Adding characteristic (for purpose of long read) fails

[Aurabindo]

I've added a custom service for transferring data. It shall contain one characteristic that will notify when data is available, and the other should be a long read characteristic (no notify). sd_ble_gatts_characteristic_add is failing (return 0x7) when I add the second characteristic.

I've made attr_char_value.max_len to 512. I am enabling attr_md.rd_auth = 1 to handle the long read as mentioned in the post devzone.nordicsemi.com/.../

Is this the right way, or are there any new preferred way to enable long read ?

Could someone please point out what is the invalid parameter ? I'm using sdk 14.1 with GCC compiler. The code is below:

static uint32_t sensor_data_char_add(ble_ksh_t * p_ksh, ble_ksh_init_t const * p_pke_init) {
ble_gatts_char_md_t char_md;
ble_gatts_attr_md_t cccd_md;
ble_gatts_attr_t    attr_char_value;
ble_uuid_t          ble_uuid;
ble_gatts_attr_md_t attr_md;

extern uint8_t sensor_data_buffer[BLE_KSH_MAX_DATA_CHAR_LEN];

memset(&cccd_md, 0, sizeof(cccd_md));

BLE_GAP_CONN_SEC_MODE_SET_OPEN(&cccd_md.read_perm);

cccd_md.vloc = BLE_GATTS_VLOC_STACK;

memset(&char_md, 0, sizeof(char_md));

char_md.char_props.read = 1;
char_md.p_char_user_desc  = NULL;
char_md.p_char_pf         = NULL;
char_md.p_user_desc_md    = NULL;
char_md.p_cccd_md         = &cccd_md;
char_md.p_sccd_md         = NULL;

ble_uuid.type = p_ksh->uuid_type;
ble_uuid.uuid = BLE_UUID_KSH_DATA_CHARACTERISTIC;

memset(&attr_md, 0, sizeof(attr_md));

BLE_GAP_CONN_SEC_MODE_SET_OPEN(&attr_md.read_perm);

attr_md.vloc    = BLE_GATTS_VLOC_STACK;
attr_md.rd_auth = 1;
attr_md.wr_auth = 0;
attr_md.vlen    = 1;

memset(&attr_char_value, 0, sizeof(attr_char_value));

attr_char_value.p_uuid    = &ble_uuid;
attr_char_value.p_attr_md = &attr_md;
attr_char_value.init_len  = sizeof(uint8_t);
attr_char_value.init_offs = 0;
attr_char_value.max_len   = BLE_GATTS_VAR_ATTR_LEN_MAX;
attr_char_value.p_value   = &sensor_data_buffer[0];

return sd_ble_gatts_characteristic_add(p_ksh->service_handle,
                                       &char_md,
                                       &attr_char_value,
                                       &p_ksh->data_handles);

}

[Hung Bui]

Hi Aura,

There is no relation between read blob and read with authorization . Have a look here.

The reason you have error #7 is that you don't set a write permission for CCCD which is required. Try adding this:

BLE_GAP_CONN_SEC_MODE_SET_OPEN(&cccd_md.write_perm);

[Aurabindo]

Thank you for the response @Hung Bui, I have some questions:

1. Are there any other setup necessary to do related to GATT Client during initialization?

2. Is the value 512 valid for the max length of the characteristic?

3. I'm unable to understand the reason why cccd needs the write permission, when there is no need for a peer to write to it, since it doesnt support enabling notification. Are there other reasons cccd needs to be written ?

[Aurabindo]

4. Even though both read blob and read with authorization are not related, is this not the only way to send to the peer a long read sequence when the peer does a normal read ? My assumption was that I will have to enable read with authorization for this characteristic, and just before I reply, I will have to call the corresponding api that does long read (which turns out to be sd_ble_gattc_read instead of sd_ble_gatts_value_get from your answer). Is this the right way to make a long read happen ?

5. Is my understanding correct that the peer has only one kind of read; and its the peripheral that decides to do a long/normal read ?

[Aurabindo]

6. Inorder to tell the stack that I have fresh data to be sent from the peripheral next time when a read is performed by a peer, the function to be used to set the fresh data is sd_ble_gatts_set_data() ? If so, this function takes a pointer to data with its length and offset. What significance does it have wrt the data pointer that was initially setup for this characteristic when it was created/added ? Can i point to this same buffer ?

[Aurabindo]

@hungbui knock knock