BLE notification sending invalid data

[Keith V]

I'm trying to implement a Write / Notify type of GATT Characteristic to return 1 item out of an array of items.

So the Central sends an index, and the peripheral sends back a notification with the data at that index

the index is 1 byte, the data is 5 bytes.

I AM sending the notification in the context of BLE_GATTS_EVT_WRITE. But I see other examples doing that, like app_pwr_mgmt_main example.

The problem is it returns the wrong data.

I am expecting: 02, 01F4, 07D0 ( as output by NRF_LOG_INFO() )

I am getting: 00 04 00 20 81 ( as displayed by nRFConnect ) This never changes and I see the same values on other Characteristics that do the same thing

Code:

catch BLE_GATTS_EVT_WRITE

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

   ble_mtgw_t * p_mtgw = (ble_mtgw_t *)p_context;

   switch (p_ble_evt->header.evt_id)
   {
      case BLE_GAP_EVT_CONNECTED:
         on_connect(p_mtgw, p_ble_evt);
         break;

      case BLE_GATTS_EVT_WRITE:
         on_write(p_mtgw, p_ble_evt);
         break;

Parse the handle and the index

Get the array data and send it as a notification

static void on_write(ble_mtgw_t * p_mtgw, ble_evt_t const * p_ble_evt)
{
    ret_code_t                    err_code;
    ble_mtb_evt_t                 evt;
    ble_mtb_client_context_t    * p_client;
    ble_gatts_evt_write_t const * p_evt_write = &p_ble_evt->evt.gatts_evt.params.write;
.
.
.
 else if (p_evt_write->handle == p_mtgw->Led_ReadTime_handle.value_handle)
    {
      uint8_t data[LED_TIME_LENGTH];
      uint16_t Period;
      uint16_t On;

      uint8_t Type = p_evt_write->data[0]; // GetValue8(p_ble_evt->evt.gatts_evt.conn_handle, p_evt_write->handle);

      GetLEDTimes(Type, &Period, &On);

      NRF_LOG_INFO("LED Time, %d, %04x, %04x", Type, On, Period);

      data[0] = Type;
      uint16_big_encode(Period, &data[1]);
      uint16_big_encode(On, &data[3]);

      //notify send 5 bytes 
      SendNotify(p_ble_evt->evt.gap_evt.conn_handle,p_mtgw->Led_ReadTime_handle.value_handle, data, LED_TIME_LENGTH);
    }

Send the notification

static void SendNotify(uint16_t conn_handle, uint16_t char_handle, uint8_t *data, uint16_t len)
{
   
	ble_gatts_hvx_params_t  hvx_params;
	memset(&hvx_params, 0, sizeof(hvx_params));

	hvx_params.handle = char_handle;
	hvx_params.type   = BLE_GATT_HVX_NOTIFICATION;
	hvx_params.offset = 0;
	hvx_params.p_len  = &len;
	hvx_params.p_data = data; 

	ret_code_t err_code = sd_ble_gatts_hvx( conn_handle, &hvx_params);
	if ((err_code != NRF_SUCCESS) && (err_code != NRF_ERROR_INVALID_STATE) && (err_code != NRF_ERROR_RESOURCES) && (err_code != BLE_ERROR_GATTS_SYS_ATTR_MISSING))
	{
		APP_ERROR_HANDLER(err_code);
	}

   NRF_LOG_RAW_INFO("Notify, %d, ", err_code);
   for(int i=0; i< len; i++)
   {
      NRF_LOG_RAW_INFO("%02x", data[i]); 
   }
   NRF_LOG_RAW_INFO("\n");
}

