Reding GATT Characterictic value.

RE: Reding GATT Characterictic value.

Einar Thorsrud — Tue, 13 Feb 2024 13:04:53 GMT

I am not sure of all the code on the central (here) and peripheral (GATT server) so there may be something I am not aware of regardign what this data represetn, but in any case, the read callback holds the data. It might be easier to test with something like the NUS central and peripheral samples, where you clearly have just a data "channel".

RE: Reding GATT Characterictic value.

Gotak — Tue, 13 Feb 2024 05:10:40 GMT

Hi I actually use the central heart rate monitor example before posting this ticket,

I am able to hear the GATT data from the peripheral hr example by giving the HRS UUID to read the GATT characteristic value for the heart rate monitor.

I wanted to use this example to read by customised gatt UUID characteristics, so I modified the GATT UUID as required and I am able to read some gatt value which is as told in the above reply

3 bytes + Characteristic UUID

And as you told they these prefixed 3 bytes are the characteristic value but when I verified it in the nRF mobile app the same characteristic value is like : (0x) 01 which is not equal to the 3 bytes I got in my nRF gatt reading code output...

RE: Reding GATT Characterictic value.

Gotak — Mon, 12 Feb 2024 12:05:05 GMT

But in the nRF mobile app if I see the characteristic value it is like

Value: (0x) 01

Are they both considered same values?

RE: Reding GATT Characterictic value.

Einar Thorsrud — Mon, 12 Feb 2024 11:51:35 GMT

[quote user="Gotak"]

But what is 10 1B 00??

Is it the characteristic value?

[/quote]

Yes, it is.

RE: Reding GATT Characterictic value.

Gotak — Mon, 12 Feb 2024 11:01:20 GMT

Hi,

I have tried the below logic for reading:

static void read_cb(struct bt_conn *conn, uint8_t err,

struct bt_gatt_read_params *params,

const void *data, uint16_t length)

{

printk("\nread_cb");

if (err)

{

printk("Read failed (err %u)\n", err);

return;

}

else{

printk("\nGATT_READ");

}

printk("Read successful. Value: ");

for (int i = 0; i < length; i++)

{

printk("%02X ", ((uint8_t *)data)[i]);

}

printk("\n");

//bt_conn_disconnect(conn, BT_HCI_ERR_REMOTE_USER_TERM_CONN);

}

The output: Read successful. Value: 10 1B 00 37 2A

37 2A --> are the characteristic UUID which I can recognise

But what is 10 1B 00??

Is it the characteristic value?

RE: Reding GATT Characterictic value.

Einar Thorsrud — Mon, 12 Feb 2024 08:48:03 GMT

Hi,

The characteristic value is accessible by the data pointer. For some reason this sample prints the pointer to the data and the length, and not the data itself. But the data is there.

To decode the data, you can refer to the Heart Rate Service specification. Or more easily, you can refer to Bluetooth: Central Heart Rate Monitor with Coded PHY (and the acompanying Bluetooth: Peripheral Heart Rate Monitor with Coded PHY). This is a much more complete example, and the central here will decode and print all the information in the received notification. So I would say this is a significatnly better sample to refer to. (If you don't want coded PHY).

RE: Reding GATT Characterictic value.

Gotak — Sun, 11 Feb 2024 15:55:08 GMT

Hi,

static uint8_t notify_func(struct bt_conn *conn,
			   struct bt_gatt_subscribe_params *params,
			   const void *data, uint16_t length)
{
	if (!data) {
		printk("[UNSUBSCRIBED]\n");
		params->value_handle = 0U;
		return BT_GATT_ITER_STOP;
	}

	printk("[NOTIFICATION] data %p length %u\n", data, length);

	return BT_GATT_ITER_CONTINUE;
}

The structure "struct bt_gatt_subscribe_params *params"

from the above function contains the parameters:

uint16_t value;

does this store the GATT characteristic value?