How to use GRTC.CC[0] with NRFX?

[Mark Rages]

Hi,

I need to use GRTC channel 0 in a nRF54L15 project.

Channel 0 is special.  It is the channel that the INTERVAL register controls.

`zephyr/boards/nordic/nrf54l15dk/nrf54l_05_10_15_cpuapp_common.dtsi` contains this excerpt:

grtc {
        owned-channels = <0 1 2 3 4 5 6 7 8 9 10 11>;
        /* Channels 7-11 reserved for Zero Latency IRQs, 3-4 for FLPR */
        child-owned-channels = <3 4 7 8 9 10 11>;
        status = "okay";
};

So I was hoping that I could override this declaration in the devicetree,  removing channel 0, which would leave it available for my own use outside of NRFX.

But this doesn't work.  It fails this assertion in `nrfx_grtc.c`:

        NRFX_STATIC_ASSERT(NRFX_GRTC_CONFIG_ALLOWED_CC_CHANNELS_MASK &
                           GRTC_CHANNEL_TO_BITMASK(MAIN_GRTC_CC_CHANNEL));

It seems channel 0 is always used by NRFX.  It is `MAIN_GRTC_CC_CHANNEL` which is `NRF_GRTC_MAIN_CC_CHANNEL` which is hardcoded as "4" for FLPR code and "0" for main processor code.

The nrfx code unilaterally allocates the first channel in `nrfx_grtc_syscounter_start()`.  Additionally `nrfx_grtc_channel_alloc()` allocates the channels in ascending numerical order.

How can I configure a nrfx project to let me use channel 0?

(Previous discussion GRTC Periodic interval + nrfx but this questions isn't answered there.)

[dejans]

Hi,

Which NCS version do you use?

I need to use GRTC channel 0 in a nRF54L15 project.

Can you provide some details about your project? Why do you need to use GRTC channel 0?

It fails this assertion in `nrfx_grtc.c`

Can you provide full log which shows the error?

Best regards,
Dejan

[Mark Rages]

> Which NCS version do you use?

NCS 2.9.2, as pinned by Ant.

> Can you provide some details about your project? Why do you need to use GRTC channel 0?

I will PPI the CC[0] event to a GPIOTE toggle task. This is the only way to make an unjittered square wave of arbitrary frequency with the nRF54.

> Can you provide full log which shows the error?

It is a static assert. You want the build log?

[dejans]

Hi,

Channel 0 is always used by nrfx. For now, it is not possible to configure nrfx project in such a way that you use GRTC channel 0. As a workaround, you could try to change main channel number here.

Best regards,
Dejan

[Mark Rages]

Thank you, I was wondering if I have to modify the SDK.

Is there any plan to fix this in future releases to allow the NRF_GRTC->INTERVAL feature to be used?

Even wrapping that #define with "#ifndef ..." would be sufficient, as I could add the definition as compiler arguments.

[dejans]

Hi,

Mark Rages said:

Is there any plan to fix this in future releases to allow the NRF_GRTC->INTERVAL feature to be used?

There is an internal feature request that will be considered. For specific future plans and timelines when this might get fixed, please contact your regional sales manager.

Best regards,
Dejan