MATLAB: Problem in converting for loop in matlab into vhdl code

for loopHDL CoderMATLABsigmoidunbounded loopvhdl

I am trying to implement sigmoid function by converting it from matlab into vhdl. Since , sigmoid function is not supported by hdl coder, I am using following code:
function [ g ] = sigmoid_exp( z )
    y = 1;
for I = 1:z
    y = y * 2.718; 
end
y = 1 / y;
g = 1.0 / (1.0 + y);
end
But I am getting this error: Found an unsupported unbounded loop structure. This loop may be user written or automatically generated due to the use of specific vector expressions or functions. For more information on unsupported loop structures, please refer to the documentation.
I know it can be directly implemented in vhdl but I want to know about this error.
Thankyou

Best Answer

Hi Amanpreet,
This error is because of the following:
1. In code generation, unbounded 'for' loops are not supported.
2. During compile time, HDL Coder does not know the value of z as it depends on the value of the input argument. Since
I = 1:z
is used in the 'for' loop, HDL Coder does not know what the upper bound is.
The workaround is to provide a hard coded value like, say,
I = 1:3
In fact, if you create a variable, say k, and assign it a value of 3 in the function 'sigmoid_exp' and then enter
I = 1:k
this will not result in the error and will generate code successfully.
Hope this helps,
Cheers!
Abhiram.
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