PRBS Generator:

      Pseudo random binary sequence is essentially a random sequence of binary numbers. So PRBS generator is nothing but random binary number generator.  It is ‘random’ in a sense that the value of an element of the sequence is independent of the values of any of the other elements. It is 'pseudo' because it is deterministic and after N elements it starts to repeat itself, unlike real random sequences. The  implementation  of  PRBS  generator  is  based  on  the  linear  feedback  shift  register (LFSR). The PRBS generator produces a predefined sequence of 1's and 0's, with 1 and 0 occurring with the same probability. A sequence of consecutive n*(2^n -1) bits comprise one data pattern, and this pattern will repeat itself over time.

A PRBS generator is implemented in VHDL, using LFSRs.(Linear feedback Shift Registers which is made up of D-Flip-flops).Here is an example of How it can be done using VHDL.

 

VHDL Code:
library IEEE; use IEEE.STD_LOGIC_1164.ALL; use IEEE.STD_LOGIC_ARITH.ALL; use IEEE.STD_LOGIC_UNSIGNED.ALL;     entity dff is Port ( CLK : in std_logic; RSTn : in std_logic; D : in std_logic; Q : out std_logic); end dff;     architecture Behavioral of dff is begin process(CLK) begin if CLK'event and CLK='1' then if RSTn='1' then Q <= '1'; else Q <= D; end if; end if; end process; end Behavioral;       VHDL CODE FOR PRBS Generator using LFSR:       library IEEE; use IEEE.STD_LOGIC_1164.ALL;     entity lfsr is Port ( CLK : in STD_LOGIC; RSTn : in STD_LOGIC; data_out : out STD_LOGIC_VECTOR (15 downto 0)); end lfsr;   architecture Behavioral of lfsr is   component dff Port ( CLK : in std_logic; RSTn : in std_logic; D : in std_logic; Q : out std_logic); end component;   signal data_reg : std_logic_vector(15 downto 0); signal tap_data : std_logic;   begin process(CLK) begin tap_data <= (data_reg(1) xor data_reg(2)) xor (data_reg(4) xor data_reg(15)); end process;   stage0: dff port map(CLK, RSTn, tap_data, data_reg(0));   g0:for i in 0 to 14 generate   stageN: dff port map(CLK, RSTn, data_reg(i), data_reg(i+1)); end generate; data_out <= data_reg after 3 ns; end Behavioral;   VHDL Test Bench for Simulation:   LIBRARY ieee; USE ieee.std_logic_1164.ALL;   ENTITY testprbs IS END testprbs;   ARCHITECTURE behavior OF testprbs IS -- Component Declaration for the Unit Under Test (UUT) COMPONENT lfsr PORT( CLK : IN std_logic; RSTn : IN std_logic; data_out : OUT std_logic_vector(15 downto 0) ); END COMPONENT; signal CLK : std_logic := '0'; signal RSTn : std_logic := '0'; signal data_out : std_logic_vector(15 downto 0); -- Clock period definitions constant CLK_period : time := 10 ns;   BEGIN -- Instantiate the Unit Under Test (UUT) uut: lfsr PORT MAP ( CLK => CLK, RSTn => RSTn, data_out => data_out ); CLK_process :process   begin CLK <= '0'; wait for CLK_period/2; CLK <= '1'; wait for CLK_period/2; end process;   -- Stimulus process stim_proc: process begin wait for 10 ns; wait for CLK_period*1; RSTn <= '1'; wait for CLK_period*1; RSTn <= '0'; wait; end process;   END;