Using fi for modeling precision in hardware model design

I'm modeling a hardware block in Simulink. One of the multiplier modules works with unsigned fixed length bit vectors.

output err: 6 bits

input u: 6 bits

input wt: 4 bits with 3 bits for fractional part

On command line, the following code snippet works:

>> err = fi(0,0,6,0); %an unsingned bit vector of 6 bits

>> u = fi(43,0,6,0);

>> wt = fi(0.25,0,4,3);

>> err.Value = u * wt

err =

11

DataTypeMode: Fixed-point: binary point scaling

Signedness: Unsigned

WordLength: 6

FractionLength: 0

>> err = u * wt

err =

10.7500

DataTypeMode: Fixed-point: binary point scaling

Signedness: Unsigned

WordLength: 10

FractionLength: 3

From a hardware design perspective, I want the first operation.

But if I write the matlab function like:

function err = fcn(u, wt)

err = fi(0,0,6,0); % using this as a variable declaration statement

err.Value = u * wt;

I get the diagnostic error:

Writing properties to 'embedded.fi' is not supported for code generation.

I could re-write the function as below which works. But if I have to assign err multiple times in the function, I need to specify the redundant information (0,6,0) all the time which is inefficient and error prone.

err = fi( (u * wt), 0, 6, 0);

Are there any suggestions to rewrite this code while keeping variable declaration and assignment separated?

Thanks!

Answer 1

[Shivansh]

Hi Anshul,

You can try to pass a prototype of the desired datatype as an input to the function and then use it to initialize the variable.

You can refer to the following code snippet for a better understanding of the approach.

% Define a prototype fi object with the desired properties
proto = fi([], numerictype(0, 6, 0));

function err = fcn(u, wt, proto)
% Use the prototype to initialize the err variable
err = proto;
% Perform the multiplication and assign the result to err
err(:) = u * wt;
end

You can refer to the following documentation for more information about fi data type: https://www.mathworks.com/help/fixedpoint/ref/embedded.fi.html.

I hope this helps resolve the issue!