I2S programming on nRF52840

[Rahul Bedarkar]

Hi,

I am trying to write 16bit, 16Khz, 2 channel PCM data to I2S. nRF52840 DK board is connected to speaker via I2S player/decoder board. I am able to hear audio but it is not that good. Looks bit of noisy.

I have seen I2S loopback example in SDK 15. Based on that I have written this new application. I want to understand how frequently I need to write to I2S buffers for above configuration. Also is there any delay we need to give after writing some samples.

Following is code sinnpet from I2S loopback example in which I have some questions:

283 do {
284 // Wait for an event.
285 __WFE();
286 // Clear the event register.
287 __SEV();
288 __WFE();
289
290 if (mp_block_to_fill)
291 {
292 prepare_tx_data(mp_block_to_fill);
293 mp_block_to_fill = NULL;
294 }
295 if (mp_block_to_check)
296 {
297 check_rx_data(mp_block_to_check);
298 mp_block_to_check = NULL;
299 }
300 } while (m_blocks_transferred < BLOCKS_TO_TRANSFER);

1. Why we need to wait for event

2. Since we are not waiting to finish samples here, how much samples I2S can buffer internally. Can it be overriden incase we are write too fast.

3. Can we drive I2S when using softdevice? Do we need __WFE() in case of soft device?

Thanks

Rahul

[Kenneth]

Hello,

1. This is just for power optimization, to keep the CPU in idle when there is nothing to do (waiting for an event), you can remove it if you like.

2. The I2S module is double buffered, you can prepare next buffer while current is written, the max size of each buffer is 8192bytes:
http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52840.ps/i2s.html?cp=2_0_0_5_10_6#concept_gq4_jly_vr 
http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52840.ps/i2s.html?cp=2_0_0_5_10_9_16#register.RXTXD.MAXCNT 

3. The I2S can be used with softdevice yes, but in that case you can call sd_app_evt_wait() instead of __WFE().

Best regards,
Kenneth

[Rahul Bedarkar]

Hi Kenneth,

Thanks for your reply.

I am using sd_app_evt_wait() before writing next buffer. But mp_block_to_fill is NULL in most of cases when sd_app_evt_wait()  returns. This means that some other interrupt has come. I have BLE connection active, may be there is some activity going on. How to wait only for I2S transfer complete such cases?

Thanks,

Rahul

[MartinBL]

Hi,

The code vill return from sd_app_evt_wait() on any interrupt that is being forwarded to your application (BLE, button presses, timers, etc.). Can you post your code?