Using a different frequency LFXO with nRF53

[Stephan Walter]

Hi everyone,

We are using the nRF5340 and need to use an LFXO, not just for BLE, but also for the real-time clock that our product should provide.

Due to other requirements, the common frequency of 32768 Hz is not very convenient for us.

Is it possible to run the LF clock with a crystal frequency of 38400Hz?  This would suit us better as it is divisible by typical "old-school" communication baud rates such as 1200 baud, 1600 baud, 2400 baud.

We would expect the LF oscillator circuit in the nRF53 to be able to handle this frequency. We are aware that this is outside of specification, but are there any specific reasons that would prohibit this?

However we see another problem with the BLE stack, namely the use of the LF clock for scheduling connection intervals.

Does the Nordic Softdevice stack support an LFXO clock different from 32768Hz? We only found the option to choose between LFXO and LFRCO, but is there a way to set a different frequency?

Alternatively, we might use the Zephyr Open-Source stack. The file zephyr/subsys/bluetooth/controller/ll_sw/nordic/hal/nrf5/ticker.h contains constants and conversion macros. Is it enough to change these, or are there other places where the stack assumes a 32768Hz clock?

Any hints are appreciated.

Best regards,

--Stephan Walter

[sipan112]

The application core clocks can be scaled from their respective HFCLK clocks.

The application core clock HCLK128M can at any time be scaled from the HFCLK128M clock using the HFCLKCTRL register.

The HCLK192M clock can be scaled from the HFCLK192M clock using the HFCLK192MCTRL register.

Note: Settings Div1 and Div2 in HFCLK192MCTRL register will result in increased power consumption.

The ACLK audio clock cannot be scaled from the HFCLKAUDIO clock. Instead, its frequency can be configured in the relevant peripherals. Refer to the Audio oscillator for more information on the audio clock and related peripherals.

Note: It is possible to scale the application core clocks at any time, for instance when a clock has already has been started, without having to stop it first.

[Stephan Walter]

Hi sipan112, I don't see how any of this is relevant to my question.

[JONATHAN LL]

Hi,

The nrf5349 needs to have the LFXO stay within the spec, https://infocenter.nordicsemi.com/topic/ps_nrf5340/chapters/oscillators/doc/oscillators.html?cp=3_0_0_3_11_4_1#unique_63981518 , so no you cant change the Hz. 

32768 Hz is not very convenient for us.

Why is this?

Regards,
Jonathan

[Stephan Walter]

Hi Jonathan,

as I said we would like to have the frequency divisible by 1200Hz, 1600Hz and 2400Hz.

We are trying to sample a datastream which can have one of these baud rates (using oversampling with an integer factor). With a 32768 Hz LFXO, we would need to use a RTC compare interrupt and reload the compare register with values just above or below the target interval, which would not be an integer.

This would require waking the core for each sample and would also introduce jitter to the sample time. Using an LFXO with a multiple of the listed frequencies would allow us to avoid these disadvantages.

Best regards,

--Stephan Walter

[JONATHAN LL]

Thanks for the explanation, but no you would not be able to successfully use anything other then 32.768 kHz crystal. 

Regards,
Jonathan