Using PPI with Zboss zigbee stack

[Gueston]

Hi,

In my project I make use of the PPI to connect the comparator with the counter (timer). But as soon as I enable the zigbee stack, especially when invoking the zboss_main_loop_iteration()
in the main loop, it looks like the PPI connection is not working.

To test this, I 'connected' button4 on my devkit with this counter. There I can also see, that when I don't invoke zboss_main_loop_iteration() the counter will count everytime I push the button.

So I suspect that the zigbee stack uses the PPI that I'm using. According to this list the zigbee stack uses channels 0 - 12. When I allocate the PPI channel with this code:

    err_code = nrfx_ppi_channel_alloc(&ppi_channel);
    APP_ERROR_CHECK(err_code);
    NRF_LOG_INFO("Assigned PPI channel: %d", ppi_channel);

I see in the debug log, that channel 5 is used. Which is strange, because the zigbee stack uses this. I call this after initiating the zigbee stack. So I don't know how to change it such that it will choose the right PPI channel when using the zigbee stack?

[Jørgen Holmefjord]

Hi,

The PPI 0-5 channels are only used by the 802.15.4 radio driver if debugging of radio events on GPIOs are enabled (by ENABLE_DEBUG_GPIO symbol). As far as I know, this is not enabled by default in the Zboss stack. It should therefor be ok to use the PPI channels for other purposes.

I'm not sure if the nrfx_ppi driver is aware of the PPI channels used by Zigbee stack, I could not find any sources in the SDK that specify channels used by this stack (we have defines for the Softdevice and ESB/Gazell protocols). You can add awareness of the PPI channels used by Zigbee stack by modifying sdk_resources.h to include PPI channel mask for used channels in NRF_PPI_CHANNELS_USED define.

Best regards,
Jørgen

[Gueston]

Adding that awareness indeed solved the problem. Shouldn't this be added by default?

[Gueston]

By the way: I discovered when I add for instance 0x1F as a mask to the NRF_PPI_CHANNELS_USED, I read in the log that it uses PPI channel 10. Then it works, but according to that list the zigbee stack is using PPI 0-12, so how can PPI channel 10 work?

[Jørgen Holmefjord]

Did you enable the PPI channels in your application before or after initializing the Zigbee stack?

I'm not sure if all channels are used by the 802.15.4 driver in all states, the channel may not be used at the point where you tested. You can see how the channels are used in the driver source. There is also a define you can use to mask out the used channels.