PDM to PCM via DFSDM getting nothing but static

[RBair.3]

On the BL47E-IOT01A2 there are two mems microphones (MP34DT01) both connected to DFSDM1_DATIN2. I'm trying to extract the data from just one of those boards and convert it to PCM using a DFSDM filter. I've configured DFSDM Channel 2 to connect to PDM/SPI and trigger on the rising edge, and filter 0 I've configured in one-shot mode, connected to DMA, with an oversampling value of 154, and an integral oversampling value of 3. The internal SPI clock on the board is configured to 1/4 the system clock which is 80MHz, thus the SPI clock should be 20MHz. The code to capture the audio is here:

void HAL_DFSDM_FilterRegConvCpltCallback(DFSDM_Filter_HandleTypeDef *hdfsdm_filter) {
	micAudioBuf[micAudioBufIdx] = micRecSample << 8;
	micAudioBufIdx++;
	if (micAudioBufIdx >= AUDIO_LEN) {
		micAudioBufIdx = 0;
		audioFilterCplt = TRUE;
	} else {
		micReset(hdfsdm_filter);
	}
 }
 
void micReset(DFSDM_Filter_HandleTypeDef *hdfsdm_filter) {
	HAL_DFSDM_FilterRegularStop_DMA(hdfsdm_filter);
	HAL_DFSDM_FilterRegularStart_DMA(hdfsdm_filter, &micRecSample, 1);
}

And the transmission code is in the main loop here:

  while (1) {
    // toggle LED2 to indicate activity
    HAL_GPIO_TogglePin(LED2_GPIO_Port, LED2_Pin);
    // if programming the transmitter
    if (audioFilterCplt == TRUE) {
      // reset the flag
      audioFilterCplt = FALSE;
      CDC_Transmit_FS((uint8_t*)micAudioBuf, AUDIO_LEN);
      // Reset the mic and start recording again
      micReset(&hdfsdm1_filter0);
    }
}

I'm able to get the data by connecting to the serial port in python, but the output is just static. Here is the code I'm using to capture the output:

import serial
import wave
import struct
import numpy as np
 
if __name__ == '__main__':
 
    # Replace '/dev/ttyACM0' with the appropriate device path for your system
    ser = serial.Serial('/dev/ttyACM1', timeout=1)
 
    # Configure the WAV file settings
    num_channels = 1
    sample_width = 2  # 16-bit audio
    frame_rate = 44100
    num_frames = 0  # Will be updated as data is received
    compression_type = 'NONE'
    compression_name = 'not compressed'
 
    # Create a new WAV file
    wav_file = wave.open('output.wav', 'wb')
    wav_file.setparams((num_channels, sample_width, frame_rate, num_frames, compression_type, compression_name))
 
    try:
        while True:
            # Read data from the serial port
            data = ser.read(2)  # Read 2 bytes at a time (16-bit samples)
 
            if data:
                # Unpack the received data as a signed 16-bit integer
                sample = struct.unpack('<h', data)[0]
 
                # Write the sample to the WAV file
                wav_file.writeframesraw(struct.pack('<h', sample))
            else:
                break
    finally:
        # Close the serial port and WAV file
        ser.close()
        wav_file.close()

I'm not sure what else to try at this point

[Federica Bossi]

Hi @RBair.3​ ,

Have you set a CLK > 2MHz?

In addition I suggest you to use the channel 1 of DFSDM (DFSDM1_CKOUT).

If not, please try to do it and let me know if you solve the problem 🙂