Sequential Statement

In a process, the execution of sequential statements will be initiated when there is a change in the signals listed in the sensitivity list. In general, the execution of statements in the process body will be carried out continuously until the end of the process sequentially. There are two types of sequential statements that will be discussed in this module:
● If statement
The if statement is used to create a branch in the execution flow of sequential statements. In VHDL, the if statement can only be used inside the process body. Example of a NAND gate with if statement:

library ieee;
use ieee.std_logic_1164.all;

entity nand_gate is
    port(
        A, B : IN STD_LOGIC;
        Y    : OUT STD_LOGIC
    );
end nand_gate;

architecture behavioral of nand_gate is
begin
    nand_proc : process (A, B) is
    begin
        if (A = '1' AND B = '1') then
            Y <= '0';
        else
            Y <= '1';
        end if;
    end process nand_proc;
end behavioral;

● Case statement
The case statement works in a way similar to the previous if statement. The difference is that the case statement will be more efficient to use when there are many variations of values. Example of a NAND gate using case statement:

library ieee;
use ieee.std_logic_1164.all;

entity nand_gate is
    port(
        A, B : IN STD_LOGIC;
        Y    : OUT STD_LOGIC
    );
end nand_gate;

architecture behavioral of nand_gate is
begin
    AB <= A & B;  -- combining signals A and B

    nand_proc : process (A, B) is
    begin
        case (AB) is
            when "11"    => Y <= '0';
            when others  => Y <= '1';
        end case;
    end process nand_proc;
end behavioral;

When using behavioral style, nested sequential statements are common and often used. This is also what makes behavioral style more powerful than data-flow style. However, even though behavioral style statements are used like programming in general, it should be noted that VHDL is a hardware description language, not a programming language. Also, try to always keep the process statement in the description you create as simple as possible to make hardware design easier when the circuit becomes more complex.