Ways of sending a Float array as a value of a characteristic

RE: Ways of sending a Float array as a value of a characteristic

Hung Bui — Mon, 12 Dec 2016 11:42:55 GMT

You mean you can send more than 20 bytes via notification ? It could be that the phone actually support long ATT MTU. With Bluetooth 4.2 the ATT MTU can get upto 512 bytes instead of 23 bytes as before. The longest notification can send is ATT MTU -3. That's why I said it was 20 bytes. We need to see the sniffer trace to check if actually long ATT MTU is used.

It's pretty new features, we only supported that from S132 v3.x

RE: Ways of sending a Float array as a value of a characteristic

nicko_f — Sun, 11 Dec 2016 06:03:04 GMT

Hi sir, thank you for the response. When I try sending more than 20 bytes (e.g. 50 bytes) to my Android Device for that single characteristic I was able to still receive it. Why is that? I thought the maximum was 20 bytes. I am using the nrf52. Thanks again. Nick

RE: Ways of sending a Float array as a value of a characteristic

Hung Bui — Wed, 30 Nov 2016 13:56:55 GMT

Hi Nicko,

In our example we have to do with IEEE-11073 float (with 24 bit mantissa and 8 bit exponent) because it's defined in the spec as in here.

If you are fine with C float, you can simply convert it to 4 bytes like what you are doing now. Note that each float requires 4 bytes, if you have 180 values, you would need 720 bytes. This may exceed the max size of characteristic we support now. I would suggest you to send them in 20 bytes trunk, so you keep the characteristic size at 20 bytes, and send the result with 36 notification packets.

RE: Ways of sending a Float array as a value of a characteristic

AlekseyJ — Tue, 29 Nov 2016 17:05:44 GMT

Read about floating point. They just encode floating value (temp_in_celcius) into the 32 bit single precision format.

encoded_temp = ((p_hts_meas->temp_in_celcius.exponent << 24) & 0xFF000000) |
                       ((p_hts_meas->temp_in_celcius.mantissa <<  0) & 0x00FFFFFF);

Another very simple way is multiply your values float temperature[2]= {1.1,2.2}; by 10 to create 2 numbers 11 and 22 and then on a receiving side divide by 10.