How to make GATT client aware of changes to a characteristic?

[john kotkin]

Hi,

I trying to write an application with android for my nRF51822 device I use Battery service example but every time i update battery level i need to request for changed value from my android program, how to make battery service example push updated value to the air?

[Victor Nascimento]

For this you have to use a Notification / Indication. Please have a look on the UART example. The TX characteristic has notification properties. On the App side, you have to subscribe to a particular characteristic and you will get an event callback whenever the BLE module writes to that characteristic.

[Victor Nascimento]

To send data with notification, you have to do: memset(&hvx_params, 0, sizeof(hvx_params)); hvx_params.handle = p_nus->rx_handles.value_handle; hvx_params.p_data = p_string; hvx_params.p_len = &length; hvx_params.type = BLE_GATT_HVX_NOTIFICATION; sd_ble_gatts_hvx(p_nus->conn_handle, &hvx_params);

Take a look at the service file on the UART example. The file is ble_nus.c. There you can find this function to send a string.