Why do the data become zero when using the function fi?

2 min readSep 14, 2021

fm = get_fimath();

idx = fi(1,0,1,0,fm);
a = (idx+fi(2,0,2,0,fm))*fi(1/3,0,16,17,fm);
k = fi(a,0,17,0,fm)

function fm = get_fimath()
	fm = fimath('RoundingMethod', 'Floor',...
	     'OverflowAction', 'Wrap',...
	     'ProductMode','FullPrecision',...
	     'MaxProductWordLength', 128,...
	     'SumMode','FullPrecision',...
	     'MaxSumWordLength', 128);
end

This code is generated when using the Matlab Coder . I want to know why is k equal to zero? Is it because of division 1/3?

ANSWER

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It's just like scientific notation

is the short answer to "Why FractionLength can be bigger than WordLength?".

The long answer is the following.

The concept of a binary-point is very useful for initial understanding of fixed-point types. Similarly, the concept of a decimal-point is useful for understanding values beyond integers. But using decimal-points becomes very cumbersome for very big or very small numbers. To make it easy to represent very big or very small values, scientific notation is super valuable.

verySmallNumber = 3e-200;
veryBigNumber = 7e123;

In essence, this notation breaks the value into two parts, a mantissa and an integer exponent for the given base.

Y = mantissa .* 10.^exponent

Fixed-point follows the same concept except that

base is 2
mantissa must be an integer
exponent is fixed, i.e. it is part of the variables type and does not change for the life of the variable

Y = intMantissa .* 2^FixedExponent

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