Frequency counter for RPi that samples at 24.576Mhz and is accurate up to several Mhz, uses very little CPU (500Khz input uses 9% of one CPU core on a RPI4)

RPiI2sFrequencyCounter.c

/*
RPi frequency counter

We use the RPi's I2S input (on pin 38)

Setting up I2S input:
https://learn.adafruit.com/adafruit-i2s-mems-microphone-breakout/raspberry-pi-wiring-test

Compile kernel module, modprobe it as described
(for RPI4 use 'modprobe snd-i2smic-rpi rpi_platform_generation=2')

I set up .asoundrc as
---
pcm.dmic_hw {
        type hw
        card sndrpii2scard
        channels 2
        format S32_LE
}
---
then 'arecord -l' should show a device 'sndrpii2scard'

Finally, hook the input signal (3v3 max!) up to the I2S DIN input (GPIO20) - no other signals (apart from gnd) are required..


So.. we set up a sound device that reads a 384khz stereo 32 bit I2S input; this gives a sample rate of
384khz = 24576000hz! (can use lower, e.g. 48khz or whatever)


Just pipe the i2s input into this program... i.e.

arecord -D dmic_hw -c2 -r384000 -f S32_LE -t raw -v - | TestFrequencyCounter

*/
#include <stdint.h>
#include <stdio.h>

#define SAMPLE_RATE 384000  //sample the input at 24.576mhz
#define BLOCK_SIZE 8000 //should evenly divide SAMPLE_RATE
uint32_t buf[BLOCK_SIZE*2];  //stereo 32 bit


void processSamples(void)
{
	uint32_t f=0;
	uint32_t state=0;
	while(1){
		f=0;
		uint32_t count=SAMPLE_RATE/BLOCK_SIZE;
		while(count--){
			fread(buf,sizeof(buf),1,stdin);
			uint32_t t=BLOCK_SIZE*2;
			uint32_t *ptr=buf;
			uint32_t s=0;
			while(t--){
				// This uses some performance tricks but is basically just counting +ve edges in the data
				s=*ptr++;
				s=s^state;
				if (!s) continue;
				uint32_t mask=(1<<31);
				if (s < 0x10000) mask=(1<<16);
				while(mask){
					if (s & mask){
						state=0xffffffff^state;
						if (state) f++; 	//+ve edge
						s^=(mask-1); 						
						s&=(mask-1); 
						if (!s) break; //no more edges remain, get out						
					}
					mask>>=1;
				}

			}
		}
		printf("%dHz\n",f);
	}
}

int main(int argc,char **argv)
{
	processSamples();
}