Model a Counter for HDL Code Generation

This design pattern shows a MATLAB^® example of a counter, which is suitable for HDL code generation.

MATLAB Counter

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

Initialize persistent variables to a specific value. In this example, an if statement and the isempty function initialize the persistent variable. If the persistent variable is not initialized then HDL code cannot be generated.
Inside a function, read persistent variables before they are modified, in order for the persistent variables to be inferred as registers.

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

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 four 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.

The following section of code defines the mldhlc_counter function.

%#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

Documentation