sd_ble_gatts_hvx() always send data from 0x00 address

[Jakub]

Hello, I have a problem with Indicate Characteristic. When I try to send data using sd_ble_gatts_hvx() function, it always sends 20bytes from address 0x00. This is my code for sending data:

        uint16_t               hvx_len;
    ble_gatts_hvx_params_t hvx_params;
		
    memset(&hvx_params, 0, sizeof(hvx_params));
			
			hvx_len = 20;
		
    hvx_params.handle = p_dt->tx_handles.value_handle;
    hvx_params.type   = BLE_GATT_HVX_INDICATION;
    hvx_params.offset = 0;
    hvx_params.p_len  = &hvx_len;
    hvx_params.p_data = data
    
    err_code = sd_ble_gatts_hvx(p_dt->conn_handle, &hvx_params);

err_code is 0. 'data' is pointer to data buffer. In debugger, I chech hvx_params struct, it contains proper data. And this is code to setting up Characteristic Value:

attr_char_value.p_uuid       = &ble_uuid;
attr_char_value.p_attr_md    = &attr_md;
attr_char_value.init_len     = 20;
attr_char_value.init_offs    = 0;
attr_char_value.max_len      = 0;
attr_char_value.p_value      = NULL;

What could be wrong?

[muhkuhns]

Why did you set attr_char_value.max_len = 0; and not 20?

[Jakub]

I was trying different settings to get things work or just understand what is happening. Normally it was set to 20, and it didn't change anything.

[muhkuhns]

What a pity. Because I had the same problem with the hvx command only sending 0x00 bytes and for me it was caused by this little setting I just overlooked in the hectic.

Could you post the variable you store your data in, please?

I use this (but for notification) and it works:

uint8_t value[8] = {0};
hvx_params.p_data = (uint8_t*)&value;

I just read about indications now and it looks like this part differs to notifications as they don't queue data.. sry..

[Jakub]

I found a solution. Reason was that I chose to store characteristic value in application memory (VLOC_USER). Therefore characteristic should be given a buffer to data. In characteristic initialization:

attr_char_value.p_uuid       = &ble_uuid;
attr_char_value.p_attr_md    = &attr_md; 
attr_char_value.init_len     = 20;
attr_char_value.init_offs    = 0;
attr_char_value.max_len      = 0; 
attr_char_value.p_value      = buffer;

In function which sends data:

    hvx_params.handle = p_dt->tx_handles.value_handle;
    hvx_params.type   = BLE_GATT_HVX_INDICATION;
    hvx_params.offset = 0;
    hvx_params.p_len  = &hvx_len;
    hvx_params.p_data = NULL;   //it means that SD will use pointer given in init
    
    err_code = sd_ble_gatts_hvx(p_dt->conn_handle, &hvx_params);

[Stian Røed Hafskjold]

Thank you for answering your own question!