sd_ble_gatts_hvx indication sets len 0 and doesn't call onCharacteristicChanged on Adroid App

RE: sd_ble_gatts_hvx indication sets len 0 and doesn't call onCharacteristicChanged on Adroid App

x_arrange — Sun, 19 Jan 2020 20:16:29 GMT

result is the same: if len = MTU, nothing sent

RE: sd_ble_gatts_hvx indication sets len 0 and doesn't call onCharacteristicChanged on Adroid App

Susheel Nuguru — Sat, 18 Jan 2020 18:41:11 GMT

Sorry. Yes you are right

RE: sd_ble_gatts_hvx indication sets len 0 and doesn't call onCharacteristicChanged on Adroid App

x_arrange — Fri, 17 Jan 2020 19:29:17 GMT

Have you ment "params.p_len = &len"?

RE: sd_ble_gatts_hvx indication sets len 0 and doesn't call onCharacteristicChanged on Adroid App

Susheel Nuguru — Fri, 17 Jan 2020 14:35:57 GMT

Looking at the implementation of HVX inside softdevice, the documentation looks correct. So I am not sure why you see that behaviour.

Can you try the below instead in your write_response and see what happens?

static void write_response(uint16_t conn_handle, uint16_t char_handle, uint16_t len, uint8_t *data){
    ble_gatts_hvx_params_t params;
    uint16_t wr_len =0;
    memset(&params, 0, sizeof(ble_gatts_hvx_params_t));
    params.handle = char_handle;
    params.type = BLE_GATT_HVX_INDICATION;
    params.p_len = len;
    params.p_data = data;
    params.offset = 0;    

    ble_gatts_value_t pvalue;
    memset(&pvalue, 0, sizeof(ble_gatts_value_t));
    pvalue.len = len;
    pvalue.offset = 0;
    pvalue.p_value = data;

    uint32_t err = sd_ble_gatts_value_set(conn_handle, char_handle, &pvalue);
    if (err != NRF_SUCCESS) printf("\tsd_ble_gatts_value_set ERROR: %x\n",err); // else printf("\tsd_ble_gatts_value_set OK: %d\n",pvalue.len);
    
    printf("\twrite_response: conn_h: %d, cchar_h: %d, len=%d\n", conn_handle, char_handle, len);
    err = sd_ble_gatts_hvx(conn_handle, &params);
    if (err != NRF_SUCCESS) printf("\tsd_ble_gatts_hvx ERROR: %x\n",err); else printf("\tsd_ble_gatts_hvx OK: %d\n",wr_len);
   
}

RE: sd_ble_gatts_hvx indication sets len 0 and doesn't call onCharacteristicChanged on Adroid App

x_arrange — Thu, 16 Jan 2020 17:07:50 GMT

... and another discovery that is not documented!

apparently ble_gatts_hvx_params_t member p_len had to be set to less or equal to current MTU-3. In this case indication will be sent

Why is that? And why documentation doesn't say a word about it?

RE: sd_ble_gatts_hvx indication sets len 0 and doesn't call onCharacteristicChanged on Adroid App

x_arrange — Thu, 16 Jan 2020 15:59:13 GMT

So... looks like I found a bug:

Comments in ble_gatts.h says:

that means my write_response function is OK.

But whe I set params.p_data = data (existing pointer) then I do get onCharacteristicChanged called on Android. BUT with zero-length byte array

So I decided to set wr_len = len hoping that in this case I'll get data in Android but I didn't - onCharacteristicChanged again not called.

I might say - API documentation/comments are very unclear..