RE: nRF52832 SPI master read on external interrupt

Michael — Thu, 21 Jul 2016 13:50:31 GMT

Thanks! I believe this is enough for me to get started.

RE: nRF52832 SPI master read on external interrupt

Øyvind Karlsen — Wed, 20 Jul 2016 12:06:22 GMT

Hi,

Real time events that require high priorities in the CPU is a hurdle, since the SoftDevice uses the highest priorities. However, your issue can be done almost completely without using CPU.

You can set this up using PPI, which is a hardware short between peripherals, to trigger on the input event from the ADC on a pin configured with GPIOTE. Using PPI you can trigger a SPI with EasyDMA task which will transfer directly to RAM. Then you can set a low priority interrupt flag and automatically deal with the data (if needed) when you have time.

A very simple PPI example is provided below. The example gives a 8 MHz output on pin 18 using the capture compare event from the TIMER to trigger toggling of a GPIOTE output via PPI.

int main(void)
{
    // Set up GPIO as output
    nrf_gpio_range_cfg_output(17, 18);
    nrf_gpio_pin_clear(17); // Light LED 1 to indicate that the code is running

    // Start high frequency clock
    NRF_CLOCK->TASKS_HFCLKSTART = 1;
    while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0)
    {
        // Wait for clock to start
    }
    NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;

    // Configure GPIOTE to toggle pin 18 
    NRF_GPIOTE->CONFIG[0] = GPIOTE_CONFIG_MODE_Task << GPIOTE_CONFIG_MODE_Pos |
                            GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos |
                            18 << GPIOTE_CONFIG_PSEL_Pos | 
                            GPIOTE_CONFIG_OUTINIT_Low << GPIOTE_CONFIG_OUTINIT_Pos;

    // Set up timer
    NRF_TIMER1->PRESCALER = 0;
    NRF_TIMER1->CC[0] = 2; // Adjust the output frequency by adjusting the CC.
    NRF_TIMER1->SHORTS = TIMER_SHORTS_COMPARE0_CLEAR_Enabled << TIMER_SHORTS_COMPARE0_CLEAR_Pos;
    NRF_TIMER1->TASKS_START = 1;

    // Set up PPI to connect the timer compare event with the GPIOTE toggle task
    NRF_PPI->CH[0].EEP = (uint32_t) &NRF_TIMER1->EVENTS_COMPARE[0];
    NRF_PPI->CH[0].TEP = (uint32_t) &NRF_GPIOTE->TASKS_OUT[0];

    NRF_PPI->CHENSET = PPI_CHENSET_CH0_Enabled << PPI_CHENSET_CH0_Pos;

    while (true)
    {

    }
}

Make sure that you read the module descriptions thoroughly, also consult the SoftDevice specification, especially the part on which hardware peripherals have restricted access.

Best regards,

Øyvind