nrf5340 External high-speed clock source & Lowpower

[Zion]

Hello,

I have a few questions:
1. How to add a prj.conf or an overlay file to enable peripherals to use an external high-frequency crystal oscillator as the clock source?
        I tested by configuring the device tree as follows, but the high-frequency clock did not take effect. It only works when I add the following code to the application code.

&clock {
    status = "okay";
    lfxo: lfxo {
        // source = "LFXO";
        compatible = "nordic,nrf-lfxo";
        #clock-cells = <0>;
        clock-frequency = <32768>;
        // zephyr,pm-device-runtime-auto;
        status = "okay";
    };

    hfxo: hfxo {
        // source = "HFXO";  
        compatible = "nordic,nrf-hfxo";
        #clock-cells = <0>;
        clock-frequency = <DT_FREQ_M(32)>;
        // zephyr,pm-device-runtime-auto;
        status = "okay";
    };  
};

void hfxo_start(void) {
    // Enable external high-speed 32MHz crystal oscillator
    nrf_clock_task_trigger(NRF_CLOCK, NRF_CLOCK_TASK_HFCLKSTART);

    // Wait for the HFXO to stabilize
    while (!nrf_clock_event_check(NRF_CLOCK, NRF_CLOCK_EVENT_HFCLKSTARTED)) {
        // Wait for the event flag
    }
}

2. How can I configure the system to enable the high-frequency external clock to automatically turn on when peripherals are in use and turn off when not in use for low-power management?

[Vidar Berg]

Hello,

The crystal oscillator is only active when requested by the application or one of the SDK modules. For example, the Bluetooth stack requests the crystal oscillator during protocol events but releases it afterward to allow the HF clock to shut down in sleep mode (System ON mode) to reduce the idle current. If you want to keep the crystal oscillator always on, you can request it using the onoff API as shown in this sample: https://github.com/nrfconnect/sdk-nrf/blob/ea2994fb4458302e5c3bb8404eabad28c1def943/samples/peripheral/radio_test/src/main.c#L18 

Best regards,

Vidar

[Zion]

So when I use the UART, do I need to enable the external high-speed clock in the application when needed and disable it when not? Does enabling the external high-speed crystal clock mean that all peripherals are currently using the high-speed external clock?

[Vidar Berg]

Hi,

Yes, the HFXO should be enabled when while UART is enabled. I have created a request internally that we should implement this in our UART examples. I posted a demo sample that does this here:  RE: Suspending UART Async using PM_DEVICE ACTION_SUSPEND causes crash. 

Zion said:

that all peripherals are currently using the high-speed external clock?

Yes, correct.

[zy_wen]

I use the CONFIG_PM_DEVICE_RUNTIME=y, pm_device_runtime_enable(uart_dev); to lower the power consumption of serial ports. It shouldn't be affected, right?

[Vidar Berg]

It should be fine as long as the application does not release its request for HFXO while UART is enabled.

[zy_wen]

Another question, regarding the SDK modules you mentioned, when using LORA or QSPI flash, does the driver clock under it switch to the external high-speed clock?

[Vidar Berg]

You typically don't need a high-accuracy clock source for serial interfaces such as QSPI, SPI, or TWI, as they use a shared clock. However, UART is asynchronous and relies on the clocks on both sides to not drift too far apart.