Ble Data Transfer Integrity

[Nuri]

I am trying to send data from a phone to the NRF51822. The data is being sent from the phone in 20 byte chunks to a write characteristic. When the event handler for receiving data is called, I use sd_ble_gatts_value_get() to get the data and copy it to a buffer. The problem I see is that part of the previous packet is returned instead of the current one, but it does have the right length. For example, if I sent 5 20-byte packets numbered 0 - 99, the first 60 would have the right number, but byte 61 might have the number 56 instead of 60. Are there any methods to improve data integrity over BLE?

Thanks!

[Kenneth]

All data transferred over BLE is reliable, so I suspect you have some issues other place, for instance in the application handling in the central or peripheral. To check where the problem may be you might need to create a sniffer log and send to me when the problem occurs.

https://www.nordicsemi.com/Software-and-Tools/Development-Tools/nRF-Sniffer 

[Nuri]

I have run the sniffer with my application, and I can tell that the correct data is being sent to the NRF51822. Here is the sniffer output if you are curious. The following code is called when BLE_GATTS_EVT_WRITE event occurs. 

#define MAX_PACKET_SIZE 20
static uint8_t data_buffer[MAX_PACKET_SIZE];
void onCharWrite(ble_evt_t * p_ble_evt)
{
    // Populate ble_gatts_value_t structure to hold received data and metadata.
    ble_gatts_value_t rx_data;
    rx_data.len = MAX_PACKET_SIZE;
    rx_data.offset = 0;
    rx_data.p_value = data_buffer;
    ble_gatts_evt_write_t * p_evt_write = &p_ble_evt->evt.gatts_evt.params.write;
    uint32_t error = NRF_SUCCESS;
    const uint16_t WriteHandle = p_evt_write->handle;

    if (WriteHandle == CharWriteHandles.value_handle)
    {
        // Get data
        memset(data_buffer, 0, MAX_PACKET_SIZE);
        error = sd_ble_gatts_value_get(zwConnection, CharWriteHandles.value_handle, &rx_data);
        APP_ERROR_CHECK(error);
        
        print_hex_buffer(&data_buffer[rx_data.offset], rx_data.len);
        
        //Copying to buffers and other processing
    }
}

Is there something wrong with the way this is being handled?

[Kenneth]

It may be that rx_data is only valid in the onCharWrite() context, so I assume then that print_hex_buffer() is blocking here to make sure the entire buffer is output before you return from onCharWrite()?

Also, I believe you don't need to use sd_ble_gatts_value_get(), I assume you are in the BLE event you should be able to use:

p_ble_evt->evt.gatts_evt.params.write.len;
p_ble_evt->evt.gatts_evt.params.write.data;

Best regards,
Kenneth

[Nuri]

Using p_ble_evt->evt.gatts_evt.params.write.len and p_ble_evt->evt.gatts_evt.params.write.data instead of sd_ble_gatts_value_get() worked. Thanks for the help.