nRFX based PDM to PCM implementation (I2S example)

[Nikhil Komalan]

Hello,

I am currently interfacing my nRF52840 DK with i2S based MP34DT05 sensor. I am using nrfx i2s driver's for interfacing with sensor. I am using master clk of 2MHz, LCLK 16KHZ. 

My sensor output PDM data. In my project I am recording audio samples on left channel, so I am currently getting 2*16bit of left data per 32bits as shown in nrf docs. I have got 16bit recorded samples in buffer, but to how to convert this pdm data in pcm format. 

Normally we need to divide the PDM data with a decimation factor, but in I2S we already are dividing the bit sampling rate with ratio value. So is the output from the I2S itself is decimated PCM ?  

Or can anyone give a example of how to convert the PDM to PCM data. Any sort of help is dually appreciated. 

[Audun]

Hello,

I see a couple of problems here, some from my code reference in earlier reply:

1) You don't need both "K_MEM_SLAB_DEFINE" and "struct k_mem_slab audio_slab;". Same for FIFO

2) The mem slab needs to be at least 3 blocks long. The PDM driver will request 2 blocks right away, and request a third before you will have had time to process the first released one:

//Memory slab
K_MEM_SLAB_DEFINE(audio_slab, sizeof(struct data_item_t), 3, 4);

//FIFO to hold pointers to memory slab
K_FIFO_DEFINE(my_fifo);

3) No double-pointer to k_fifo_put. It should be like this:

k_fifo_put(&my_fifo, filled_block);

4) k_sleep() must be removed from Microphone_Start_Record(). Otherwise slab will be emptied while the main thread is sleeping. If you want to skip the first X seconds of audio, you need to free the first Y blocks in main() without printing them out first (k_mem_slab_free)