MATLAB: Matlab Fixed Point – Suggesting Fraction length larger than Word Length

simulink fixed point

I am confused by how matlab is suggesting a Fixed point number of sfix32_En44. This means it has a word length of 32bits but a fraction length of 44. With standard Binary Point this is impossible. The fraction length is always less than the word length.
Is matlab trying to tell me my word size is not big enough?

Best Answer

The fraction length does not have to be shorter than the word length if you allow for (implicit) leading zeros. So if it is suggesting a 32 bit word length with 44 bits fraction length, then that implies that with one bit for the sign, all 31 of the remaining bits are fraction; and in addition, there must be 13 leading zeros to the right of the binary point prior to the 31 significant binary digits.
So the word consists of 1 sign bit followed by the 31 significant binary digits, and 13 implicit leading zeros (or ones if the sign bit is negative) between the binary point and the first significant binary digit.
HTH.
Rick
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