External crystal substitution question for nRF51822AC

[Questchin]

Hi, All,

I am currently developing with nRF51822AC. Due to some issue, we would like to replace the current 16MHz crystal with a crystal other than this frequency. We would like to ask if it is possible to configure the PLL so that we can work with a crystal of different frequency. Since we are using Softdevice (s110 8.0.0), we want to know whether the substitution of the crystal will cause any issues to the SD. I notices that the Softdevice seems to be using the 16MHz as the 0th instance of NRF_DRV_TIMER. Also, will this replacement trigger any other issue for the components such app_timer or scheduler? If not, what configuration should be applied accordingly?

I am new to this field, sorry to bother. Thanks in advance.

[Hung Bui]

@Questchin: As you can find at section 13.1.1 in the nRF51 reference manual, the chip only accept either 16MHz crystal or 32 MHz crystal. The oscillator also work with 2 alternative external source:

• 16 MHz rail to rail clock
• 16 MHz low swing clock.

If you try to apply other frequency than 16MHz (or 32MHz), I'm pretty sure the radio won't work with correct frequency, even though the CPU may still work.

[Questchin]

Thank you for the reference! If I choose to use 32MHz, what parameters I need to set correspondingly to switch over from 16MHz to this frequency? I am using app_timer as well as nrf_drv_timer. Thanks!

[RK]

See Clock Management, Section 13 in the nRF51 manual. As long as the chip is configured to use the 32MHz crystal, there's nothing to do.

[Questchin]

Thanks for the input. I guess you mean the XTALFREQ register? I found this is in the nrf51_bitfields which is a locked library. How may I change this? Also, as the nrf_drv_config.h I applied, the nrf_timer_frequency_t enum has the highest frequency of 16MHz. Is it really okay to just change the XTALFREQ and nothing else?

[Hung Bui]

@Questchin: what we do on our chip when running on 32MHz is to use a divider and convert the 32MHz frequency to 16MHz. So basically the chip is still running on 16MHz oscillator. You can have a look at this answer to know how to let the chip run with 32MHz crystal: https://devzone.nordicsemi.com/question/33972/use-external-32mhz-crystal/

Just XTALFREQ in UICR need to be updated, nothing else need to be change.