Converting two decimal numbers into binary, concatenating the resulting 8 bit integers to get 16 bit signed integer and converting to signed float (IMU)

So I have this 6DOF IMU that spits out data packets as follows:

sensorData = [85 13 109 85 12 147 12 0 156 0 71 255 229 10 144 252];

The acceleration in x axis for example, would be found by converting 12 and 147 to 8 bit binary numbers, concatenating them to get signed 16 bit binary number and converting them to signed floating point number.

I am not sure how to properly write the code to do the job. Here's my code for example, which doesn't work as expected:

sensorData = read(characteristic(b, "FF06", "FF08"));
z(i,1) = (typecast(uint16(bin2dec(strcat(dec2bin(sensorData(5),8), dec2bin(sensorData(6),8)))),'int16'))/SFacc; %SFacc = 8192 LSB/ms^-2 

Am I doing something wrong? The values I receive are not what I am expecting.

Answer 1

[Walter Roberson]

double( swapbytes(typecast(uint8(sensorData), 'int16')) ) / SFacc

[Swapnil Sayan Saha]

Thanks. Does this take groups of two bytes (e.g. 85-13, 109-85, 12-147 ...... 144-252) and convert them to double? The output seems to do so.

[Swapnil Sayan Saha]

If so, then it works!

    sensorData = read(characteristic(b, "FF06", "FF08"));
    %acceleration values given in terms of g.
    %acceleration values given in terms of g.
    temp =  double(swapbytes(typecast(uint8(sensorData), 'int16')));
    z(i,1) = temp(3)/SFacc;
    z(i,2) = temp(4)/SFacc;
    z(i,3) = temp(5)/SFacc;
    %angular velocity given as deg/s.
    z(i,4) = temp(6)/SFgyr;
    z(i,5) = temp(7)/SFgyr;
    z(i,6) = temp(8)/SFgyr;

[Walter Roberson]

Yes, it takes groups of two uint8 to form the int16.

All supported versions of MATLAB for a number of years have been on Intel architecture, which is "little endian". For little endian, the byte pair A, B when bundled together into a single 16 bit entity, are mathematically A + 256*B not 256*A + B . The swapbytes() reorders the byte pairs within the 16 bit numbers.

[Swapnil Sayan Saha]

The concatenation indeed follows little endian (as per the datasheet of the IMU).