Gatt Setup

    memset(&add_char_params, 0, sizeof(add_char_params));
    add_char_params.uuid_type                = m_mtb.uuid_type;
    add_char_params.max_len                  = BLE_SCAN_CHAR_LEN;
    add_char_params.init_len                 = BLE_SCAN_CHAR_LEN;
    add_char_params.is_var_len               = false;
    add_char_params.char_props.write         = 1;
    add_char_params.char_props.read          = 0;
    add_char_params.char_props.notify        = 1;
    
    add_char_params.read_access              = SEC_OPEN;
    add_char_params.write_access             = SEC_OPEN;
    add_char_params.cccd_write_access        = SEC_OPEN;
    add_char_params.is_value_user            = false;
 
   // Add the LED Blink Time Characteristic.
    descript.size                            = strlen("Get LED Times");
    descript.p_char_user_desc                = "ReGetad LED Times";
    descript.max_size                        = descript.size;

    add_char_params.uuid                     = BLE_UUID_LEDTIME_READ_CHAR;
    add_char_params.max_len                  = LED_TIME_LENGTH;
    add_char_params.init_len                 = 2;
    err_code =  characteristic_add(m_mtb.service_handle, &add_char_params, &m_mtb.Led_ReadTime_handle);

Thanks!

Keith Vasilakes

[Edvin]

Hello,

So you are sending this data from a peripheral and receiving it on a central, the central is a computer or a phone?

Can you please try to debug, and compare the data buffers (length and actual data) when you send it and when it is received?  I see that you are printing:

   NRF_LOG_RAW_INFO("Notify, %d, ", err_code);
   for(int i=0; i< len; i++)
   {
      NRF_LOG_RAW_INFO("%02x", data[i]); 
   }
   NRF_LOG_RAW_INFO("\n");

But you say that you see:

"02, 01F4, 07D0 ( as output by NRF_LOG_INFO() )"

Is that what the Notify print is printing? If so, shouldn't it just output hexadecimal values, and not the commas?

It is not clear to me where you see the. Is the 02, 01F4, 07D0 actually from the NRF_LOG_INFO()?

And when you say nRF Connect, is it the desktop app, or on a phone?

Best regards,

Edvin

[Keith V]

Thanks Edvin,

My device is the peripheral and nRFConnect ( desktop ) is the central

I print in 2 places, the on_write() and  SendNotify(), the commas are not printed no, here's whats actually printed: I've changed the value 01f4 to 03E8  for other reasons....

nRFConnect should show "(0x) 01-07-D0-03-e8

<info> MTserv: BLE_GATTS_EVT_WRITE: 60<
<info> MTserv: LED Time, 1, 03E8, 07D0
Notify, 0, 0107D003E8

the data buffer looks correct ( same data as notify )

I'm using nRF connect on the PC, but tried it on my phone with the same results(0x) 00-04-00-20-81

This is from the PC

[Edvin]

Hello,

I don't know what your custom functions do. All I can see is that you are sending LED_TIME_LENGTH bytes (I guess LED_TIME_LENGTH is 5, since you are receiving 5 bytes), and the first byte is the parameter "Type" = data = p_evt_write->data[0];

However, it looks like it should be correct from where you print using NRF_LOG_RAW_INFO().

Is it possible for me to reproduce this on a DK and a phone? If so, would it be possible to share the project?

BR,

Edvin

[Keith V]

my custom functions are listed in the message.

LED_TIME_LENGTH is 5 yes

I did use a phone, what difference would a dev kit make?

is there an example showing how to do this?

it can't be a new idea

[Edvin]

Keith V said:

my custom functions are listed in the message.

Yes, but not all functions that you have used to replicate the issue. Particularly the initialization of the service. Besides. I can set up a characteristic that sends this sort of data (or I can demonstrate it using the ble_app_uart example), but that will not replicate the issue that you are seeing.

Keith V said:

I did use a phone, what difference would a dev kit make?

No difference. A phone should be fine.

Keith V said:

is there an example showing how to do this?

How to do what, exactly? The ble_app_uart example is quite versatile, as you can use it to send a data array over BLE. 

If you upload your project, so that I can simply compile it and run it to replicate the issue, I can have a look at what the issue might be. I can't tell from the snippets you have provided. But perhaps debugging can give a better idea. 

Best regards,

Edvin