Excessively long boot time

RE: Excessively long boot time

V.Lorz — Wed, 18 May 2022 08:34:45 GMT

Thanks Einar for mentioning CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC.

RE: Excessively long boot time

Einar Thorsrud — Wed, 18 May 2022 07:57:38 GMT

Hi,

As you note, system OFF is a special low power mode that is only suitable for some applications in some specific cases. With system on low power mode you also get a sleep current town to a few micro amps when you disable all peripherals you don't need. Wakeup from system on is fast, and there is no reset involved.

A wakeup from system OFF on the other hand always involves a reset, and this means reconfiguring the IC from scratch. It also involves starting the 32.768 kHz oscillator again, which takes a long time if you use the (optional) 32.768 kHz crystal (instead of LFRC). This has a typical startup time of 250 ms, but it can also be longer. I expect this constitutes the vast majority of the startup time you are seeing (you can check this by testing with CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y to see if is reduced).

RE: Excessively long boot time

V.Lorz — Wed, 18 May 2022 07:13:49 GMT

I'll share what I've found about this.

System OFF is, to my opinion, appropriated for applications like a mouse or a keyboard, where a 300ms boot time poses little to no inconvenient for the first move or keystroke, but not for other applications, like mine.

It takes so long to boot due to the time required for initialising everything and for starting the 32KHz crystal oscillator. It is possible to obtain better boot times using the internal RC oscillator, still not good for my application.

The alternative is to use System ON low power mode, shutting down all non required peripherals. Using zephyr, this is mode is entered automatically when all tasks are put to sleep. This option is working for me, yet current consumption is a bit higher.