | ./pdm | aplay -t raw -f S16_LE -r 16000 -c 1 -

record to wav file:
sigrok-cli -d fx2lafw --continuous --config samplerate=8MHz -O binary | ./pdm | sox -t raw -r 16000 -b 16 -c 1 -L -e signed-integer - record.wav
*/

#include <stdio.h>
#include <limits.h>

int integrator_cascade(int x)
{
	static int sum0, sum1, sum2;
	return sum2 += (sum1 += (sum0 += x));
}

int comb_cascade(int x)
{
	static int prv2, prv1, prv0;
	int tmp = x;
	tmp -= prv0; prv0 = x; x = tmp;
	tmp -= prv1; prv1 = x; x = tmp;
	tmp -= prv2; prv2 = x; x = tmp;
	return x;
}

int block_dc(int x)
{
	static int avg;
	avg = (15 * avg + x) / 16;
	return x - avg;
}

int clamp(int x)
{
	return x < SHRT_MIN ? SHRT_MIN : x > SHRT_MAX ? SHRT_MAX : x;
}

int main()
{
	for (int chr = 0, pclk = 0, pdat = 0, num = 0; chr >= 0; chr = getchar()) {
		int sel = 0;
		int clk = (chr & 1) ^ sel;
		int dat = (chr >> 4) & 1;
		if (!pclk && clk) {
			int intp = integrator_cascade(pdat);
			if (++num == 125) {
				num = 0;
				int norm = clamp(block_dc(comb_cascade(intp)));
				putchar(norm & 255);
				putchar((norm >> 8) & 255);
			}
		}
		pclk = clk;
		pdat = dat;
	}
	return 0;
}