What cause the empty PDU

[Tom.Fu]

Dear Support Team

I use the nRF5 SDK's ble_app_uart_c sample and ble_app_uart sample to test peripheral send data to central. On the application layer, I called ble_nus_data_send() founction to send one packet which 10bytes per 20ms and the BLE connection interval I set is 20ms. Below is my 20ms callback function implementation.

static void Test_20ms_founction(void * p_context) 
{
  ret_code_t err_code;
  if((SEND_RDY == 1) && (CONN_RDY == 1))
  {  
     call_counter_start = 1;
     do
        {
            uint8_t Test_array[] = {0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0xaa, 0xbb};
            uint16_t Test_lenght = sizeof(Test_array);
            err_code = ble_nus_data_send(&m_nus, Test_array, &Test_lenght, m_conn_handle);
            if ((err_code != NRF_ERROR_INVALID_STATE) &&
                (err_code != NRF_ERROR_RESOURCES) &&
                (err_code != NRF_ERROR_NOT_FOUND))
            {
                APP_ERROR_CHECK(err_code);
            }
        } while (err_code == NRF_ERROR_RESOURCES);
        SEND_RDY = 0;
        packet_counter++;
        NRF_LOG_INFO("packet_counter = %d, SEND_RDY = %d", packet_counter, CONN_RDY);
        if(packet_counter == 0xffff) packet_counter = 0;
  }
  if(call_counter_start == 1)
  {
    call_counter++;
    if(call_counter == 0xffff) call_counter = 0;
    NRF_LOG_INFO("Test_20ms_founction call time:%d",call_counter)
  }
}

But I used the Sniffer to catch the packet on the air, But there are empty PDUs in some connection event like picture below, I would like to know what cause this and how to void these empty packet. Because l would like to send one packet per 20ms on the air but in fact it send one packet per 40ms .

The enclosed file is my code. many thanks.

4466.ble_app_uart_c.7z

[Håkon Alseth]

Hi,

 

Unfortunately, it seems that the peripheral code was not uploaded properly. Could you please re-upload this?

 

Without having tested or looked deeper into the implementation, I would recommend that you use deferred logging, as logging in the handler will delay the function.

Either set sdk_config.h::NRF_LOG_DEFERRED to 1, or try to limit log printing in your callback handler to see if this has an impact on the behavior.

 

Kind regards,

Håkon

[Tom.Fu]

Hi Håkon 

Sorry for that, I upload the peripheral code again.

But I found that the NRF_LOG_DEFERRED has been set to 1 both in these two sample by default.

Many thanks.

1067.ble_app_uart.7z

[Håkon Alseth]

Hi,

 

Thanks for sharing. 

I think you want the do-while to be checked with this:

do {
...
} while (err_code != NRF_ERROR_RESOURCES);

 

You should also set sdk_config.h::NRF_SDH_BLE_GAP_DATA_LENGTH to 27 or higher. It was set to 23 by default.

To send more packets per connection interval, you can also consider adjusting sdk_config.h::NRF_SDH_BLE_GAP_EVENT_LENGTH to a higher number.

The 40 ms comes from the RTC prescaler. You should set the sdk_config.h::APP_TIMER_CONFIG_RTC_FREQUENCY to '0' to fix this.

 

Here's a small screenshot from the sniffer after making the adjustments above:

 

Kind regards,

Håkon

[Tom.Fu]

Hi Håkon 

Many thanks for your supporting.

But this part is the sample implementation by default. By the way, what is the correct way to call the ble_nus_data_send() in this case? Many thanks.

[Håkon Alseth]

It depends on if you want to send as much as possible or not.

It is up to you how you want it to behave.

 

Kind regards,

Håkon

[Tom.Fu]

Hi Håkon 

I set NRF_SDH_BLE_GAP_DATA_LENGTH to 27 and the APP_TIMER_CONFIG_RTC_FREQUENCY to 0　and I tried sending data in the ways separately like the screenshot below. But it still catch the empty PDU like before.

 

[Håkon Alseth]

Hi,

 

Sorry for not seeing this earlier, but the SEND_RDY flag is the reason why you're seeing every other payload. 

When your function is called, you are effectively queueing one payload to the SoftDevice, and for an ACK (ie. evt BLE_NUS_EVT_TX_RDY occurs, which sets SEND_RDY=1 again), that will happen when a notification has completed.

When receiving GATT event "BLE_GATTS_EVT_HVN_TX_COMPLETE", which is the same as BLE_NUS_EVT_TX_RDY, it means that the TX has successfully occurred. This comes with a count field, which shows how many packets has been successfully sent in this specific interval, as described by my colleague here:  RE: Missing BLE_GATTS_EVT_HVN_TX_COMPLETE events 

 

Kind regards,

Håkon

[Tom.Fu]

Hi Håkon

Many thanks for the detail explanation.

Is there any other way to avoid or reduce the empty PDU in some connection? In our application, we would like to send 10bytes a packet to the peer every 20ms accurately to control the motor, and the connection interval I set is 20ms, so it requires that a packet must be sent in every connection. Is there any way to achieve this? Many thanks.