Time gap between each team sending data via BLE nrf52840

[MinhDuc]

Good day support team, 

In my project, I save the packet of 60bytes then send it via BLE with ble_nus_data_send(). (20kHz sampling rate)

I set the timer = 50us.

uint16_t count;
void timer_led_event_handler(nrf_timer_event_t event_type, void* p_context)
{
        UNUSED_PARAMETER(event_type);
        count++;
}

I want to check the time gap between each time BLE send. Instead of saving data, I save the count timer value then send it via BLE. 

y-axis is count value with unit : 50us

x-axis is data number

I expect if ble_nus_data_send() process fast enough ( < 50us), I will see the linearity in the graph but there was delay between each packet.

The time gap I mentioned above happened random. 

I appreciate any suggestion and explanation.

Thank you very much.

Best,

 

[Susheel Nuguru]

Hi,

I do not see the code snippet where you actually send the BLE packet.

• Is it inside the timer handler?
• what is the priority of the timer interrupt?
• What is the processing time of the handler in which ble_nus_data_send is being called?
• 50uS can be very ambitious speed if you have something other than the timer handler running in the background (for example you have other contexts where you get and process sensor data)
• You need to add some GPIO toggles in the handler start and end where you call ble_nus_data_send to know the actual latencies of your handlers. You should do the same with another gpio for other contexts you have in the same/higher priority as the priority of the handler you can ble_nus_data_send.

If you can share all your application handler contexts and the interrupt priorities and the GPIO toggle timing for each of this context, then we will have enough information to see if you can transmit 60Bytes of data with 20KHz frequency of incoming sampled data. 

Also check your MTU size and see to it that it is bigger than 60 bytes so that every data is sent in one packet and not in multiple packets.

[MinhDuc]

Glad to hear you Nuguru.

Here is my main while loop to send BLE.

Actually I sent the packet of 240 data bytes and 3 bytes header. 60 bytes/channel

while(true)
{

if (BLE_send_state == true && ADC_SPI_state == true && index_buffer < sizeof(data_save_buffer))
{
            uint16_t ADC_4 = read_ADC_reg(false,false,true,false,CONVERT_6);
            uint16_t ADC_5 = read_ADC_reg(false,false,true,false,CONVERT_7);
            uint16_t ADC_6 = read_ADC_reg(false,false,true,false,CONVERT_8);
            uint16_t ADC_7 = read_ADC_reg(false,false,true,false,CONVERT_4);
            uint16_t ADC_8 = read_ADC_reg(false,false,true,false,CONVERT_5);
            data_save_buffer[index_buffer] = (ADC_4 >> 0) & 0xFF;
            data_save_buffer[index_buffer+1] = (ADC_4 >> 8) & 0xFF;
            data_save_buffer[index_buffer+2] = (ADC_5 >> 0) & 0xFF;
            data_save_buffer[index_buffer+3] = (ADC_5 >> 8) & 0xFF;
            data_save_buffer[index_buffer+4] = (ADC_6 >> 0) & 0xFF;
            data_save_buffer[index_buffer+5] = (ADC_6 >> 8) & 0xFF;
            data_save_buffer[index_buffer+6] = (ADC_7 >> 0) & 0xFF;
            data_save_buffer[index_buffer+7] = (ADC_7 >> 8) & 0xFF;
            data_save_buffer[index_buffer+8] = (ADC_8 >> 0) & 0xFF;
            data_save_buffer[index_buffer+9] = (ADC_8 >> 8) & 0xFF;
            index_buffer+=10;
            ADC_SPI_state = false;

}

if( index_buffer == sizeof(data_save_buffer))
{
            uint32_t err_code;
            uint16_t index_data_send = sizeof(data_send_buffer);
            do
            {
            err_code = ble_nus_data_send(&m_nus, data_save_buffer, &index_data_send, 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);
            index_buffer = 0;
}

}

Is it inside the timer handler?

Save data in buffer --> yes. send BLE --> No

what is the priority of the timer interrupt?

Default : 3

What is the processing time of the handler in which ble_nus_data_send is being called?

I think it is around 50us 

#define MIN_CONN_INTERVAL  7.5ms

#define MAX_CONN_INTERVAL  7.5ms

BLE_GATT_ATT_MTU_DEFAULT = 243

Best,