How to change nrf52840 CPU clock to 32MHz or even lower ?

[JY wu]

Hello~

I am developing nrf52840 with zephyr ncs tool chain v2.7.0 and SDK v2.6.1.

Our pre-production PCB power consumption still too high, so we are looking for a way to lower nrf52840 CPU clock.

For example: change from 64MHz to 32MHz or 16MHz

The Xtal in our pre-production PCB is 32MHz.

Would you please tell us "how to change nrf52840 CPU frequency from 64MHz to 32MHz or even lower" ?

[Vidar Berg]

Hello,

The CPU frequency is fixed at 64 MHz on the nRF52 series and cannot be changed. This is also the frequency the CPU has been optimized for. The idea of having a relatively fast CPU clock is to allow the CPU to complete its task faster and allow it to spend more time in sleep. But are you sure the high current consumption is caused by the CPU run current and not something else. You should be able to match the numbers from our online power profiler at  Online Power Profiler for Bluetooth LE if you are only using BLE.

Best regards,

Vidar 

[JY wu]

Hi~

Thanks for your quick help.

For the question : "are you sure the high current consumption is caused by the CPU run current and not something else?"

No...not sure.

Would you please help me to clarify:

Can these two settings helps to lower down power consumption?

CONFIG_PM_CPU_OPS=y

CONFIG_BOARD_ENABLE_DCDC=y

It says using DCDC will save more power than LDO. 

[Vidar Berg]

Hi,

CONFIG_BOARD_ENABLE_DCDC will reduce the peak currents if not enabled already (you can check if it is selected for your build by seeing if the symbol is seleced in the generated .config file). Note: make sure your board include the DCDC inductors before enabling this. Otherwise, the CPU will not receive a stable supply voltage.

Generated configuration file:

Do you have measuring equipment to measure the current waveform like the PPK2? It would help to know what idle current is. 

[JY wu]

Hi~

Thanks for your detail answer, we do have this two DCDC inductors and not enable DCDC yet.

For question "Do you have measuring equipment to measure the current waveform like the PPK2? "

Yes, we have.

We will measure current again after change DCDC to enable.