Max interval between BLE data send

[Ian]

Hi,

I use NRF51822 + S130 + SDK12 platform, my device work in CENTRAL mode, I want send data to peripheral every 10ms, code like this:

while (1)
{
   while (m_free_tx_packet_count == 0)
   {
         //wait free send packet
         //m_free_tx_packet_count will increase when BLE_EVT_TX_COMPLETE event received 
      
         if (timeout)
         {
                break;
         }
   }       

   if (m_free_tx_packet_count != 0)   
    {
        if（NRF_SUCCESS == ble_nus_c_string_send(...))  //send some datas
        {
            m_free_tx_packet_count--;
        }
    }
    
    nrf_delay_ms(10);
}

My system is a realtime system, the peripheral need data received at least once every 300 milliseconds, so I measure the max interval of two ble_nus_c_string_send function call, then I found sometimes i must wait free send packet more than 400ms, this will cause my system not work well.

So,is there any way to reduce the time between two BLE data send oprator? Or how can i build a real-time system use BLE?

[Ian]

The parameters is used when my device work as GAP Central, although when it work as GAP Peripheral use the same parameters. iOS 11 has a BLE bug,and Apple company suggest use min interval 12ms and max 24ms to resolve it.

I measure the time each part of code cost,and found the code wait m_free_tx_packet_count increase cost almost all the "delays" time. That means since ble_nus_c_string_send function be called and return NRF_SUCCESS to receive BLE_EVT_TX_COMPLETE event cost over 400ms~ So i think that's 'SoftDevce' cost the time and there's nothing i can do to reduce the time.

[endnode]

Sorry but this doesn't make sense. We all use Nordic Soft Device with similar connection parameters and we are achieving bandwidths of >kbps and latency <50ms even with iPhones. It more indicates bug in your code (or rather misunderstanding of how SD API works?)

[Ian]

My device doesn't connect to iPhone in this case, it work in GAP Central role and the peer is also nrf51822 which work in GPA Peripheral role. I guess was it because I used two interrupts in my code. I use ADC and app_timer, in ADC ISR,it restart the ADC conversion so i can get the ADC value continuously. In the app_timer ISR, I parse data received from UART and BLE then put the it to a FIFO.

[Ian]

But that still doesn't make sense. I refer to the examples in the SDK,and I didn't find what's wrong in my code

[Ian]

I just call ble_nus_c_string_send function, and wait BLE_EVT_TX_COMPLETE event, i can't imagine what bug it would have...

In case the SVC interrupt to be preempted and cause miss BLE_EVT_TX_COMPLETE event, i call ble_nus_c_string_send even wait free tx packet timeout.