Implementation of Periodic SPI Operations with nrfx

[BENBF]

Good evening,

I would like to execute an SPI operation every 20ms using an interruption. I saw that it was recommended utilizing nrfx timer and PPI to achieve this, ensuring that the SPI writing process does not encounter timeouts. But I have difficulties implementing this solution, and don't manage to find a lot of documentation about it, and online samples seem to be a bit outdated.

Would you be able to help me implementing this, either with a small sample or some guidelines? Thank you very much and have a good day.

[PabloL]

Hi,

1) Set a timer in timer mode:

#define TIMER1_INST_IDX 0
#define TIMER_COMPARE_COUNT 320000  // 16000000 * 0.02s

// Timer instance
const nrfx_timer_t timer1_inst = NRFX_TIMER_INSTANCE(TIMER1_INST_IDX);

void timer_init(void)
{
    nrfx_err_t err;
    (void)err;
    
    // Configure Timer
    uint32_t base_frequency = NRF_TIMER_BASE_FREQUENCY_GET(timer1_inst.p_reg);
    nrfx_timer_config_t config = NRFX_TIMER_DEFAULT_CONFIG(base_frequency);
    config.bit_width = NRF_TIMER_BIT_WIDTH_32;
    config.mode = NRF_TIMER_MODE_TIMER;
    config.p_context = &timer1_inst;
    err = nrfx_timer_init(&timer1_inst, &config, NULL);
    NRFX_ASSERT(err == NRFX_SUCCESS);
    
    #if defined(__ZEPHYR__)
    IRQ_DIRECT_CONNECT(NRFX_IRQ_NUMBER_GET(NRF_TIMER_INST_GET(TIMER1_INST_IDX)), IRQ_PRIO_LOWEST,
    NRFX_TIMER_INST_HANDLER_GET(TIMER1_INST_IDX), 0);
    #endif
    
    /* Setting the timer (in timer mode) channel NRF_TIMER_CC_CHANNEL0 in the extended compare
     * mode to clear the timer and trigger an event if internal counter register is equal
     * to TIMER_COMPARE_COUNT.
     */
    nrfx_timer_extended_compare(&timer1_inst, NRF_TIMER_CC_CHANNEL0, TIMER_COMPARE_COUNT,
    NRF_TIMER_SHORT_COMPARE0_CLEAR_MASK, false);
}

2) Set SPIM:

// SPIM instance to be used
#define SPIM_INST_IDX 4

// SPI instance
nrfx_spim_t spim_inst = NRFX_SPIM_INSTANCE(SPIM_INST_IDX);

// SPI Configuration
void SPI_Init(void)
{
    nrfx_spim_config_t spim_config = NRFX_SPIM_DEFAULT_CONFIG(SCK_PIN,
                                                              MOSI_PIN,
                                                              MISO_PIN,
                                                              CS_PIN);

	spim_config.ss_pin = CS_PIN;
	spim_config.use_hw_ss = true;		// hardware SS (SPIM4 only)

    // Enable IRQ if necessary
	// IRQ_CONNECT(DT_IRQN(DT_NODELABEL(spi4)),
	// 			DT_IRQ(DT_NODELABEL(spi4), priority),
	// 			nrfx_isr, nrfx_spim4_irq_handler, 0);

    err = nrfx_spim_init(&spim_inst, &spim_config, NULL, NULL);
    NRFX_ASSERT(err != NRFX_SUCCESS);
}    

3) Configure DPPI:

uint8_t ppi_channel1;

void ppi_init(void)
{
	nrfx_err_t err;

	/* Allocate a (D)PPI channel. */
	err = nrfx_gppi_channel_alloc(&ppi_channel1);
	if (err != NRFX_SUCCESS) {
		LOG_ERR("nrfx_gppi_channel_alloc error: 0x%08X", err);
		return;
	}
	
