Custom UART BLE Read fails

[Lodge]

Hello, I have a custom program based on the uart example. I can write to the device, but sending to the master fails. I call the SendString code, but p_nus->is_notification_enabled is always false. Looking through the code, the only place this flag gets set true is in:

static void on_write(ble_nus_t * p_nus, ble_evt_t * p_ble_evt)
{
    ble_gatts_evt_write_t * p_evt_write = &p_ble_evt->evt.gatts_evt.params.write;
    if ((p_evt_write->handle == p_nus->rx_handles.cccd_handle)  && (p_evt_write->len == 2))
    {
        if (ble_srv_is_notification_enabled(p_evt_write->data))
        {
            p_nus->is_notification_enabled = true;
        }
        else
        {
            p_nus->is_notification_enabled = false;
        }
    }
    else if ((p_evt_write->handle == p_nus->tx_handles.value_handle) && (p_nus->data_handler != NULL))
    {
        p_nus->data_handler(p_nus, p_evt_write->data, p_evt_write->len);
    }
    else
    {
        // Do Nothing. This event is not relevant for this service.
    }

Can anyone explain where the message with p_evt_write->handle == p_nus->rx_handles.cccd_handle and with a length of 2 comes from? If this part of the code is never called, then i cant send any data to the master. Much appreciated.

[Alex]

That part of the code called when you enable notification from master. For example in Master Control Panel in Android connect to your device, choose service and then on right hand side of characteristic you will see three down arrows. Press on them and notification will be enabled. If you are using your own application then you need to enable notification in your application before the first SendString used in the device

[Lodge]

Hi Alex, Thanks for the reply. Does this mean that I have to enable notifications from the master side before I can send data to it? What I am doing at the moment is sending data to the device from a custom windows 8 application with a read command. I then want to send about 5 packets of data back to the master. This is failing when I call the SendString command for the above reason. Is there a way to do this without enabling notifications? Thanks very much, Lodge.

[Alex]

Why don't you enable notification in Windows 8 application? This is how data send from device to master, either by notification or indication. Give me some time, I'll find the command which enables notification in our W8 code

[Alex]

In W8 you can enable notification by:

gatt_characteristic.WriteClientCharacteristicConfigurationDescriptorAsync(
                                            CHARACTERISTIC_NOTIFICATION_TYPE);

where

gatt_characteristic = service.GetCharacteristics(UUIDofcharacteristic)[0];

and service = await GattDeviceService.FromIdAsync(deviceId);

[Lodge]

Hi Alex, Thanks again. I will try that. Out of interest, you mention you can send data to master either by notification or indication. Could you explain how indication works? Much appreciated.

[Alex]

Short and nice explanation is here