Undocumented I2S ratios

[Charlie]

I'm trying to achieve as close as possible to 48KHz (or 24KHz) LRCLK rate, in 24-bit mode (MCK/LRCLK = 48), on an nRF52832. With the values defined in `nrf_i2s_mck_t` the closest I can get is 32DIV15 (44.4 KHz, -7%) or 32DIV30 (22.2 KHz, -7%). However I can get closer if I use `0x12000000` (measured at 47.44 KHz, -1%) or `0x09000000` (measured at 23.74 KHz, -1%). 

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.

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`.

[Charlie]

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

Håkon Alseth said:

If we write 0x12000000 to this register, we should get ~2.2857 MHz

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?

[Håkon Alseth]

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