	/* Configure endpoints of the channel so that the TIMER1, CCCH0 event is
	 * connected with the SPIM task.
	 */
	nrfx_gppi_channel_endpoints_setup(ppi_channel1,
			nrfx_timer_compare_event_address_get(&timer1_inst, NRF_TIMER_CC_CHANNEL0),
			nrfx_spim_start_task_address_get(&spim_inst));
	
	// Enable the channel
	nrfx_gppi_channels_enable(BIT(ppi_channel1));
}

4) Configure xfer and start timer:

    uint8_t tx_data[TX_DATA_SIZE]; // FILL with data to send
    uint8_t rx_data[RX_DATA_SIZE];
    
    // SPIM Descriptor
    nrfx_spim_xfer_desc_t spim_xfer_desc = NRFX_SPIM_XFER_TRX(tx_data, sizeof(tx_data), rx_data, sizeof(rx_data));

	// Configure SPIM transfer, hold for an event, increment rx pointer after transfer
	// Note: Should reset the pointer at some point
	err = nrfx_spim_xfer(&spim_inst, &spim_xfer_desc, NRFX_SPIM_FLAG_HOLD_XFER|NRFX_SPIM_FLAG_REPEATED_XFER|NRFX_SPIM_FLAG_RX_POSTINC|NRFX_SPIM_FLAG_NO_XFER_EVT_HANDLER);
    NRFX_ASSERT(err != NRFX_SUCCESS);

	
	nrfx_timer_clear(&timer1_inst);
    nrfx_timer_enable(&timer1_inst);

Enjoy!

[BENBF]

Hello, thank you for your answers.

In the end it appear that I can't use PPI for my application as I need to have fine control over SPI transactions...

I tried to perform my SPI transactions in a dedicated thread that is triggered by a 50 Hz timer.
Unfortunately, my SPI thread is not triggered every time that timer callback is called. Do you have nay idea of what be the cause of this issue ?

I've tried to trigger the thread with either semaphore or thread management (with k_thread_resume()) but the observed behaviour is the same

This is the test code I'm using:

void t_test(void){
    while(1){
        //k_sem_take(&_sem, K_FOREVER);
        printk("Second Thread\n");
        k_thread_suspend(t_thread);
    }
}



void main(void)
{
    //k_sem_init(&_sem, 0, 1);

    t_thread = k_thread_create(&t_k_thread, _stack, STACKSIZE, t_test, NULL, NULL, NULL, -2, 0, K_NO_WAIT);
    k_msleep(1000);
    printk("Starting timer\n");
    meas_timer_init();

    while (1) {
        printk("Main thread: still alive\n");
        k_msleep(1000);
    }
}

And this is the timer part :

ISR_DIRECT_DECLARE(meas_timer_handler)
{

    if (NRF_TIMER1->EVENTS_COMPARE[0]) {
        NRF_TIMER1->EVENTS_COMPARE[0] = 0;
        printk("Timer\n");
        //k_sem_give(&_sem);
        k_thread_resume(t_thread);
       
    }
    return 0;
}

void meas_timer_init(void)
{
    IRQ_DIRECT_CONNECT(TIMER1_IRQn, 4, meas_timer_handler, 0);
    NRF_TIMER1->BITMODE = 32;
    NRF_TIMER1->PRESCALER = 8;
    NRF_TIMER1->MODE = TIMER_MODE_MODE_Timer;
    NRF_TIMER1->CC[0] = 1250;
    NRF_TIMER1->SHORTS = 0;
    NRF_TIMER1->SHORTS = TIMER_SHORTS_COMPARE0_CLEAR_Enabled << TIMER_SHORTS_COMPARE0_CLEAR_Pos;
    NRF_TIMER1->INTENSET = TIMER_INTENSET_COMPARE0_Enabled << TIMER_INTENSET_COMPARE0_Pos;
    NRF_TIMER1->TASKS_START = 1;
    irq_enable(TIMER1_IRQn);
}

Thank you,
Best regards

[ovrebekk]

Hi

We are severely understaffed this week because of the Christmas holidays, and Kenneth will have to get back to you on this next week. Sorry for the inconvenience.

Best regards
Torbjørn