change fixed point number signedness

Hi, I have a fixed point number which is unsigned. How do I cast it to a signed fi by taking the bits literally:

eg.

two cases:

a=fi(3,0,3,0); %case1: a='011'

a=fi(4,0,3,0); %case2: a='100'

Now define a new b=fi(....) based on 'a' such that

case1: b.bin will be '011' and is signed

case2: b.bin will be '100' and is signed

Thanks.

Answer 1

[stozaki]

Hi scc28x,

>> b1 = fi(3,1,3,0)
b1 = 
     3
          DataTypeMode: Fixed-point: binary point scaling
            Signedness: Signed
            WordLength: 3
        FractionLength: 0
        
>> bin1 = b1.bin
bin1 =
    '011'
    

>> b2 = fi(4,1,4,0)
b2 = 
     4
          DataTypeMode: Fixed-point: binary point scaling
            Signedness: Signed
            WordLength: 4
        FractionLength: 0

>> bin2 = b2.bin
bin2 =
    '0100' 

b2 is carried by sign extension.

Regards,

stozaki

[scc28x]

Thank you for your reply. Perhaps I was not too clear about my question. What I wanted to convey is that I want to convert an offset binary number to its 2's complement equivalent binary:

eg. 'a' can take the value of:

11, 10, 01, 00 ----> 3, 2, 1, 0

I want to create 'b' by flipping the msb to get:

01, 00, 11, 10 ----> 1,0,-1,-2

I was going to do:

b=fi(a-2,1,2,0);

But just wondering if I could operate in the binary domain, that is:

1. flip msb
2. recast the new binary as a signed binary

In case you wonder why I want to do it the long way. The reason is that I want to code it in this manner so it is easy for the digital engineer to convert the matlab code to HDL without much thought.

Thanks.

Answer 2

[Andy Bartlett]

reinterpretcast is the solution

u = fi(129,0,8,0)
ntu = numerictype(u)
nty = numerictype(ntu,'SignednessBool',true)
y = reinterpretcast(u,nty)
[u.bin;y.bin]

which outputs

u = 
   129
      numerictype(0,8,0)
ntu =

          DataTypeMode: Fixed-point: binary point scaling
            Signedness: Unsigned
            WordLength: 8
        FractionLength: 0
nty =

          DataTypeMode: Fixed-point: binary point scaling
            Signedness: Signed
            WordLength: 8
        FractionLength: 0
y = 
  -127
      numerictype(1,8,0)
ans =
  2×8 char array
    '10000001'
    '10000001'