Can a peripheral read a CCCD value?

[pbirkner]

Hi,

I am wondering if it is possible for a peripheral device to interpret and react to a specific value written to it's CCCD by a central device. In all of the examples I looked at, it appears that this characteristic can simply be NULL or != NULL. From the bas example:

            // CCCD written, call application event handler
        if (p_bas->evt_handler != NULL)
        {
            ble_bas_evt_t evt;

            if (ble_srv_is_notification_enabled(p_evt_write->data))
            {
                evt.evt_type = BLE_BAS_EVT_NOTIFICATION_ENABLED;
            }

            else
            {
                evt.evt_type = BLE_BAS_EVT_NOTIFICATION_DISABLED;
            }

            p_bas->evt_handler(p_bas, &evt);
        }

is it possible to create code where a central device would write 01:00 to the peripheral's CCCD to flash the peripheral's LED1 and write a value of 02:00 to flash the peripheral's LED2?

[Vyacheslav Lebets]

Hi. First: CCCD is an abbreviation for Client Characteristic Configuration Descriptor. By writing the data to CCCD, the server allows the peripheral to make notifications or indications.

Second: You can read this value for example like this:

static void on_write(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 == YOUR_CHARACTERISTIC_HANDLE.cccd_handle)
    {
            /*
             then in "p_evt_write->data" we have cccd value
             and in "p_evt_write->len" we have value len, usually is 2 byte len.
            */
    }
}

And, of course, you may compare p_evt_write->data with 01:00 or 02:00.

Best, VL.

[pbirkner]

Thanks! Still a bit confused tho. How can I actually read an integer value from p_evt_write->data ?

[Vyacheslav Lebets]

You may use "uint16_decode()" function in app_util.h like this:

uint16_t aaa = uint16_decode(p_evt_write->data);

or:

uint16_t aaa = (uint16_t )*p_evt_write->data;

[Turbo J]

The latter method with the pointer cast could crash on NRF51x due to mis-alignment.