【VHDL記述】

library IEEE;

use IEEE.std_logic_1164.all;

entity COUNTER_4 is

port( CK, RESET, X : in std_logic;

Y : out std_logic );

end COUNTER_4;

architecture STATE_MACHINE of COUNTER_4 is

type STATE is ( Q0, Q1, Q2, Q3 );

signal CURRENT_STATE, NEXT_STATE : STATE;

begin

STORAGE : process ( RESET, CK ) begin

if ( RESET = '1' ) then

CURRENT_STATE <= Q0;

elsif ( CK'event and CK = '0' ) then

CURRENT_STATE <= NEXT_STATE;

end if;

end process;

COMB : process ( CURRENT_STATE, X ) begin

case CURRENT_STATE is

when Q0 => if ( X = '1' ) then

NEXT_STATE <= Q1; Y <= '0';

else

NEXT_STATE <= Q0; Y <= '0';

end if;

when Q1 => if ( X = '1' ) then

NEXT_STATE <= Q2; Y <= '0';

else

NEXT_STATE <= Q1; Y <= '0';

end if;

when Q2 => if ( X = '1' ) then

NEXT_STATE <= Q3; Y <= '0';

else

NEXT_STATE <= Q2; Y <= '0';

end if;

when Q3 => if ( X = '1' ) then

NEXT_STATE <= Q0; Y <= '1';

else

NEXT_STATE <= Q3; Y <= '0';

end if;

end case;

end process;

end STATE_MACHINE;

【Verilog-HDL記述】

module COUNTER_4 (

CK, RESET, X,

Y

);

input CK, RESET, X;

output Y;

parameter Q0 = 2'b00, Q1 = 2'b01, Q2 = 2'b10, Q3 = 2'b11;

reg[ 1 : 0 ] CURRENT_STATE;

wire[ 1 : 0 ] NEXT_STATE;

always @ ( negedge CK or posedge RESET ) begin

if ( RESET ) begin

CURRENT_STATE <= Q0;

end else begin

CURRENT_STATE <= NEXT_STATE;

end

end

assign NEXT_STATE = FUNC_NS( CURRENT_STATE, X );

assign Y = (( CURRENT_STATE == Q3 ) & X ) ? 1'b1 : 1'b0;

function[ 1 : 0 ] FUNC_NS;

input[ 1 : 0 ] CURRENT_STATE;

input X;

begin

case ( CURRENT_STATE )

Q0: FUNC_NS = X ? Q1 : Q0;

Q1: FUNC_NS = X ? Q2 : Q1;

Q2: FUNC_NS = X ? Q3 : Q2;

default: FUNC_NS = X ? Q0 : Q3;

endcase

end

endfunction

endmodule