How to do a GATT read for multiple characterisitics in nRF Connect SDK

[db_lw]

Hello all,

I'm working on a BLE central application based on nRF Connect SDK V1.6.1. The application connects to certain BLE peripheral devices and does a service discovery for the device information service. If discovered I would like the application to read out multiple characteristics of the DIS but when I do so I get error 6 in my read attribute callback.

I adopted the ble_discovery module from the nrf_desktop application sample to my needs.

How can I do a GATT read for multiple characteristics? I couldn't find any example that does this but according to the zephyr documentation it should be possible.

Below are the relevant code parts. The error is catched in line 5.

static uint8_t read_attr(struct bt_conn *conn, uint8_t err,
		      struct bt_gatt_read_params *params,
		      const void *data, uint16_t length)
{
	if (err) {
		LOG_ERR("Problem reading GATT (err:%" PRIu8 ")", err);
		peer_disconnect(conn);
		return BT_GATT_ITER_STOP;
	}

    __ASSERT_NO_MSG(data != NULL);
	const uint8_t *data_ptr = data;

	switch (state) {
	case DISCOVERY_STATE_DIS_FW_VER:
		read_dis(data_ptr, length);
		break;

	default:
		__ASSERT_NO_MSG(false);
	}

	k_work_submit(&next_discovery_step);

	return BT_GATT_ITER_STOP;
}

static uint16_t handles[2];

static void discovery_completed(struct bt_gatt_dm *dm, void *context)
{
	__ASSERT_NO_MSG(dm != NULL);
	__ASSERT_NO_MSG(discovering_peer_conn == bt_gatt_dm_conn_get(dm));
	const struct bt_uuid *model_num_uuid = dis_uuids[0];
    const struct bt_uuid *fw_uuid = dis_uuids[1];
	const struct bt_gatt_dm_attr *attr;
	int err;

	attr = bt_gatt_dm_char_by_uuid(dm, model_num_uuid);

	if (!attr) {
		LOG_ERR("Characteristic not found - disconnecting");
		err = bt_gatt_dm_data_release(dm);
		if (err) {
			goto error;
		}
		peer_disconnect(bt_gatt_dm_conn_get(dm));
		return;
	}

    handles[0] = attr->handle + 1;

    attr = bt_gatt_dm_char_by_uuid(dm, fw_uuid);

	if (!attr) {
		LOG_ERR("Characteristic not found - disconnecting");
		err = bt_gatt_dm_data_release(dm);
		if (err) {
			goto error;
		}
		peer_disconnect(bt_gatt_dm_conn_get(dm));
		return;
	}

    handles[1] = attr->handle + 1;

    static struct bt_gatt_read_params rp =  {
		.func = read_attr,
		.handle_count = 2,
	};

    rp.handles = handles;

	err = bt_gatt_read(bt_gatt_dm_conn_get(dm), &rp);
	if (err) {
		LOG_ERR("GATT read problem (err:%d)", err);
		peer_disconnect(bt_gatt_dm_conn_get(dm));
	}

	err = bt_gatt_dm_data_release(dm);
	if (err) {
		goto error;
	}

	return;
error:
	LOG_ERR("Discovery data release failed (err:%d)", err);
	module_set_state(MODULE_STATE_ERROR);
}

Thanks in advance!

[Simonr]

Hi

A limitation of the GATT discovery manager that the BLE discovery module you're using is that it can only do one discovery procedure at the same time, so you will have to do this multiple times to get multiple characteristics. Please note that the function to access characteristic vvalues is the structbt_gatt_chrc*bt_gatt_dm_attr_chrc_val(conststructbt_gatt_dm_attr*attr), so please try calling that to get characteristic values

Best regards,

Simon

[db_lw]

Thanks Simon for the fast reply!

Simonr said:

A limitation of the GATT discovery manager that the BLE discovery module you're using is that it can only do one discovery procedure at the same time, so you will have to do this multiple times to get multiple characteristics.

Ok, good to know then I will do that.

Simonr said:

Please note that the function to access characteristic vvalues is the structbt_gatt_chrc*bt_gatt_dm_attr_chrc_val(conststructbt_gatt_dm_attr*attr), so please try calling that to get characteristic values

Do you mean I should use this function to replace the part with the attribute handles incrementation?

    handles[0] = attr->handle + 1;

[Simonr]

Yes, I think you should use the bt_gatt_dm_attr_chrc_val to access characteristic value saved with a characteristic attribute. Refer to the GATT discovery manager for details.

Best regards,

Simon