MTU exchange issue – ble_app_uart_c / ble_app_uart with MTU 247

[Mark]

I am using the nRF5 SDK, nRF52833, and nRF52832.

I flashed the following two demos:

• nRF5_SDK_17.1.0_ddde560\examples\ble_central\ble_app_uart_c\pca10100\s122 (nRF52833)

• nRF5_SDK_17.1.0_ddde560\examples\ble_peripheral\ble_app_uart\pca10040\s132 (nRF52832)

I changed BLE_GATT_ATT_MTU_DEFAULT to 247 in both projects.

However, the MTU exchange does not happen. In the following function:

void gatt_evt_handler(nrf_ble_gatt_t * p_gatt, nrf_ble_gatt_evt_t const * p_evt)
{
    if (p_evt->evt_id == NRF_BLE_GATT_EVT_ATT_MTU_UPDATED)
    {
        NRF_LOG_INFO("ATT MTU exchange completed.");
        m_ble_nus_max_data_len = p_evt->params.att_mtu_effective - OPCODE_LENGTH - HANDLE_LENGTH;
        NRF_LOG_INFO("Ble NUS max data length set to 0x%X(%d)", m_ble_nus_max_data_len, m_ble_nus_max_data_len);
    }
}

These two log messages do not appear.

In actual data transmission, I can only send 20 bytes. Sending 23 bytes causes an error.

Could you please help me understand why the MTU exchange is not taking place?

Thank you.

[Vidar Berg]

Hello,

I have not tried to reproduce this yet, but as a test, please try set NRF_BLE_GATT_MTU_EXCHANGE_INITIATION_ENABLED to "0" in your ble_app_uart build so that the MTU exchange is only initiated by the central. 

Best regards,

Vidar

[Mark]

Hi Vidar,

Thank you for your suggestion. Setting NRF_BLE_GATT_MTU_EXCHANGE_INITIATION_ENABLED to 0 on the peripheral side worked – the MTU exchange now happens successfully.

However, I have a follow-up question. Looking at the logs:

00> <debug> nrf_ble_gatt: max_rx_octets: 27
00> 
00> <debug> nrf_ble_gatt: max_tx_octets: 27

Why is it showing 27 bytes?

Here is the full log for reference:

00> <info> app_timer: RTC: initialized.
00> 
00> <info> app: Debug logging for UART over RTT started.
00> 
00> <info> app: Advertising_start.
00> 
00> <debug> nrf_ble_gatt: Updating data length to 251 on connection 0x0.
00> 
00> <info> app: Connected
00> 
00> <debug> nrf_ble_gatt: Peer on connection 0x0 requested an ATT MTU of 247 bytes.
00> 
00> <debug> nrf_ble_gatt: Updating ATT MTU to 247 bytes (desired: 247) on connection 0x0.
00> 
00> <info> app: Data len is set to 0xF4(244)
00> 
00> <debug> app: ATT MTU exchange completed. central 0xF7 peripheral 0xF7
00> 
00> <debug> nrf_ble_gatt: Data length updated to 27 on connection 0x0.
00> 
00> <debug> nrf_ble_gatt: max_rx_octets: 27
00> 
00> <debug> nrf_ble_gatt: max_tx_octets: 27
00> 
00> <debug> nrf_ble_gatt: max_rx_time: 328
00> 
00> <debug> nrf_ble_gatt: max_tx_time: 328
00> 
00> <debug> app: ATT MTU exchange completed. central 0xF7 peripheral 0xF7
00> 

The ATT MTU exchange shows 247 bytes (0xF7), but the data length update shows only 27 octets. I expected the data length to be updated to a larger value as well.

Could you please explain why the data length remains at 27 bytes in this case?

Thank you for your help.

[Vidar Berg]

Hi,

Please make sure that NRF_SDH_BLE_GAP_DATA_LENGTH is set to "251" in the sdk_config.h header in both projects. The radio packet length should be updated from 27 bytes to 251 with this setting.

Best regards,

Vidar

[Mark]

Hi Vidar,

Thank you for the suggestion.

I have checked both projects (central and peripheral), and NRF_SDH_BLE_GAP_DATA_LENGTH is already set to 251 in both sdk_config.h files:

// <i> Requested BLE GAP data length to be negotiated.
#ifndef NRF_SDH_BLE_GAP_DATA_LENGTH
#define NRF_SDH_BLE_GAP_DATA_LENGTH 251
#endif

However, the log still shows max_rx_octets: 27 and max_tx_octets: 27 after the data length update.

Could there be another configuration or setting that is limiting the data length to 27 bytes?

Thank you for your continued support.