Read added custom characteristic value from peripheral

Question

Hi, 
 I am developing a central application that will connect to my custom made peripheral device and continuously read a characteristic available on the peripheral device . I have currently managed to find and connect to my peripheral device, and also request to read the device name characteristic without any problems. However, when I start reading the custom service that is added on the peripheral device, with the sd_ble_gattc_char_value_by_uuid_read function, it returns err_code 7, which I assume is invalid params. 
 On the peripheral I can see that there are two characteristics that I can read through the nrf connect application. 
 I had my code in the following way when requesting to read the device name characteristic: 
 static void repeated_timer_handler(void * p_context)
{
 ret_code_t err_code;
 
 ble_uuid_t p_uuid = {.uuid = BLE_UUID_GAP_CHARACTERISTIC_DEVICE_NAME, .type = BLE_UUID_TYPE_BLE};
 ble_gattc_handle_range_t p_handle_range = {.start_handle = 0x0001, .end_handle = 0xFFFF};

 err_code = sd_ble_gattc_char_value_by_uuid_read(connection_handle, &p_uuid, &p_handle_range);
 if(err_code != NRF_SUCCESS)
 {
 NRF_LOG_INFO("Failed to send read request to peer error = %d", err_code);
 }
} 
 And when I now want to read my service characteristic I instead now changed the uuid and type to following: 
 static void repeated_timer_handler(void * p_context)
{
 ret_code_t err_code;
 
 ble_uuid_t p_uuid = {.uuid = 0xF011, .type = BLE_UUID_TYPE_VENDOR_BEGIN};
 ble_gattc_handle_range_t p_handle_range = {.start_handle = 0x0001, .end_handle = 0xFFFF};
 err_code = sd_ble_gattc_char_value_by_uuid_read(connection_handle, &p_uuid, &p_handle_range);
 if(err_code != NRF_SUCCESS)
 {
 NRF_LOG_INFO("Failed to send read request to peer error = %d", err_code);
 }
} 
 Am I calling the wrong function or do I have to perform some other function call in order to be able of requesting to read the following service? 
 I assume that I don't have to use the base uuid to call the specific service, or do I have to? I have seen that some examples perform a service discovery when initiating the connection, but if I know the characteristic uuid, do I still have to call this function? I tried adding this sd_ble_gattc_primary_services_discover function but didn't solve the problem. 
 Update: I have seen that some have used the sd_ble_gattc_read (connection_handle, 0xF011 , 0 ) function. When I use this instead, I get no error but the value I get in the event handler has a length of 0. 
 
 Thanks in advance! 
 /Hadi

Edvin · Accepted Answer

Hello Hadi, 
 Check the (short) description on line 90 in ble_gattc.h. (At least it is found here in SDK15.3.0). 
 BLE_GATTC_EVT_CHAR_VAL_BY_UUID_READ_RSP, /**< Read By UUID Response event. 
 See @ref ble_gattc_evt_char_val_by_uuid_read_rsp_t. */

You can also look at the struct that belongs to the BLE_GATTC_EVT_CHAR_VAL_BY_UUID_READ_RSP on line 257 -> 265 in ble_gattc.h: 
 /**@brief Event structure for @ref BLE_GATTC_EVT_CHAR_VAL_BY_UUID_READ_RSP. */
typedef struct
{
 uint16_t count; /**< Handle-Value Pair Count. */
 uint16_t value_len; /**< Length of the value in Handle-Value(s) list. */
 uint8_t handle_value[1]; /**< Handle-Value(s) list. To iterate through the list use @ref sd_ble_gattc_evt_char_val_by_uuid_read_rsp_iter.
 @note This is a variable length array. The size of 1 indicated is only a placeholder for compilation.
 See @ref sd_ble_evt_get for more information on how to use event structures with variable length array members. */
} ble_gattc_evt_char_val_by_uuid_read_rsp_t; 
 So you need to use sd_ble_gattc_evt_char_val_by_uuid_read_rsp_iter() to read the actual value (snippet from ble_gattc.h line 669): 
 * \code
 * ble_gattc_handle_value_t iter;
 * memset(&iter, 0, sizeof(ble_gattc_handle_value_t));
 * while (sd_ble_gattc_evt_char_val_by_uuid_read_rsp_iter(&ble_evt.evt.gattc_evt, &iter) == NRF_SUCCESS)
 * {
 * app_handle = iter.handle;
 * memcpy(app_value, iter.p_value, ble_evt.evt.gattc_evt.params.char_val_by_uuid_read_rsp.value_len);
 * }
 * \endcode
 *
 * @retval ::NRF_SUCCESS Successfully retrieved the next Handle-Value pair.
 * @retval ::NRF_ERROR_NOT_FOUND No more Handle-Value pairs available in the list.
 */ 
 After the memcpy(), you should have the data in app_value. It is copied from iter.p_value, and has a length of o_ble_evt.evt.gattc_evt.params.char_val_by_uuid_read_rsp.value_len.

I haven't seen this been used much. Notifications are a bit easier to work with, in my opinion.

Read added custom characteristic value from peripheral

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