How to increase the packet size for sd_ble_gattc_write

[Haresh05]

We are using nrf52832 with S132 for our application.

Central device connect to Peripheral using sd_ble_gap_connect() api and then uses sd_ble_gattc_write() to write the data.

We are currently able to send only 20 bytes per packet but we want to send upto 128 bytes of data.

Can anyone help me to achieve this?

[Einar Thorsrud]

Hi,

You typically want to both enable larger GATT packets and larger GAP packets using data length extension. I suggest you take a look at the Nordic UART Service Client and UART/Serial Port Emulation over BLE from nRF5 SDK 17.1.0 for examples that do this on the central and peripheral respectively.

Note particularily the value of NRF_SDH_BLE_GATT_MAX_MTU_SIZE (247) and NRF_SDH_BLE_GAP_DATA_LENGTH (251) in sdk_config.h for both examples, and that in the central example, nrf_ble_gatt_att_mtu_central_set() is called.

[Haresh05]

Hi,

Sorry I forgot to mention that we are using nRF5_SDK_13.0.0

I have found the  Nordic UART Service Client for central device and will follow that.

Can you please let me know which example to follow for peripheral device?

[Einar Thorsrud]

Hi,

You can refer to the Nordic UART Service (NUS) examples in SDK 13.0.0 as well for how to set the GATT max MTU. For the GAP part, you can see that the peripheral example calls sd_ble_gap_data_length_update() for a BLE_GAP_EVT_DATA_LENGTH_UPDATE_REQUEST event. This is for responding do a DLE update procedure. On the other side you should call the same function to initiate a data length update procedure.

[Haresh05]

Hi

I have made the changes as per example but I am not getting the BLE_GAP_EVT_DATA_LENGTH_UPDATE_REQUEST in peripheral.

Do I need to call any specific API on EVT_CONNECTED from central device?

[Einar Thorsrud]

You need to initiate the data length update procedure on one of the sides (assuming the other handles the request). This is done with sd_ble_gap_data_length_update() (the same function as is used to respond). This can also be called from the SDK GATT module, if you take a look at the nrf_ble_gatt.c implementation in SDK 13. If you use that library, it will initiate a data length update procedure when handing the EVT_CONNECTED, as long as NRF_BLE_GATT_MAX_MTU_SIZE is set high. You can also take a look at the implementationin nrf_ble_gatt.c to see how you can do it manually if you do not use the SDK GATT module.

[Haresh05]

It is working now.

I had also changed the BLE_GATT_ATT_MTU_DEFAULT value in nrf5_sdk/softdevice/s132/headers/ble_gatt.h and due to that it was not initiating the data length update procedure.

Thank you for your support