qtalker---/firmware/esp32p4-sensor-node/main/mems_mic.c

// I2S MEMS microphone driver — see mems_mic.h for wiring and honesty notes.
//
// UNVERIFIED AGAINST REAL HARDWARE. Written against ESP-IDF's documented
// `driver/i2s_std.h` API (the current idiomatic I2S driver, superseding the
// older monolithic `driver/i2s.h`) and the INMP441 family's well-documented
// output format: 24-bit signed PCM, MSB-first, left-justified in a 32-bit
// I2S slot (Philips/standard I2S timing). The right-shift-by-8 used below
// to recover the 24-bit sample from the 32-bit slot, and the dBFS
// reference level (2^23, a 24-bit signed sample's full-scale magnitude),
// are the commonly-documented values for this exact mic family — but
// "commonly documented" is not "verified against this specific board," so
// treat the very first real readings as a sanity check, not a given: talk
// near the mic and confirm the reported level actually rises before
// trusting it unattended.

#include <string.h>
#include <stdbool.h>
#include <math.h>
#include <stdlib.h>
#include "mems_mic.h"
#include "driver/i2s_std.h"
#include "esp_log.h"
#include "freertos/FreeRTOS.h"
#include "freertos/task.h"

static const char *TAG = "mems_mic";

// dBFS reference: full-scale magnitude of a 24-bit signed sample.
#define FULL_SCALE_24BIT (8388608.0) // 2^23

static i2s_chan_handle_t s_rx_chan = NULL;
static bool s_ready = false;
static int32_t *s_sample_buf = NULL; // heap-allocated, MEMS_MIC_SAMPLES_PER_READ entries

esp_err_t mems_mic_init(void) {
    s_sample_buf = (int32_t *)malloc(MEMS_MIC_SAMPLES_PER_READ * sizeof(int32_t));
    if (s_sample_buf == NULL) {
        ESP_LOGE(TAG, "sample buffer allocation failed");
        return ESP_ERR_NO_MEM;
    }

    i2s_chan_config_t chan_cfg = I2S_CHANNEL_DEFAULT_CONFIG(MEMS_MIC_I2S_PORT, I2S_ROLE_MASTER);
    esp_err_t err = i2s_new_channel(&chan_cfg, NULL, &s_rx_chan);
    if (err != ESP_OK) {
        ESP_LOGE(TAG, "i2s_new_channel failed: %s", esp_err_to_name(err));
        free(s_sample_buf);
        s_sample_buf = NULL;
        return err;
    }

    i2s_std_config_t std_cfg = {
        .clk_cfg = I2S_STD_CLK_DEFAULT_CONFIG(MEMS_MIC_SAMPLE_RATE_HZ),
        .slot_cfg = I2S_STD_PHILIPS_SLOT_DEFAULT_CONFIG(
            I2S_DATA_BIT_WIDTH_32BIT, I2S_SLOT_MODE_MONO),
        .gpio_cfg = {
            .mclk = I2S_GPIO_UNUSED,
            .bclk = MEMS_MIC_I2S_BCLK_GPIO,
            .ws = MEMS_MIC_I2S_WS_GPIO,
            .dout = I2S_GPIO_UNUSED, // RX-only channel, no data output pin
            .din = MEMS_MIC_I2S_DIN_GPIO,
            .invert_flags = {
                .mclk_inv = false,
                .bclk_inv = false,
                .ws_inv = false,
            },
        },
    };
    // Left channel per this driver's documented default wiring (mic's L/R
    // pin tied to GND) -- change to I2S_STD_SLOT_RIGHT to match a mic
    // wired the other way.
    std_cfg.slot_cfg.slot_mask = I2S_STD_SLOT_LEFT;

    err = i2s_channel_init_std_mode(s_rx_chan, &std_cfg);
    if (err != ESP_OK) {
        ESP_LOGE(TAG, "i2s_channel_init_std_mode failed: %s", esp_err_to_name(err));
        free(s_sample_buf);
        s_sample_buf = NULL;
        return err;
    }

    err = i2s_channel_enable(s_rx_chan);
    if (err != ESP_OK) {
        ESP_LOGE(TAG, "i2s_channel_enable failed: %s", esp_err_to_name(err));
        free(s_sample_buf);
        s_sample_buf = NULL;
        return err;
    }

    s_ready = true;
    ESP_LOGI(TAG, "I2S mic init ok (%d Hz, port %d)", MEMS_MIC_SAMPLE_RATE_HZ, MEMS_MIC_I2S_PORT);
    return ESP_OK;
}

esp_err_t mems_mic_read(sensor_reading_t *out, size_t max_out, size_t *out_count) {
    *out_count = 0;
    if (!s_ready) {
        return ESP_ERR_INVALID_STATE;
    }
    if (max_out < 1) {
        return ESP_ERR_NO_MEM;
    }

    size_t bytes_to_read = MEMS_MIC_SAMPLES_PER_READ * sizeof(int32_t);
    size_t bytes_read = 0;
    esp_err_t err = i2s_channel_read(s_rx_chan, s_sample_buf, bytes_to_read,
                                      &bytes_read, pdMS_TO_TICKS(500));
    if (err != ESP_OK) {
        ESP_LOGW(TAG, "i2s_channel_read failed: %s", esp_err_to_name(err));
        return err;
    }

    size_t n_samples = bytes_read / sizeof(int32_t);
    if (n_samples == 0) {
        ESP_LOGD(TAG, "no I2S samples this cycle");
        return ESP_OK;
    }

    // RMS over the block. The mic's 24-bit sample is left-justified in the
    // 32-bit I2S slot -- shift right 8 to recover it before squaring, so
    // the magnitude lines up with FULL_SCALE_24BIT below.
    double sum_sq = 0.0;
    for (size_t i = 0; i < n_samples; i++) {
        double sample = (double)(s_sample_buf[i] >> 8);
        sum_sq += sample * sample;
    }
    double rms = sqrt(sum_sq / (double)n_samples);

    // dBFS: 20*log10(rms / full_scale). A true-silent input gives rms=0,
    // which is -inf in dB -- clamp to a floor rather than emit a value the
    // JSON encoder/backend can't handle.
    double dbfs;
    if (rms < 1.0) {
        dbfs = -120.0; // effective noise floor
    } else {
        dbfs = 20.0 * log10(rms / FULL_SCALE_24BIT);
        if (dbfs < -120.0) dbfs = -120.0;
    }

    memset(&out[0], 0, sizeof(out[0]));
    strncpy(out[0].sensor_type, "evp", SENSOR_READING_TYPE_MAXLEN - 1);
    out[0].value = dbfs;
    strncpy(out[0].unit, "dbfs", SENSOR_READING_UNIT_MAXLEN - 1);
    out[0].metadata = NULL;

    *out_count = 1;
    return ESP_OK;
}