Unable to receive UART-BLE from peripheral

Question

Hi there! I'm using the nRF Toolbox's UART to test the receiving of my data stream. I'm trying to send a data array collected (from spi) in my main loop and send it via UART-BLE to my mobile phone. However, on the Toolbox app, I'm not receiving any string from my peripheral even though I called ble_nus_string_send. 
 //Global variable

bool ble_connected = false;


----------


static void on_ble_evt(ble_evt_t * p_ble_evt)
{
	
	
	switch (p_ble_evt->header.evt_id)
	{
 case BLE_GAP_EVT_CONNECTED:
 ble_connected = true;
 m_conn_handle = p_ble_evt->evt.gap_evt.conn_handle;
 break;
 
 case BLE_GAP_EVT_DISCONNECTED:
 ble_connected = false;
 m_conn_handle = BLE_CONN_HANDLE_INVALID;
 break;
 }
}


----------


int main (void) 
{
 
/* SPI data comes in and gets stored into array data_array */
 
 data_array[0] = something;
 data_array[1] = something;
 data_array[2] = something;

 if(ble_connected)
 
 ble_nus_string_send(&m_nus, data_array, length_of_data_array)
}

FormerMember · Accepted Answer

To be able to transmit data from your device (server) to the phone (client), the phone will first have to enable notifications on the server side. A server is not allowed to transmit notifications unless the client has enabled it. Enabling of notifications happens after a connection has been established. 
 To enable notifications, the client has to write to the CCCD of the characteristic in question. Part 1 of this video tutorial introduces CCCDs at approximately 17:00, and continues to talk about data transfer/notifications from 19:30 to 22:40. In part 2 of the tutorial, the code walk through goes through how notifications are enabled at the server time, that part starts at approximately 14:50.