PDM working example

[Vishnu Pradeep]

hi, Is there any working PDM example for nRF52832? or can anyone give a sample initialization code for PDM with easyDMA using the inbuilt nrf_pdm_drv functions?

Regardss, Vishnu

[Sigurd]

Hi,

We don't have any full examples yet. But here is some code to get you started:

#include "nrf_drv_pdm.h"

#define CONFIG_PDM_BUFFER_SIZE_SAMPLES 320

// <o> PDM Decimation Filter Gain <0x00-0x50>
// <i> For details on the PDM decimation filter, see the 'Decimation filter' section in the nRF52 Product Specification document.
#define CONFIG_PDM_GAIN 0x28


// <o> PDM CLK Pin
#define CONFIG_IO_PDM_CLK 0x06

// <o> PDM DATA Pin
#define CONFIG_IO_PDM_DATA 0x05

// <o> PDM Microphone Power Control Pin
#define CONFIG_IO_PDM_MIC_PWR_CTRL 0x1A


/**@brief Audio buffer handler. */
typedef void (*drv_audio_buffer_handler_t)(int16_t *p_buffer, uint16_t samples);



static int16_t                      m_pdm_buff[2][CONFIG_PDM_BUFFER_SIZE_SAMPLES];

static drv_audio_buffer_handler_t   m_buffer_handler;


static void m_audio_buffer_handler(int16_t *p_buffer, uint16_t samples)
{
    // Audio processing
}

static void drv_audio_pdm_event_handler(uint32_t *p_buffer, uint16_t length)
{
	m_buffer_handler((int16_t *)p_buffer, length);
}



uint32_t drv_audio_init(drv_audio_buffer_handler_t buffer_handler)
{
    nrf_drv_pdm_config_t pdm_cfg = NRF_DRV_PDM_DEFAULT_CONFIG(CONFIG_IO_PDM_CLK,
                                                              CONFIG_IO_PDM_DATA,
                                                              m_pdm_buff[0],
                                                              m_pdm_buff[1],
                                                              CONFIG_PDM_BUFFER_SIZE_SAMPLES);
    
    if (buffer_handler == NULL)
    {
        return NRF_ERROR_INVALID_PARAM;
    }
    
    m_buffer_handler    = buffer_handler;
    pdm_cfg.gain_l      = CONFIG_PDM_GAIN;
    pdm_cfg.gain_r      = CONFIG_PDM_GAIN;
    

    return nrf_drv_pdm_init(&pdm_cfg, drv_audio_pdm_event_handler);
}






/**
 * @brief Function for application main entry.
 */
int main(void)
{
    uint32_t err_code;
    
    err_code = drv_audio_init(m_audio_buffer_handler);
    APP_ERROR_CHECK(err_code);
    
    err_code =  nrf_drv_pdm_start();
    APP_ERROR_CHECK(err_code);
    while (true)
    {
        // Do nothing.
    }
}
/** @} */

I also recommend taking a look at the PDM driver. It's located in SDK_folder\components\drivers_nrf\pdm

[Vishnu Pradeep]

Thanks Sigurd. That helped me a lot. The program executes into the "drv_audio_pdm_event_handler" but I am unable to capture any data from the "p_buffer". It returns either zero sometimes or 0xFFF8FFF8. I am using SPK0838HT4H PDM microphone in my application. The configuration is mono. I have made the(L/R)select pin as ground(which is R). The configuration is set as same as above. I have changed the frequency with all the three options. But there is no convincing result. What are the things I should look into?

[Sigurd]

Are you samling data on the correct edge of PDM_CLK?

Try to set pdm_cfg.edge = NRF_PDM_EDGE_LEFTRISING; in drv_audio_init()

[Vishnu Pradeep]

edge settings is currently default NRF_PDM_EDGE_LEFTFALLING, which is what the application note says. I tried with NRF_PDM_EDGE_LEFTRISING as well but similar result. It is going into the drv_audio_pdm_event_handler. What is the significance of CONFIG_PDM_BUFFER_SIZE_SAMPLES ?

[Vishnu Pradeep]

I am not able to move any further from this. Is there anything I can check with?

[Sigurd]

Do you see any difference if you change the Right/Left output gain adjustment(CONFIG_PDM_GAIN) ?