How to send notification for a large custom attribute

[Rbai]

I am using nRF52 S132 and latest SDK V11. I defined a custom service with a custom characteristic which is 509 bytes (based on your tutorial). Read operation is fine from a client. But when using sd_ble_gatts_hvx() to send notification, only the first 20 bytes are received. The return code of this function is 0x3401, which means BLE_ERROR_GATTS_SYS_ATTR_MISSING. But it does not make sense to me. So I reduce the size from 509 to 10. The notification is sent and received. But the error code is still 0x3401. How to send notification properly?

[Tony Garland]

Another approach which seems to work, is to queue up multiple notifications using a characteristic with short length. By updating the characteristic value between each call to sd_ble_gatts_hvx() it results in multiple notifications, each with the portion you want to send.

        uint16_t len = BLOCK_SIZE;
        ble_gatts_hvx_params_t hvx_params;
        hvx_params.handle = m_p_resp->handles.value_handle;
        hvx_params.type   = BLE_GATT_HVX_NOTIFICATION;
        hvx_params.offset = 0;
        hvx_params.p_data = NULL;
        hvx_params.p_len  = &len;
        // Notify full value as three individual piece notifications.
        for (int i = 0 ; i < (3 * BLOCK_SIZE) ; i += len)
        {
            static uint8_t fill = 0;
            memset(m_response, fill++, sizeof(m_response));

            uint32_t err_code = sd_ble_gatts_hvx(
                p_ble_event->evt.gatts_evt.conn_handle,
                &hvx_params
                );
            APP_ERROR_CHECK(err_code);
        }

Another way to do this would be to break the longer characteristic into multiple short characteristics and notify on each one.

[Christian Waldmann]

Hi Tony Very informativ your addition comments. I was very irritated by the completely different meaning of the parameter offset and p_data for notification (only for updating the internal value, before sending always the first bytes) and reading (ask for a special part of a long value). Thanks Chris