Configuring nRF52840 for counting input frequency of upto 5MHz.

[Aliasgar Surti]

Hello,

I want to configure my nRF52840 development kit for counting input pulses on one of the GPIOs (Pin 13) in this case. I am using SEGGER Embedded Studio as IDE.

Here I am using PPI, GPIOTE and TIMER for this configuration. I have configured GPIOTE as an input event. As I want to count the rising edge on that particular GPIO, I have configured that GPIO to sense Low to High signal. According to this event, gpiote event handler is called and the static counter is incremented. 

As I searched in nordic forum, PPI is capable for operating in 8MHz. But when I apply 2MHz clock as input to the above mentioned GPIO, I don't get exact count in the watch window. As per the count, nRF kit is only able to count pulses at about 120kHz frequency.

If I apply clock below 120kHz it is counting perfectly. And when I increase the frequency, for example 1MHz or something, I get maximum count of 120kHz.

Is there anything missing here? Is my configuration correct? Is there a specific GPIO which has to be used for this purpose?

I want to capture the input frequency of about 5MHz. Is it possible with the nRF52840 dev kit?

Below is the configuration for PPI channel as well as timer0 settings.

static void gpiote_event_handler(nrfx_gpiote_pin_t pin, nrf_gpiote_polarity_t action)
{
++m_gpiote_counter;
}

static void ppi_init(void)
{
uint32_t err_code = NRF_SUCCESS;
nrf_drv_gpiote_in_config_t config = NRFX_GPIOTE_RAW_CONFIG_IN_SENSE_LOTOHI(true);
config.pull = NRF_GPIO_PIN_NOPULL;
uint32_t gpiote_event_addr;
uint32_t PIN_NUMBER = 13;

err_code = nrf_drv_ppi_init();
APP_ERROR_CHECK(err_code);

err_code = nrf_drv_gpiote_init();
APP_ERROR_CHECK(err_code);
nrfx_gpiote_in_uninit(PIN_NUMBER);

err_code = nrf_drv_ppi_channel_alloc(&m_ppi_channel1);
APP_ERROR_CHECK(err_code);

err_code = nrf_drv_gpiote_in_init(PIN_NUMBER, &config, gpiote_event_handler);
APP_ERROR_CHECK(err_code);

nrf_drv_gpiote_in_event_enable(PIN_NUMBER, true);

gpiote_event_addr = nrf_drv_gpiote_in_event_addr_get(PIN_NUMBER);

err_code = nrf_drv_ppi_channel_assign(m_ppi_channel1,
gpiote_event_addr,
nrf_drv_timer_task_address_get(&m_timer0,
NRF_TIMER_TASK_COUNT));
APP_ERROR_CHECK(err_code);

// Enable configured PPI channels
err_code = nrf_drv_ppi_channel_enable(m_ppi_channel1);
APP_ERROR_CHECK(err_code);

}

static void timer0_init(void)
{
// Check TIMER0 configuration for details.
nrf_drv_timer_config_t timer_cfg = NRF_DRV_TIMER_DEFAULT_CONFIG;
timer_cfg.frequency = NRF_TIMER_FREQ_16MHz;
timer_cfg.mode = NRF_TIMER_MODE_COUNTER;
ret_code_t err_code = nrf_drv_timer_init(&m_timer0, &timer_cfg, timer0_event_handler);
APP_ERROR_CHECK(err_code);

nrf_drv_timer_extended_compare(&m_timer0,
NRF_TIMER_CC_CHANNEL0,
nrf_drv_timer_us_to_ticks(&m_timer0,
1),
NRF_TIMER_SHORT_COMPARE0_CLEAR_MASK,
true);
}

Thanks.