I2S Master Clock always runs at same speed as Bit Clock

RE: I2S Master Clock always runs at same speed as Bit Clock

Edvin — Tue, 16 Apr 2024 07:38:20 GMT

Yes. Did you see my previous reply from yesterday?

RE: I2S Master Clock always runs at same speed as Bit Clock

sthon — Tue, 16 Apr 2024 06:11:32 GMT

Hi Edvin,

any updates?

Kind regards,

Steinar

RE: I2S Master Clock always runs at same speed as Bit Clock

Edvin — Mon, 15 Apr 2024 07:48:32 GMT

Hello Steinar,

Sorry for the late replies. Karl got back, but then had to leave again.

I looked into this some more, and discussed with some of the authors of our I2S drivers. Unfortunately, there is no out of the box support to directly set the I2S Master Clock frequency in Zephyr, but it is possible to do so by either using the nrfx_i2s driver directly, or by modifying the nrfx_i2s driver sligtly.

The way that the frequency is chosen is through the find_suitable_clock() function in i2s_nrfx.c (ncs\zephyr\drivers\i2s\n2s_nrfx.c).

If you comment out the line starting with "{ 64, ..."

from here:

static void find_suitable_clock(const struct i2s_nrfx_drv_cfg *drv_cfg,
				nrfx_i2s_config_t *config,
				const struct i2s_config *i2s_cfg)
{
	static const struct {
		uint16_t        ratio_val;
		nrf_i2s_ratio_t ratio_enum;
	} ratios[] = {
		{  32, NRF_I2S_RATIO_32X },
		{  48, NRF_I2S_RATIO_48X },
		//{  64, NRF_I2S_RATIO_64X },
		{  96, NRF_I2S_RATIO_96X },
		{ 128, NRF_I2S_RATIO_128X },
		{ 192, NRF_I2S_RATIO_192X },
		{ 256, NRF_I2S_RATIO_256X },
		{ 384, NRF_I2S_RATIO_384X },
		{ 512, NRF_I2S_RATIO_512X }

This function, which finds the first suitable frequency, will not find the 64 one, and it will move on to the 128 (the 96 will not match). Then you will see that the MCK that the frequency will double.

Alternatively, you can use the nrfx driver directly, instead of the Zephyr API. If so, you can change the nrfx_i2s_config_t.mck_setup to the MCK frequency you want to use, and make sure to also adjust nrfx_i2s_config_t.ratio, to get the correct bit-clock frequency. Do this before calling nrfx_i2s_init(). It should be pretty straight forward.

Best regards,

Edvin

RE: I2S Master Clock always runs at same speed as Bit Clock

sthon — Mon, 08 Apr 2024 13:04:41 GMT

Hi Karl,

any updates?

Kind regards,

Steinar

RE: I2S Master Clock always runs at same speed as Bit Clock

sthon — Tue, 02 Apr 2024 12:36:54 GMT

Hi Edvin,

Thank you for the reply.

In addition to the code already submitted above in the original ticket, you can add the following for a minimized application to replicate the issue I am seeing:

#include <zephyr/kernel.h>

static uint8_t mem_block[BLOCK_SIZE];

int main(void)
{
	i2s_init();

	while (1)
	{
		uint32_t block_size;

		i2s_buf_read(i2s_dev_rx, &mem_block, &block_size);
	}
}

All that is needed in the prj.conf is:

CONFIG_I2S=y

The application will print the following to the terminal when started:

*** Booting nRF Connect SDK v2.5.0 ***
[00:00:00.411,102] <inf> i2s_nrfx: I2S MCK frequency: 3072000, actual PCM rate: 48000

Kind regards,

Steinar

RE: I2S Master Clock always runs at same speed as Bit Clock

Edvin — Tue, 26 Mar 2024 14:35:32 GMT

Hello Steinar,

Sorry for the late reply. Karl is out of office, so I will try to look into this ticket.

Is it possible to share a minimized application replicate the issue that you are seeing? We are a bit short staffed due to easter holidays, so if I could use this to get a flying start, that would be really helpful.

Best regards,

Edvin

RE: I2S Master Clock always runs at same speed as Bit Clock

sthon — Mon, 18 Mar 2024 08:56:42 GMT

Hi Karl,

Thank you for the help so far.

I've attached the oscilloscope traces showing master clock (MCLK) and serial clock (SCLK) being equal.

Here is a screenshot showing all I2S signals:

As can be seen, the LRCLK is close to the expected sample frequency of 48kHz.

Furthermore, using the equation for calculating SCK: SCK = 2 * LRCK * CONFIG.SWIDTH

SCK = 2 * 48 kHz * 32 = 3.072 MHz which is close to the measured SCLK = 3.012 MHz from the trace

I tried to find a setting for mck_setup that you referred or anything related to master clock, but I cannot find where in i2s_config this is?

Or is the master clock configured in the overlay somhow?

Best regards,

Steinar

RE: I2S Master Clock always runs at same speed as Bit Clock

Karl Ylvisaker — Fri, 15 Mar 2024 20:49:50 GMT

Hello Steinar,

Thank you for your patience with this.

From the nRF5340 I2S documentation we have that:

LRCK = MCK / CONFIG.RATIO

and

The serial clock (SCK), often referred to as the serial bit clock, pulses once for each data bit being transferred on the serial data lines SDIN and SDOUT.

When operating in Master mode, the SCK is generated from the MCK, and the frequency of SCK is then given as:

SCK = 2 * LRCK * CONFIG.SWIDTH

I do not however see that this should produce a SCK equal to the MCK in your case. Could you possibly share the oscilloscope traces so that I could have a look?

I also notice that you are not setting the mck_setup explicitly - could you try this, and see if it then achieves the mck frequency you were intending?

Best regards,
Karl