Can I make one of the nRF5340 pins oscillate at 12.288MHz? (Audio PLL Clock)

[DBT]

I'm trying to avoid buying a 12.288MHz oscillator if I can make one of the pins on the nRF5340 oscillate at that frequency.

It looks like the ACLK is only available to the PDM and I2S peripherals.

Is there a way to make one or both of them use ACLK as its clock source and do a divide-by-1 so that the clock output will be 12.288MHz?

(The PDM or I2S interface doesn't need to "function" as a PDM or I2S peripheral.  It just needs to provide the 12.288MHz clock signal)

[Håkon Alseth]

Hi,

 

You can output the ACLK by setting the source to ACLK and .BYPASS=1 in this register:

https://infocenter.nordicsemi.com/topic/ps_nrf5340/i2s.html?cp=3_0_0_6_14_9_26#register.CONFIG.CLKCONFIG

 

You should then get the ACLK out on the selected MCLK pin. Note that some pins on the nRF5340-PDK are not routed to the pin header (used for other purposes). I took P0.10 for testing.

 

Note that the ACLK and external HFCLK shall be started prior to enabling and starting the I2S peripheral:

 

NRF_CLOCK->TASKS_HFCLKSTART = 1;
while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0);
NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;

NRF_CLOCK->TASKS_HFCLKAUDIOSTART = 1;
while (NRF_CLOCK->EVENTS_HFCLKAUDIOSTARTED == 0);
NRF_CLOCK->EVENTS_HFCLKAUDIOSTARTED = 0;

 

Kind regards,

Håkon

[DBT]

That worked.

Here is the code that makes it work:

#define I2S_MCK_PIN 7 // MCLK pin is P0.07

#define I2S_DATA_BLOCK_WORDS 512 //How many numbers do we want. time reorded = DATA_BLOCK_WORDS / freq

static u32_t m_buffer_rx32u[I2S_DATA_BLOCK_WORDS];
static nrfx_i2s_buffers_t i2s_initial_buffers;



// I2S ISR handler
ISR_DIRECT_DECLARE(i2s_isr_handler)
{
  nrfx_i2s_irq_handler();
  ISR_DIRECT_PM();  // PM done after servicing interrupt for best latency
  return 1;         // We should check if scheduling decision should be made
}

// I2S event handler
static void i2s_data_handler(nrfx_i2s_buffers_t const *p_released, u32_t status)
{
  if (NRFX_I2S_STATUS_NEXT_BUFFERS_NEEDED == status)
  {
    nrfx_i2s_next_buffers_set(&i2s_initial_buffers);
  }
}

/*****************************************************************************
* Function: i2s_clk_init
*
* Description: provide 12.288MHz reference clock to audio ADC
*
* Parameters: none
*
* Returns: nothing
*****************************************************************************/
static void i2s_clk_init(void)
{
  // the audio clock defaults to 12.288MHz, so we don't need to adjust it

  // configure I2S
  IRQ_DIRECT_CONNECT(I2S0_IRQn, 0, i2s_isr_handler, 0);
  i2s_initial_buffers.p_rx_buffer = m_buffer_rx32u;

  nrfx_i2s_config_t i2s_cfg = NRFX_I2S_DEFAULT_CONFIG(NRFX_I2S_PIN_NOT_USED, NRFX_I2S_PIN_NOT_USED, I2S_MCK_PIN, NRFX_I2S_PIN_NOT_USED, NRFX_I2S_PIN_NOT_USED);
  i2s_cfg.clksrc = NRF_I2S_CLKSRC_ACLK; // audio clock
  i2s_cfg.enable_bypass = true;         // bypass division, so we get the raw audio clock
  nrfx_err_t err_code = nrfx_i2s_init(&i2s_cfg, i2s_data_handler);

  if(err_code != NRFX_SUCCESS) 
  {
    printf("Error initializing I2S\n");
    return;
  }

  // start i2s
  err_code = nrfx_i2s_start(&i2s_initial_buffers, I2S_DATA_BLOCK_WORDS, 0);
  if (err_code != NRFX_SUCCESS)
  {
    printf("I2S start error\n");
  }
}

[yoyou]

Hi,DBT

how to adjust audio clock to 16bit 16kHz Left mono?

thanks

[DBT]

I'm not an expert on I2S using nRF5340.  This ticket is about getting the MCLK pin to oscillate.  I wasn't trying to send or receive any I2S data.

However, at this point I am trying to receive data over I2S, so I'll probably know more within a few weeks.

Look at section 7.15.3 in the datasheet for the discussion about "mono"

By "left" do you mean left-aligned or the left channel of audio?  Look at section 7.15.6 in the datasheet.  This same section talks about 16-bit wide data.

I think for 16kHz sampling you'd want to use PCLK32M rather than ACLK as the clock source.

You'll probably want to submit a separate ticket in order to get better answers to your specific questions.