How to read characteristic value

RE: How to read characteristic value

lyrics — Thu, 29 Oct 2020 13:16:23 GMT

First, I used 'print the hexadecimal values' to read results similar to the values I wanted to see.

Thank you for your reply and solution. :)

(But I need to study more..)

[quote userid="26071" url="~/f/nordic-q-a/67709/how-to-read-characteristic-value/277533#277533"]Do you know what data the characteristic you are reading holds? Is it "E5 2F 46 C4 00 00 00"? Or is that just an example?[/quote]

Because of the nature of my device, when I put the device on my skin, I get a variety of data for about 5 seconds. One of the data is the example value. So that value is also what I want to read.

[quote userid="26071" url="~/f/nordic-q-a/67709/how-to-read-characteristic-value/277533#277533"]but not "len". Where do you get this callback?[/quote]

The variable name defined in structure 'ble_gattc_evt_read_rsp_t' was len, offset, and p_handle..

[quote userid="26071" url="~/f/nordic-q-a/67709/how-to-read-characteristic-value/277533#277533"]And does your peripheral support reading and not only notifications? What is your peripheral?[/quote]

Unfortunately, I also don't know much about my device. I just know it's a 'skin moisture measurement' beacon.

Anyway, I'm getting closer to what I want. Thank you for your constant reply. If I have any more questions while studying, I will post them again.

RE: How to read characteristic value

Henry_Chou — Thu, 29 Oct 2020 09:16:21 GMT

I saw you that you had nus_c_rx_notif_enable

That means you may read the characteristic by notify.....

Well,...You can also get the event case BLE_GATTC_EVT_HVX:

That can get the value more easy.

RE: How to read characteristic value

Edvin — Thu, 29 Oct 2020 08:46:52 GMT

Do you know what data the characteristic you are reading holds? Is it "E5 2F 46 C4 00 00 00"? Or is that just an example?

It looks like you are trying to print them as numbers when you use %d, but when you use app_uart_put() it will print the hexadecimal value, so depending on your UART terminal, it may look different due to how it interprets non-ascii values.

I am a bit confused by your parameter names. Where do you receive this event, and what parameters does that callback contain? The BLE_GATTC_EVT_CHAR_VAL_BY_UUID_READ_RSP event should contain something like a ble_evt_t, possibly called p_ble_evt, and it should contain:

p_ble_evt->gattc_evt.params.char_val_by_uuid_read_rsp.value_len
and
p_ble_evt->gattc_evt.params.char_val_by_uuid_read_rsp.handle_value,

but not "len". Where do you get this callback?

And does your peripheral support reading and not only notifications? What is your peripheral? Is it something you wrote yourself? Is it running on a Nordic chip?

Let us say that the event does contain a pointer to the data, "p_value", and a length variable "len".

If your data contains ascii characters (only) you can use:

NRF_LOG_RAW_INFO("received: ");
for (uint8_t i=0; i<len; i++)
{
    NRF_LOG_RAW_INFO("%c", p_data[i]);
}
NRF_LOG_RAW_INFO("\r\n");

If you want to print the hexadecimal values:

NRF_LOG_RAW_INFO("received hex values: 0x");
for (uint8_t i=0; i<len; i++)
{
    NRF_LOG_RAW_INFO("%02x:", p_data[i]);
}

RE: How to read characteristic value

lyrics — Thu, 29 Oct 2020 05:24:30 GMT

This is my code, and I think I defined the characteristic handle line 10..

static void ble_nus_c_evt_handler(ble_nus_c_t * p_ble_nus_c, ble_nus_c_evt_t const * p_ble_nus_evt)
{
    ret_code_t err_code;

    switch (p_ble_nus_evt->evt_type)
    {
        case BLE_NUS_C_EVT_DISCOVERY_COMPLETE:
            NRF_LOG_INFO("Discovery complete.");
            printf("Discovery\r\n");
            err_code = ble_nus_c_handles_assign(p_ble_nus_c, p_ble_nus_evt->conn_handle, &p_ble_nus_evt->handles);
            APP_ERROR_CHECK(err_code);

            /**********added**********/            
            //nrf_delay_ms(10000);
            //ble_uuid_t p_uuid = {.uuid = BLE_UUID_GAP_CHARACTERISTIC_DEVICE_NAME, .type = BLE_UUID_TYPE_BLE};
            ble_uuid_t p_uuid = {.uuid = 0x1601, .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(p_ble_nus_evt->conn_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);
                printf("\n\rFailed to send read request to peer error = %d\n\r", err_code);
            }
            /**************************/

            err_code = ble_nus_c_tx_notif_enable(p_ble_nus_c);
            APP_ERROR_CHECK(err_code);
            NRF_LOG_INFO("Connected to device with Nordic UART Service.");
            printf("connect2\r\n");     
    
            break;
    .....
}

[quote userid="93984" url="~/f/nordic-q-a/67709/how-to-read-characteristic-value/277495#277495"]you may take the reference code from SDK (Central or Peripheral) ble_app_gatts example[/quote]

I will study again with that example.

