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Model a Counter for HDL and High-Level Synthesis Code Generation

To write MATLAB® code that models hardware and is suitable for HDL and High-Level Synthesis (HLS) code generation, use this design pattern.

This design pattern demonstrates best practices for writing MATLAB code for HDL and HLS code generation:

  • Initialize persistent variables to a specific value. In this example, an if statement and the isempty function initialize the persistent variable. If you do not initialize the persistent variable, then you cannot generate HDL and HLS code.

  • Inside a function, read persistent variables before they are modified so that the persistent variables are inferred as registers.

This Simulink® model illustrates the MATLAB counter modeled in this example.

Simulink counter model.

To learn how to model the counter in Simulink, see Create HDL-Compatible Simulink Model.

MATLAB Code for the Counter

The function mlhdlc_counter is a behavioral model of a 4-bit synchronous up counter. The input signal, enable_ctr, triggers the value of the count register, count_val, to increase by one. The counter continues to increase by one each time the input is nonzero, until the count reaches a limit of 15. After the counter reaches this limit, the counter returns to zero. A persistent variable, which is initialized to zero, represents the current value of the count. Two if statements determine the value of the count based on the input.

To define the mldhlc_counter and mldhlc_counter_tb, use these codes:

MATLAB CodeMATLAB Testbench
%#codegen
function count = mlhdlc_counter(enable_ctr)
% four bit synchronous up counter

% persistent variable for the state 
persistent count_val;
if isempty(count_val)
    count_val = 0;
end

% counting up
if enable_ctr
    count_val=count_val+1;

    % limit to four bits
    if count_val>15
        count_val=0;
    end
end

count=count_val;

end

for i = 1:100
    if mod(i,5) == 0
        % do not increment the counter if mod(i,5) is zero.
        val = mlhdlc_counter(false);
    else
        val = mlhdlc_counter(true);
    end
end

See Also

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