Audio (PDM mic)

Overview

The PDM interface on the SAI clocks one or more microphone pairs and de-interleaves their bitstreams. On the WBA there is a single SAI and no hardware decimator, so the path is: SAI in PDM master-RX mode generates the bit clock and captures the raw 1-bit stream over GPDMA, then the application decimates in software. There is no Studio DSL block for audio — capture is escape-to-C; the Core layer here is the core_pdm decimator helper, with core_audio as a thin convenience wrapper that ties the SAI and the decimator together.

Capturing the bitstream

core_audio wraps the whole pipeline: it configures the SAI for the microphone’s data/clock lines, starts a circular GPDMA capture, and runs the decimator on each half-buffer, handing you finished PCM through a callback.

#include "core_audio.h"

static uint8_t  pdm[4096] __attribute__((aligned(4)));  // raw bitstream
static int16_t  pcm[256];                                // decimated output
static core_audio_t mic;

static void on_pcm(const int16_t *samples, uint32_t n, void *ctx) {
    // n PCM samples ready (per DMA half-buffer)
}

// kernel clock, PCM rate, data line (SAI_D2), clock line (CK2),
// CIC order, extra gain shift:
core_audio_init(&mic, 16000000, 16000, 2, 2, 4, 0);
core_audio_start(&mic, GPDMA1_CH0, pdm, sizeof pdm, pcm, on_pcm, NULL);
// route GPDMA1_Channel0_IRQHandler -> core_audio_irq(&mic)

Decimating to PCM

core_pdm is an integer CIC decimator (N integrator stages at the bit rate, a /R downsample, N comb stages) followed by a one-pole DC blocker. Feed it successive DMA half-buffers — filter state carries across calls, so block boundaries are seamless. The decimation factor is the PDM clock divided by the output rate (e.g. 2.048 MHz / 128 = 16 kHz).

#include "core_pdm.h"

static int16_t pcm[256];
static core_pdm_cic_t cic;

core_pdm_init(&cic, 4, 128, 0, 0);   // order 4, /128, unity gain, MSB-first

// in the SAI DMA half/complete callback:
uint32_t n = core_pdm_process(&cic, pdm_half, half_bytes, pcm);
//   gain_shift (arg 4) is an extra left shift on the output — lift it to
//   pull up a quiet mic; a single-mic capture takes the first slot byte.

Cross-architecture support

PDM capture rides on the SAI, which is present on the W5, L4, and H5 Cores (not L0). The decimator is portable integer C and runs anywhere. See the implementation status for the full matrix.

API reference

void core_pdm_init(core_pdm_cic_t * st, uint8_t order, uint16_t decimation, uint8_t gain_shift, uint8_t lsb_first);

uint32_t core_pdm_process(core_pdm_cic_t * st, const uint8_t * pdm, uint32_t nbytes, int16_t * out);