RE: How to read characteristic value

Henry_Chou — Thu, 29 Oct 2020 02:00:17 GMT

If the event is the BLE_GATTC_EVT_CHAR_VAL_BY_UUID_READ_RSP. You may setup the characteristic handle during discovery event. In your case,maybe like this

if (p_evt->evt_type == BLE_DB_DISCOVERY_COMPLETE &&
p_evt->params.discovered_db.srv_uuid.uuid == Your_UUID_SERVICE &&
p_evt->params.discovered_db.srv_uuid.type == BLE_UUID_TYPE_BLE)
{
ble_gatt_c_evt_t evt;

evt.evt_type = BLE_GATT_C_EVT_DISCOVERY_COMPLETE;
evt.conn_handle = p_evt->conn_handle;

uint32_t i;
for (i = 0; i < p_evt->params.discovered_db.char_count; i++)
{
const ble_gatt_db_char_t * p_char = &(p_evt->params.discovered_db.charateristics[i]);
switch (p_char->characteristic.uuid.uuid)
{
case Your_NUS_UUID:
evt.params.peer_db.gatt_handle = p_char->characteristic.handle_value;

;
break;

default:
break;
}
}

If you don't define the characteristics handle, the event case will not match the characteristic uuid which you want.

RE: How to read characteristic value

Henry_Chou — Thu, 29 Oct 2020 01:27:14 GMT

Hey!Lyrics:

the ble_app_uart.c dosen't have the code exactly. The code is build in my project case.

However, you may take the reference code from SDK (Central or Peripheral) ble_app_gatts example. The gatt_cache_manager.c

void gcm_ble_evt_handler(ble_evt_t const * p_ble_evt)

function. case BLE_GATTC_EVT_READ_RSP:

You may get the answer which you need. However nus(Nordic Uart service) is also a kind of gatt. So the solution is the same.

My SDK version is 17.0.2

RE: How to read characteristic value

lyrics — Thu, 29 Oct 2020 00:36:00 GMT

Hi, Henry

The 'ble_app_uart_c' example does not have 'on_read_rsp', so I compiled the code.

[quote userid="93984" url="~/f/nordic-q-a/67709/how-to-read-characteristic-value/277306#277306"]evt->gattc_evt.params.char_val_by_uuid_read_rsp.handle_value[?][/quote]

ble_gattc_evt_read_rsp_t const * p_read_rsp = &p_ble_evt->evt.gattc_evt.params.read_rsp;

I tried to access value with 'p_read_rsp' by defining like this.

RE: How to read characteristic value

lyrics — Thu, 29 Oct 2020 00:27:58 GMT

Hi, Edvin

[quote userid="26071" url="~/f/nordic-q-a/67709/how-to-read-characteristic-value/277386#277386"]What is the p_read_rsp->offset when you hit this for-loop?[/quote]

I just don't understand what offset means, so I tried 'for-loop'.

[quote userid="26071" url="~/f/nordic-q-a/67709/how-to-read-characteristic-value/277386#277386"]Have you tried to debug and see what the data in the p_read_rsp->data looks like?[/quote]

When I tried to see my device name which is 'MyService',

I can see my device name through 'p_read_rsp->data'.

The picture is the result below.

case BLE_GATTC_EVT_CHAR_VAL_BY_UUID_READ_RSP :
    printf("Read length: %d, Read offset: %d\r\n", p_read_rsp->len, p_read_rsp->offset);
    printf("Read %d\r\n", p_read_rsp->data);
           
    for(uint32_t j = 0; j <=  p_read_rsp->len; j++) {
        for (uint32_t i = 0; i < p_read_rsp->offset; i++)
        {
            while(app_uart_put( p_read_rsp->data[i]) != NRF_SUCCESS);
        }
        printf("\r\n");       
    }
    break;

(I don't understand why offset fits the length of the data...)

I tried to show the characteristic value, just like the name but it's not coming out properly.

RE: How to read characteristic value

Edvin — Wed, 28 Oct 2020 12:34:29 GMT

Have you tried to debug and see what the data in the p_read_rsp->data looks like? What is the p_read_rsp->offset when you hit this for-loop?

RE: How to read characteristic value

Henry_Chou — Wed, 28 Oct 2020 09:23:04 GMT

You may read characteristic from your gatt evt handle like this.

void ble_gatt_c_on_ble_evt(ble_evt_t const * p_ble_evt, void * p_context)
{
if ((p_context == NULL) || (p_ble_evt == NULL))
{
return;
}

ble_gatt_c_t * p_ble_gatt_c = (ble_gatt_c_t *)p_context;

switch (p_ble_evt->header.evt_id)
{
case BLE_GATTC_EVT_HVX:
on_hvx(p_ble_gatt_c, p_ble_evt);
break;

case BLE_GATTC_EVT_CHAR_VAL_BY_UUID_READ_RSP:

on_read_rsp(p_ble_gatt_c, p_ble_evt);
break;

case BLE_GATTC_EVT_WRITE_RSP:

on_write_rsp(p_ble_gatt_c, p_ble_evt);
break;

case BLE_GAP_EVT_DISCONNECTED:
on_disconnected(p_ble_gatt_c, p_ble_evt);
break;

default:
break;
}
}

In on_read_rsp code

evt->gattc_evt.params.char_val_by_uuid_read_rsp.handle_value[?]

to get your data.

By the way, your GAP service uuid=0x1601.

should be setup in your handle initial.