Dear all,
I have some trouble setting up the correct acquisition of the MEMS microphone over I2S.
Consider the STM32F401RE Nucleo board connected to the IMP34DT05 PDM microphone with L/R to GND.
Through Mx, the I2S peripheral is configured as follows:
Here is the skeleton of the code:
uint16_t pdmRxBuf[128];
uint16_t MidBuffer[16];
uint16_t pcm_data[4096];
uint8_t txstate = 0;
uint8_t rxstate = 0;
int main(void)
{
/* USER CODE BEGIN 1 */
/* USER CODE END 1 */
/* MCU Configuration--------------------------------------------------------*/
/* Reset of all peripherals, Initializes the Flash interface and the Systick. */
HAL_Init();
/* USER CODE BEGIN Init */
/* USER CODE END Init */
/* Configure the system clock */
SystemClock_Config();
/* USER CODE BEGIN SysInit */
/* USER CODE END SysInit */
/* Initialize all configured peripherals */
MX_GPIO_Init();
MX_DMA_Init();
MX_I2S2_Init();
MX_I2S3_Init();
MX_CRC_Init();
MX_PDM2PCM_Init();
/* USER CODE BEGIN 2 */
pcm_index = 0;
/* USER CODE END 2 */
HAL_I2S_Receive_DMA(&hi2s2, &pdmRxBuf[0],128);
/* Infinite loop */
/* Infinite loop */
/* USER CODE BEGIN WHILE */
while (1)
{
if (rxstate==1) {
PDM_Filter(&pdmRxBuf[0],&MidBuffer[0], &PDM1_filter_handler);
for(uint8_t i = 0; i < 16 ; i++){
pcm_data[pcm_index] = mid_buffer[i];
pcm_index++;
}
rxstate=0;
}
if (rxstate==2) {
PDM_Filter(&pdmRxBuf[64],&MidBuffer[0], &PDM1_filter_handler);
for(uint8_t i = 0; i < 16 ; i++){
pcm_data[pcm_index] = mid_buffer[i];
pcm_index++;
}
rxstate=0;
}
if(pcm_index==4096){
//Stop the DMA
HAL_I2S_DMAPause(&hi2s2);
// Send to UART or Store Data
pcm_index = 0;
}
/* USER CODE END 3 */
}
void HAL_I2S_RxHalfCpltCallback(I2S_HandleTypeDef *hi2s){
rx_state = 1;
}
void HAL_I2S_RxCpltCallback(I2S_HandleTypeDef *hi2s){
rx_state = 2;
}The program is very stupid, tries to get 4096 samples into the pcm_data to be saved or sent through the UART.
Having configured the DMA as a circular buffer, I would expect that at every interrupt Half of the buffer is filled with new data, while the other half is used to run the PDM_Filter function and complete the PCM conversion and copy the data out to the pcm_data.
However, before starting the execution I emit from the PC a sine wave at a specific frequency that should be recorded by the 4096 samples acquired by the microphone.
Unfortunately, when I reproduce the 4096 samples, there is only noise.
Did I misunderstand some configurations?
Thanks for your help
Dear @Eleon BORLINI,
The IS2 is configured for a mono microphone (L/R connected to ground).
I'm using the internal HSI (16MHz) to derive the clock (M=10), the PLLI2S is configured with N=192, R = 5 that resulting in 61.44MHz as I2S clock. This is fine with the documentation (Tab.10 AN5027) for mono configuration.
Changing the data size: 24bits on 32bit frame requires that the pdm_buffer[128], but the call to:
HAL_I2S_Receive_DMA(&hi2s2, &pdm_buffer[0], 128/2);
Requires that only half of the size is passed to the DMA because internally one left shift is done for data with 24 and 32bits.
This configuration perfectly works.
However, simply changing the data size: 16bits on 16bit (or 16bits on 32 bits) frame requires to call:
HAL_I2S_Receive_DMA(&hi2s2, &pdm_buffer[0], 128);
Passing the full size of the buffer.
However, this solution results in a distorted audio signal.
Looking with the debugger, the I2SDIV and ODD are the following:
- 16bit data on 16bit frame (I2SDIV = 0x3c and ODD = 0x00) -> Fs = 16kHz
- 24bit data on 32bit frame (I2SDIV = 0x1e and ODD = 0x00) -> Fs = 16kHz
Could you help me understand? Have the pdm_buffer the wrong size?
Thanks!
Hi @frnt ,
I got this feedback from our experts: they are referring to the X-CUBE-MEMSMIC1 library, and they are asking you whether you are using this function pack.
If so, the I2S peripheral configuration in the MEMSMIC1 with the PDMFilter function works (only) if it is set in that way (i.e. 24 bit on 32).
This because, during the data conversion, registers of specific 24-bits peripherals are used, together with local variable at 32 bits.
For this reason, if you change just a configuration parameter in the I2S configuration, then the procedure works no more.
See for example this presentation --> STM32H7-Peripheral-Digital Filter for SD Modulators interface (DFSDM)
-Eleon
