Avoid autoscaling of wordlength and fractionlength in fixed point objects

Hello, I set a fixed point object with wordlength 16 and fractionlength 4

   x=ufi(16,12,4)

but when I make an operation on the object, the wordlength and fractionlength get changed, for example

   y = sqrt(x)

scales those to 6 and 2 respectively.

How can I avoid this autoscaling?

Thanks!

[Walter Roberson]

Does it happen for other arithmetic operations, or just for sqrt() ?

Answer 1

[MathWorks Fixed Point Team]

SQRT function computes the square root of a fi object using a bisection algorithm.

You can use additional numerictype to control the output data types:

c = sqrt(a,T) returns the square root of fi object a with numerictype object T. Intermediate quantities are calculated using the fimath associated with a. See Data Type Propagation Rules.

x=ufi(16,12,4)

T = numerictype(0, 12, 4)

y = sqrt(x,T);

The choice of output data type without specifying T is documented as "internal rule" section of the function reference.