Required buffer data on cccd_handle write for INDICATION

[Mark Paul]

Hello,

I'm working with a nrf51822, SoftDevice S120 v2.0 and SDK v9.0 platform.

Specifically using it a Central role to connect to a Blood Pressure peripheral.

I connect to the peripheral, find the Service/Characteristic I need and register for an "Indication" like so:

uint32_t                 err_code;
ble_gattc_write_params_t write_params;
uint8_t                  buf[BLE_CCCD_VALUE_LEN];

buf[0] = BLE_GATT_HVX_INDICATION;
buf[1] = 0;

write_params.write_op = BLE_GATT_OP_WRITE_REQ;
write_params.handle   = p_client->srv_db.services[0].charateristics[p_client->my_char_index].cccd_handle;
write_params.offset   = 0;
write_params.len      = sizeof(buf);
write_params.p_value  = buf;

err_code = sd_ble_gattc_write(p_client->srv_db.conn_handle, &write_params);
APP_ERROR_CHECK(err_code);

Unfortunately this does not work for me.

i.e. I get a BLE_GATTC_EVT_WRITE_RSP back from the peripheral but BLE_GATTC_EVT_HVX never gets throw to indicate data has been received.

In this:

buf[0] = BLE_GATT_HVX_INDICATION;
buf[1] = 0

Can someone please tell me what buf[1] = 0 is supposed to be? I copied this from a NOTIFICATION example and I cant seem to find anywhere in the documentation what this buffer is supposed to contain. I suspect this may have something to do with the problem.

Thanks very much, Mark

[Petter Myhre]

The CCCD is a 2-byte value. So what you have done seems to be correct. Are you sure you are writing to the correct handle? Are you sure sd_ble_gatts_hvx() is successfully called in the peripheral? Have you tried to read the CCCD after you have written it? To confirm it is 0x0002.

[Mark Paul]

Yes @Petter, this is the answer to the question. After checking the Peripheral docs and the Bluetooth docs CCCD value is 2 bytes and the format is correct