Undocumented I2S ratios

RE: Undocumented I2S ratios

Håkon Alseth — Thu, 08 Apr 2021 13:56:35 GMT

Hi,

This formula will give you a ballpark number, if you change the divider to be 32M instead of 16M:

https://devzone.nordicsemi.com/f/nordic-q-a/391/uart-baudrate-register-values/2046#2046

However, since this specific module uses a pure clk division, and not a baudrate generator, it will not be accurate for all frequencies that you input. for instance, the formula does not match specifically for the frequency that you request.

Kind regards,

Håkon

RE: Undocumented I2S ratios

Charlie — Tue, 06 Apr 2021 14:30:21 GMT

Thanks for the information, this all makes sense. There's just one bit I'm unclear about

[quote userid="2115" url="~/f/nordic-q-a/73569/undocumented-i2s-ratios/303186#303186"] If we write 0x12000000 to this register, we should get ~2.2857 MHz[/quote]

How do you get from the register value (0x12000000) to the corresponding division ratio (14)? I initially thought that the register value was just the division ratio with the bits reversed, but this doesn't seem to be the case?

RE: Undocumented I2S ratios

Håkon Alseth — Tue, 06 Apr 2021 13:29:52 GMT

Hi,

The base frequency is 32 MHz, so your wanted mclk must be a integer division of this base clock.

2 ch * 24 bit * 48 kHz = 2.304 MHz.

32M / 2.304M = 13.89 (similar scenario if we look at 24 kHz sampling rate)

Rounding to the closest integer of 14.

32M/14 = 2.2857 MHz

As you have already found out:

Wrt. the register value of div14, you can look at the definition:

https://infocenter.nordicsemi.com/topic/ps_nrf52840/i2s.html?cp=4_0_0_5_10_9_13#register.CONFIG.MCKFREQ

div15 = 0x11000000

so If we write 0x12000000 to this register, we should get ~2.2857 MHz, which seems to be the case when I look at it with a logic analyzer:

[quote user=""]Is there any reason to prefer the values in `nrf_i2s_mck_t` over the experimentally-determined rates? The custom values seem to work with my amplifier in practice, I can't hear any obvious issues.[/quote]

The list was shortened to the most match the most popular settings, so it should not be an issue to use the register value that you found now.

[quote user=""]Additionally, are there any better values than the ones I found experimentally? I couldn't find any documentation about how the register value corresponds to a clock frequency, so I was just guessing based on `I2S_CONFIG_MCKFREQ_MCKFREQ` definitions in `nrf52_bitfields.h`.[/quote]

No, unfortunately. The value must be an integer division of the base clock.

Kind regards,

Håkon