Can ACLK be tuned to 12.0 MHz by setting HFCLKAUDIO.FREQUENCY to 0x8000?

[Kevin Doss]

I have an application where I would like to use ACLK at 12.0 MHz to drive the I2S MCLK signal and create an atypical audio sampling rate (46875 samples/sec). If ACLK is at 12 MHz I can then set the I2S CONFIG.RATIO register to value 6 (LRCLK = MCLK/256) to get a 46875 samples/sec LRCLK. My concern is that even though it appears ACLK can be tuned to 12 MHz by setting HFCLKAUDIO.FREQUENCY = 0x8000, there is language in the nRF5340 Product Spec that suggest ACLK can only be tuned in smaller ranges around 11.289 MHz and 12.288 MHz. If I attempted to tune ACLK to 12.0 MHz would the PLL be stable at this value?

[Maria Gilje]

Hello,

I want to let you know that I'm working on finding an answer to your questions. Thank you for your patience.

Best regards,

Maria

[Kevin Doss]

FYI, I tried setting HFCLK.FREQUENCY = 0x8000 and configuring the I2S peripheral to operate using ACLK @12 MHz and LRCLK @ 46875 (ACLK/256). The clocks appear to be at the right frequency and stable on my devkit, but what I need to know is if this will always be true or am I just getting lucky on this particular device?

[Maria Gilje]

We have no guarantee for the device functionality if you go outside the SoC or protocol specification.

A sampling rate of 46875 is not within the LE Audio specification, so if you go forward with that it will be a proprietary solution.

I have not found the right person to ask about the reasoning for the two clock frequency bands for the ACLK yet.

[Kevin Doss]

46875 samples per second is a proprietary solution. It's convenient for solutions where the main frequency reference in the system is 24 MHz (or a multiple thereof).

[Maria Gilje]

The person who can provide the necessary information on the ACLK frequency is not available this week. When they are back and has provided us with an explanation we will update you. This will likely be done by Tuesday next week.
Thank you for your patience.

Best regards,
Maria

[Naeem Maroof]

Hi Kevin,

Maria is out-of-office, and I will communicate with you regarding this ticket. I am relaying to you the response received internally regarding the query.

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The reason for the two frequency bands is purely the limitations in the analog oscillator. The two bands correlate to 44.1 kHz and 48 kHz audio frequencies. We are production trimming these two bands to make sure the oscillator operates as intended.

Using frequencies outside these two ranges are prohibited. This is because there is no guarantee that the PLL can provide correct frequency due to the limited tuning range of the internal VCO.

Meaning, if a customer tries to use 12 MHz, they might be able to do so at the bench, but over temperature, process, and voltage, the PLL will start to fail. We cannot guarantee its accuracy or correct operation.

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Regards,

Naeem