Sending SAADC data via ble_nus_data_send

Question

Hello,

I am trying to send data obtained from a SAADC port via BLE using the ble_nus_data_send function. This must happen periodically at frequencies within the Gigahertz range, ideally.

I started with the ble_app_uart example, from SDK16. Then, I initialized a timer using PPI to trigger an SAADC sampling event, such as in the saadc example.

Inside the saadc_callback function, I process the data and call the ble_nus_data_send function, as seen bellow:

void saadc_callback(nrf_drv_saadc_evt_t const * p_event)
{
    if (p_event->type == NRF_DRV_SAADC_EVT_DONE)
    {
        ret_code_t err_code;

        err_code = nrf_drv_saadc_buffer_convert(p_event->data.done.p_buffer, SAMPLES_IN_BUFFER);
        APP_ERROR_CHECK(err_code);

        saadc2ble_convert();   
        uint16_t length = (uint16_t)UART_TX_BUF_SIZE;
        err_code = ble_nus_data_send(&m_nus, adc_output, &length, m_conn_handle);                  
        APP_ERROR_CHECK(err_code);         
    }
}

the saadc2ble_convert function simply converts the data from uint16_t to uint8_t and the length of the data transmitted via ble_nus_data_send function has not been modified from the initial example:

#define UART_TX_BUF_SIZE                256                                         /**< UART TX buffer size. */
#define SAMPLES_IN_BUFFER  UART_TX_BUF_SIZE/2
static nrf_saadc_value_t adc_buf[SAMPLES_IN_BUFFER];                   //!< Buffer used for storing ADC value.
static uint8_t          adc_output[UART_TX_BUF_SIZE]; //!< Current battery level.


void saadc2ble_convert(){

   int i;

   for(i=0; i<SAMPLES_IN_BUFFER; i++){
       adc_output[i*2] = adc_buf[i] & 0xff;
       adc_output[i*2 + 1] = (adc_buf[i] >> 8) & 0xff;
   }

}

I initialize all in the main function as seen in the next code block:

int main(void)
{
    bool erase_bonds;
    ret_code_t err_code;

    // Initialize.
    uart_init();
    log_init();
    timers_init();    
    buttons_leds_init(&erase_bonds);
    power_management_init();
    ble_stack_init();
    gap_params_init();
    gatt_init();
    services_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.");



    advertising_start();

    saadc_sampling_event_enable();



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

Debugging, I see that the SAADC is reading data well, however, I couldn't get any data at the smartphone.

I am getting the error shown on the image:

Could anyone help me to find a way to solve this problem?

Karl Ylvisaker · Accepted Answer

Thiago said: I configure a timer using the function nrf_drv_timer_us_to_ticks with 1 as the second input, so I assume that the timer will repeat every 1 microsecond. I did just like the saadc example , except that I replaced the function nrf_drv_timer_ms_to_ticks in the example with nrf_drv_timer_us_to_ticks. 
 Do you intend to sample the SAADC every us ? The minimum Acquisition time for the SAADC is 3 us , and the maximal sampling frequency is therefore 200 kHz. Have you configured your SAADC channel to use 3 us acquisition time? 
 Thiago said: I believe this is how the connection interval is configured: 
 This is the connection interval range that the peripheral will accept. It is the central that determines what connection interval will actually be used. But, lets say for the calculations sake that the central determines the shortest connection interval supported by the peripheral will be used - 20 ms. 
 Thiago said: I am not sure, but I think I queue a notification every time I call the function ble_nus_data_send , if that is right, then it happens every time the timer triggers the saadc_callback functions, and as explained above, it seems to be configured to happen at every 1us. 
 This depends on the size of the provided buffer. The DONE event will be generated every time the buffer is filled - how often this is depends on the size of the buffer. If you are using the SAADC example, the default buffer size is 5. Lets say for the sake of the calculations that you were producing 1 sample every 1 us. This would mean that you are trying to queue a notification ever 5th us, but you are only actually sending notifications every 20 ms. So, with these parameters, you will have attempted to queue 4000 notifications between every connection event. I would absolutely expect this to cause an NRF_ERROR_RESOURCES. So, this will not work for multiple reasons. Let us look at how we may resolve this situation. What is your applications requirements - how often do you need to sample the SAADC and how fast do you need to get these samples to your central device? Is power consumption a part of the equation? 
 Thiago said: But I don't know how can I see the HVN TX queue size. In what file would it be defined? 
 This is set during the initialization of your BLE Nordic UART service. Looking forward to resolving this issue together! Best regards, Karl

Sending SAADC data via ble_nus_data_send

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