Generating clock to mimic 48kHz sample clock for I2S device

Question

Hi everyone, 
 I am working on interfacing the nRF52840DK with a 24-bit mems microphone INMP441 through the I2S peripheral. As multi-sources pointed out that the nRF52840 does not support 32-bit word size. (ie. WS ( LRCLK ) frame = 64 BCLK pulses wide ) and 48kHz sampling. 
 As the workaround, I configured the nRF52840 as an I2S slave and provided the BCLK (48kHz) and LRCLK(3.125MHz) externally from a signal generator. The I2S works completely fine. 
 As moving forward, I have tried to use the PMWs to mimic the BCLK (48kHz) and LRCLK(3.125MHz) by following this GitHub repo: https://github.com/gregtomasch/nRF52_24-bit-_I2S_Microphone_Audio_Recording_Utility . However, the two PMW signals are not in sync and the output is garbled. 
 
 As a result, I would like to know is there any alternative that I can generate two in-sync clock signals from PMW or other peripheral? 
 
 Many Thanks Anthony

Sigurd · Accepted Answer

Hi, 
 I took a quick look at the code in the 3rd party repo. 
 I see that the PWM clocks uses 2 different timers, and it's started like this: 
 app_pwm_enable(&PWM1);
 app_pwm_enable(&PWM2); 
 So there will be a slight delay there between when the clocks start. 
 A workaround could be to use PPI to start the timers instead. 
 Modify app_pwm_enable so that it does not start the timer immediately. 
 i.e. remove: nrf_drv_timer_enable(p_instance->p_timer); 
 and create a new function for starting the timers at the same time, a function that is called right after the two calls to app_pwm_enable(). 
 In this function, you could e.g. connect a EGU event to start the timers. E.g. connect EGU_event to timer0 start, and fork the event to start timer1 as well. Then trigger the EGU event, by writing to the corresponding EGU TASKS_TRIGGER.

Generating clock to mimic 48kHz sample clock for I2S device

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