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Digital Systems & FPGA Programming

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Behavioral statements are declared inside an always or initial block. There are three possible statements, if-else case and loop.

If-else Statements

It has the same format as many programming languages. It tests a condition and executes the code inside the if block and if the condition isn’t met the code inside the else block is executed. Example of a multiplexer:

always@(a or b or sel) begin
    if (sel)
        out = a;
    else
        out = b;
end

Case Statements

Case the variable or expression is equal some value inside the case block the statement where it is true is executed.

reg a;
case (a)
    1'b0 : statement1;
    1'b1 : statement2;
    1'bx : statement3;
    1'bz : statement4;
endcase

There are also the forms of the case block that are casez and casex. The casez treats the values as don’t cares and all ‘z’ is represented by ‘?’ . The casex is the same but treats ‘x’ and ‘z’ as don’t cares. The default option is used when any other condition is met.

always @(irq) begin
    {int2, int1, int0} = 3'b000;
    casez (irq)
        3'b1?? : int2 = 1'b1;
        3'b?1? : int1 = 1'b1;
        3'b??1 : int0 = 1'b1;
        default: {int2, int1, int0} = 3'b000;
    endcase
end

Forever and Repeat Loops

They are used for test and simulation and cannot be synthesized. The forever loop executes continuously and never stops. The loop repeat executes a finite amount of times.

forever

initial
begin
    forever
    #25 clock = ~clock;  //executes forever and at 25 periods of time clock changes state
end

repeat

if(rotate == 1)
repeat(8)            // if rotates = 1 rotates data 8 times
begin
    tmp = data[15];
    data = data << 1 + tmp;
end

While Loop

Executes the block if the condition is true, repeats the test and executes again until the condition is not met.

always@(a or b) begin

while(count < 12) begin  // count 12 times and terminate
count = count + 1;
end

end

For Loop

Executes the code a finite amount of times. If the condition is true it enters the block, after the end it repeats the test and executes again until the condition is not met. In the example, the condition is i < 16. In the beginning the i variable is declared as zero and it increases by one at the end of every loop.

always@(a or b) begin
for (i = 0; i < 16; i = i +1) begin
    a <= a + 1; // executes this code 16 times
end
end

References

Verilog HDL Basics - Altera