High-frequency GPIO

[Thiago]

Hello, all.

I am working on a project in which the ultimate goal is to read an 8-bit signal with 10kHz frequency.

To test if this would be possible, I tried to generate a digital signal using a timer. The timer is configured as below:

void digital_signal_event_init(void)
{
    ret_code_t err_code;

    nrf_drv_timer_config_t timer_cfg = NRF_DRV_TIMER_DEFAULT_CONFIG;
    timer_cfg.bit_width = NRF_TIMER_BIT_WIDTH_32;
    err_code = nrf_drv_timer_init(&TIMER_DIGITAL, &timer_cfg, digital_signal_handler);
    APP_ERROR_CHECK(err_code);

    // setup m_timer for compare event every 400ms 
    uint32_t ticks = nrf_drv_timer_us_to_ticks(&TIMER_DIGITAL, 1);
    nrf_drv_timer_extended_compare(&TIMER_DIGITAL,
                                   NRF_TIMER_CC_CHANNEL0,
                                   ticks,
                                   NRF_TIMER_SHORT_COMPARE0_CLEAR_MASK,
                                   true);
    nrf_drv_timer_enable(&TIMER_DIGITAL);
    
}

And the handle is as shown next:

void digital_signal_handler(nrf_timer_event_t event_type, void * p_context)
{
    switch (event_type)
        {
            case NRF_TIMER_EVENT_COMPARE0:                 
                if(currentSample < 10){
                  GPIO0_OUT_R = ((SineWave[currentSample])<<14);
                  sprintf(GPIO_Out_current_Value, "GPIO0_OUT register current value: %d\n", (GPIO0_OUT_R>>14)&0xFF);
                  SEGGER_RTT_WriteString(0, GPIO_Out_current_Value);
                  currentSample++;
                }
                else{
                  currentSample = 0;
                }               
                break;

            default:
                //Do nothing.
                break;
        }

}

The signal is in the array SineWave:

const uint16_t SineWave[10] = {12, 14, 14, 12, 8, 3, 1, 1, 3, 7};

The GPIO is initialized as follows:

#define IN1          NRF_GPIO_PIN_MAP(0,14)
#define IN2          NRF_GPIO_PIN_MAP(0,15)
#define IN3          NRF_GPIO_PIN_MAP(0,16)
#define IN4          NRF_GPIO_PIN_MAP(0,17)
#define IN5          NRF_GPIO_PIN_MAP(0,18)
#define IN6          NRF_GPIO_PIN_MAP(0,19)
#define IN7          NRF_GPIO_PIN_MAP(0,20)
#define IN8          NRF_GPIO_PIN_MAP(0,21)

#define IN_NUMBER                        8

static const uint8_t input_list[8] = {IN1, IN2, IN3, IN4, IN5, IN6, IN7, IN8};


for (i = 0; i < IN_NUMBER; ++i)
    {
        nrf_gpio_cfg_output(input_list[i]);  //Configure as Output
    }

But when I measure the generated signal using a DAC and an oscilloscope the frequency of the signal is as seen in the next image:

The achieved period is of 569us, which gives a 1.757kHz frequency.

I saw in other tickets people saying that GPIO frequency can go up to 16Mhz. Then, why can't I generate a high-frequency signal?

[Thiago]

Hello,

After following the suggestions above:

Dmitry said:

I think sprintf() and SEGGER_RTT_WriteString() eat most of these 57 microseconds, try to comment them out. To achieve maximum frequency, use HAL module functions (nrf_timer.h) instead of nrf_drv_timer

I got an improvement in the frequency of almost 18 times as seen in this image:

+

I used this code to initialize the clock and for the Interrupt service routine:

#define TIMER_INSTANCE NRF_TIMER2
 /*---------------------------------------------------------------------------*/
 void
 timer2_init(void)
 {
    TIMER_INSTANCE->PRESCALER = 0;

    TIMER_INSTANCE->TASKS_STOP = 1;	/* Stop timer */
    TIMER_INSTANCE->MODE = TIMER_MODE_MODE_Timer;  /* taken from Nordic dev zone */
    TIMER_INSTANCE->BITMODE = TIMER_BITMODE_BITMODE_16Bit;
    TIMER_INSTANCE->CC[0] = 1;
    TIMER_INSTANCE->TASKS_CLEAR = 1; /* Clear timer */
    TIMER_INSTANCE->INTENSET = TIMER_INTENSET_COMPARE0_Enabled << TIMER_INTENSET_COMPARE0_Pos;
    TIMER_INSTANCE->SHORTS = (TIMER_SHORTS_COMPARE0_CLEAR_Enabled << TIMER_SHORTS_COMPARE0_CLEAR_Pos);
    
    NVIC_ClearPendingIRQ(TIMER2_IRQn);
    NVIC_SetPriority(TIMER2_IRQn, 6);
    NVIC_EnableIRQ(TIMER2_IRQn);

    

    TIMER_INSTANCE->TASKS_START = 1;
 }


 void
 TIMER2_IRQHandler(void)
 {
     if (TIMER_INSTANCE->EVENTS_COMPARE[0] != 0)
    {
    	TIMER_INSTANCE->EVENTS_COMPARE[0] = 0;

        if(currentSample < 10){
            GPIO0_OUT_R &= ~(0xFF<<18);   
            GPIO0_OUT_R |= ((SineWave[currentSample])<<18);   
            //sprintf(GPIO_Out_current_Value, "GPIO0_OUT register current value: %d\n", (GPIO0_OUT_R>>18)&0xFF);
            //SEGGER_RTT_WriteString(0, GPIO_Out_current_Value);
            currentSample++;
        }
        else{
            currentSample = 0;
        }

    	TIMER_INSTANCE->SHORTS = TIMER_SHORTS_COMPARE0_CLEAR_Msk;
    }
 }

The frequency improvement is great, but the module is not advertising anymore.

If I don't make a call for the timer2_init() function in the main(), it advertises, but not otherwise.

I wrote the code based on the ble_app_uart example. I changed nothing in the BLE configuration.

The main function is looking like this now:

/**@brief Application main function.
 */
int main(void)
 {
    bool erase_bonds;   

    //NRF_CLOCK->TASKS_HFCLKSTART = 1;

    // Initialize.
    uart_init();
    log_init();
    timers_init();    
    leds_init(&erase_bonds);
    gpio_init();
    power_management_init();
    ble_stack_init();
    conn_evt_ext_enable();
    data_len_set(DATA_LENGTH_MAX);
    gap_params_init();
    gatt_init();
    services_init();
    timer2_init();
    //digital_signal_event_init();

    //saadc_init();
    //saadc_sampling_event_init();
    

    advertising_init();
    conn_params_init();

    // Start execution.
    printf("\r\nUART started.\r\n");
    NRF_LOG_INFO("Debug logging for UART over RTT started.");
    SEGGER_RTT_WriteString(0, "Debug logging for UART over RTT started.\n");


    advertising_start();

    //saadc_sampling_event_enable();



    // Enter main loop.
    for (;;)
    {
        idle_state_handle();
              
    }
}

Would anyone have a clue why the advertising stops or what could I do to find out how to solve it?

[Dmitry]

Hi,

you have set TIMER_INSTANCE->CC[0] = 1, so CPU spends all the time in TIMER2_IRQHandler, not leaving any time for low-priority processes.

[Thiago]

It works fine now. Thank you very much for your help.