About timing in mesh processing

[wataru_m]

Hello team,

I'm working on system development using Mesh SDK and I have some questions so please let me know.

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Development environment
IDE : Segger Embedded Stuido
SoC : nRF52840
SDK : Mesh v2.2.0
          normal SDK v15.0.0
Base project : light switch example (modifying for our system)
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1. If a node belonging to a group receives a message for that group address, which of the following will be performed first?
    ・Processing for messages, such as replies
    ・Transfer message to another node
    ・Anything else.


2. There is possiblity to rewrite the same variable in the receive callback of Mesh message and the timer interrupt handler, in main.c.
    As such, I do not want to generate other interrupts during this process, includes receiving mesh messages.
    How can I realize this?
    Are the following functions and macros useful?
    ・CRITICAL_REGION_ENTER () / CRITICAL_REGION_EXIT ()
    ・nrf_mesh_disable () / nrf_mesh_enable ()


3. About interrupt
    As far as the document is referred to, when the Mesh message is received during execution of the application interrupt process,
    the application interrupt is interrupted and the Mesh reception interrupt is processed.
    I think that system will return to the interrupted application interrupt after the processing of the reception interrupt is finished.
    Is this recognition correct?

    I will assume that it is correct and ask a question.
    Is there a way to see this series of processes?
    
    I put a loop processing with an end condition to make the waiting time in the timer interrupt handler.
    I tried sending Mesh messages during looping, but the timer interrupt handler was not interrupted by the receipt of Mesh messages.
    On the contrary, mesh message reception interrupt did not occur even after loop termination.
    Does that mean that the system has not received a message?
    Or did the system receive it but did not put the interrupt on hold?

    The code in the timer interrupt handler is described below for reference.

void timer_handler(void * p_context)
{
    __LOG(LOG_SRC_APP, LOG_LEVEL_INFO, "%s called.\n", __func__);
    
    for (uint8_t i = 100; i > 0; i--)
    {
        printf("loop %d\n", i);
    }
}

Thanks in advance.

Wataru

[ovrebekk]

Hi Wataru

1. First the packet will be decrypted. Then there will be a check to see if it should be relayed, after which the TTL will be decremented, the packet encrypted again, and put in the outgoing packet buffer. 

After that the packet will be processed locally. 

Still, depending on the state of the outgoing buffer there might be some delay before the packet is retransmitted, so whether or not this happens before the local processing is impossible to say. 

2. An interrupt of the same priority as another can not interrupt it. This also means that an interrupt can not interrupt itself. 

The only thing that can interrupt you when you are in an interrupt handler is another interrupt of higher priority (lower IRQ priority number). 

If the timer interrupt has a higher interrupt priority than the mesh interrupt then I would suggest disabling this particular interrupt before you access the shared variable. 

3. If an application interrupt of lower priority is interrupted by the mesh interrupt, then execution will return to the application interrupt after the mesh interrupt is completed, that is correct. An interrupt of similar priority will not be interrupted. 

You could visualize this by using a debugger with trace functionality, such as the Segger J-Trace.

Regarding your timer_handler, I expect that this is running at an interrupt priority similar or higher than that of the mesh interrupts, which means you are blocking the mesh operation while sending your messages. This is not recommended, as the mesh will not be able to operate as normal. 

Maybe you could try to defer the execution to the main context instead, either by using the app_scheduler module, or by setting a flag in the timer_handler that you later check in your main loop?

Best regards
Torbjørn

[wataru_m]

Hi, Ovrebekk

Thanks for your reply. And sorry for the late my reply.

1. My understanding that the order to execute first is not decided, is it correct?
    (Processing of messages or relaying of messages)
    Even in that case, both processes will be performed?

2. If an interrupt with the same priority occurs while processing in the interrupt handler,
    will interrupt processing that occurs later be on hold?
    In other words, will interrupt processing that occurs later be executed
    after the current interrupt handler has ended?

By the way, in the light_example example, what is the priority of mesh interrupts?
I think the priority is "0". Does that mean not?
The timer interrupt priority is set to LOWEST (7).

Although it is not intended, if you can block mesh interrupts,
do you think that blocking mesh interrupts is not a problem
if it is an algorithm that can collect data in multiple communications?

I don't want to do this now because using the app_scheduler is likely to result in many system changes.
I would like to consider again at the timing of the big update,
so please inform me about how app_scheduler works and how to use it.

Best regards
Wataru

[ovrebekk]

Hi Wataru

You are correct, the INTEN register can not be accessed directly, only indirectly through the INTENSET and INTENCLR registers. 

To read INTEN you just need to read INTENSET or INTENCLR, they will return the same thing. 

To write to INTEN you have to either set or clear bits independently by writing to INTENSET/INTENCLR. 

How do you enable TWI in your example?

Do you use one of the standard drivers?

Best regards
Torbjørn

[wataru_m]

Hi, Torbjørn

I learned how to get information about interrupts. Thank you very much.

/**
 * @brief UART initialization.
 */
void twi_init (void)
{
    ret_code_t err_code;

    const nrf_drv_twi_config_t twi_tof_config = {
//       .scl                = 3,
//       .sda                = 2,
       .scl                = ARDUINO_SCL_PIN,
       .sda                = ARDUINO_SDA_PIN,
       .frequency          = NRF_DRV_TWI_FREQ_400K,
       .interrupt_priority = APP_IRQ_PRIORITY_HIGH,
       .clear_bus_init     = false
    };

    err_code = nrf_drv_twi_init(&m_twi, &twi_tof_config, twi_handler, NULL);
    APP_ERROR_CHECK(err_code);

    nrf_drv_twi_enable(&m_twi);
}

I initialize and enable TWI in this way.
I think this is the same as the peripheral example. not right?

Best regards, 
Wataru

[ovrebekk]

Hi Wataru

If you look at the driver implementation in nrf_drv_twi.c you will see that interrupts are not enabled until you start a TWI transfer, so after init the INTEN register will still be 0. 

You can search for the nrf_twi_int_enable function in nrf_drv_twi.c to see where INTENSET is written. 

Best regards
Torbjørn

[wataru_m]

Hi Torbjørn

Thank you for your advice.
Actually, I thought the same thing after asking a question.
When I checked on the device now, I could see that the value was set in the INTENSET register immediately after the TWI communication.

Now I know where to look for information about interrupts, thank you.

Best regards
Wataru

[ovrebekk]

No problem, happy to help