fpga4student.com FPGA digital design projects using Verilog/ VHDL: A complete 8-bit Microcontroller in VHDL

Saturday, December 24, 2016

A complete 8-bit Microcontroller in VHDL

In this project, a complete 8-bit microcontroller is designed, implemented, and operational as a full design which users can program the microcontroller using assembly language. 

The microcontroller has an 8-bit processor, a 128-byte program memory, a 96-byte RAM, 16x8-bit output ports, and 16x8-bit input ports. Users can program the microcontroller by inserting opcodes and operands in the program memory.

After completing the design, the microcontroller is implemented on FPGA DE0-nano board as shown in the figure below.
Microcontroller in VHDL

Microcontroller in VHDL
Below is the top level block diagram of the 8-bit microcontroller:
Microcontroller in VHDL
The CPU as shown in the following figure has two 8-bit registers (A and B), an Arithmetic Logical Unit (ALU) to perform math and logic operations, and a control unit to handle the operations of instructions. 
Microcontroller in VHDL
  • Instruction Register (IR): contains the content of the current instruction. 
  • Memory Address Register (MAR): holds the current address to access the memory. It can be from program counter or from an operand of an instruction.
  • Program Counter (PC): holds the next location in program memory for the next instruction.
  • A & B: two 8-bit registers being controlled by users.
  • Arithmetic Logical Unit (ALU): performs math and logical operations such as addition, subtraction, logical AND, logical OR, increments, and decrements. Inputs of the ALU are from Bus1 and register B. The output of the ALU goes to the multiplexer driving Bus2. The functionality of the ALU is selected by the ALU_Sel lines (3 bits) where "000" = ADD, "001" = SUB, "010" = AND, "011" = OR, "100" = Increment, and "101" = Decrement.
  • Condition Code Register (CCR): holds the status information of the results at the ALU. It contains a negative flag (N), a zero flag (Z), a two's complement overflow flag (V), and a carry flag (C).
The memory map of the CPU is as follows:
Microcontroller in VHDL
It consists of 128 bytes ROM, 96 bytes RAM, 16 output ports, and 16 input ports.
Microcontroller in VHDL


The Instruction Set of the 8-bit computer is as follows:
Microcontroller in VHDL
The state diagram is shown in the following figure:
Microcontroller in VHDL
The following waveform is an example of the execution of the LDA_IMM instruction.
Microcontroller in VHDL
The VHDL code for the ALU of the microcontroller:
 library IEEE;  
 use IEEE.STD_LOGIC_1164.ALL;  
 USE ieee.std_logic_arith.all;  
 USE ieee.std_logic_unsigned.all;  
 entity ALU is  
 port (  
                A,B: in std_logic_vector(7 downto 0);  
                ALU_Sel:in std_logic_vector(2 downto 0);   
                NZVC: out std_logic_vector(3 downto 0);  
                Result: out std_logic_vector(7 downto 0)  
           );  
 end ALU;  
 architecture Behavioral of ALU is  
 signal ALU_Result:std_logic_vector(7 downto 0);  
 signal ALU_ADD: std_logic_vector(8 downto 0);  
 signal C,Z,V,N,add_ov,sub_ov: std_logic;  
 begin  
 process(ALU_Sel,A,B)  
 begin  
           ALU_ADD <= "000000000";  
      case(ALU_Sel) is  
           when "000" => -- ADD  
                ALU_ADD <= ('0' & A )+ ('0'& B);  
                ALU_Result <= A + B;  
           when "001" => -- SUB  
                ALU_Result <= B - A;       
                ALU_ADD      <= ('0' & B) - ('0' & A);       
           when "010" => -- AND  
                ALU_Result <= A and B;  
           when "011" => -- OR  
                ALU_Result <= A or B;            
           when "100" => -- Increment  
                ALU_Result <= A + x"01";                 
           when "101" => -- Decrement  
                ALU_Result <= A - x"01";       
           when others =>  
                ALU_Result <= A + B;  
      end case;  
 end process;  
      Result <= ALU_Result;  
      N <= ALU_Result(7);  
      Z <= '1' when ALU_Result = x"00" else  
            '0';  
 ---Overflow flag---------------------------------------  
   add_ov<= (A(7)and B(7)    and (not ALU_Result(7))) or ((not A(7))and (not B(7)) and ALU_Result(7));  
   sub_ov<= (A(7)and (not B(7)) and (not ALU_Result(7))) or ((not A(7))and B(7)    and ALU_Result(7));   
   with ALU_Sel select  
    V    <= add_ov when "000",  
          sub_ov when "001",  
          '0' when others;  
      -- Carry out flag  
   with ALU_Sel select  
    C <=  ALU_ADD(8) when "000",  
        ALU_ADD(8) when "001",  
          '0' when others;  
      NZVC <= N & Z & V & C;  
 end Behavioral;  
The VHDL code for RAM memory of the microcontroller:
 library IEEE;  
 use IEEE.STD_LOGIC_1164.ALL;  
 -----------------------------------------------------------------  
 ------------ MEMORY SYSTEM for 8-BIT COMPUTER -------------------  
 -----------------------------------------------------------------  
 entity memory is  
 port (  
                address: in std_logic_vector(7 downto 0);  
                data_in: in std_logic_vector(7 downto 0);  
                write: in std_logic;  
                port_in_00: in std_logic_vector(7 downto 0);   
      port_in_01: in std_logic_vector(7 downto 0);  
      port_in_02: in std_logic_vector(7 downto 0);    
      port_in_03: in std_logic_vector(7 downto 0);   
      port_in_04: in std_logic_vector(7 downto 0);   
      port_in_05: in std_logic_vector(7 downto 0);   
      port_in_06: in std_logic_vector(7 downto 0);   
      port_in_07: in std_logic_vector(7 downto 0);   
      port_in_08: in std_logic_vector(7 downto 0);   
      port_in_09: in std_logic_vector(7 downto 0);  
      port_in_10: in std_logic_vector(7 downto 0);   
      port_in_11: in std_logic_vector(7 downto 0);   
      port_in_12: in std_logic_vector(7 downto 0);   
      port_in_13: in std_logic_vector(7 downto 0);   
      port_in_14: in std_logic_vector(7 downto 0);    
      port_in_15: in std_logic_vector(7 downto 0);  
                clock,reset: in std_logic;  
                data_out: out std_logic_vector(7 downto 0);  
                port_out_00: out std_logic_vector(7 downto 0);   
      port_out_01: out std_logic_vector(7 downto 0);  
      port_out_02: out std_logic_vector(7 downto 0);    
      port_out_03: out std_logic_vector(7 downto 0);   
      port_out_04: out std_logic_vector(7 downto 0);   
      port_out_05: out std_logic_vector(7 downto 0);   
      port_out_06: out std_logic_vector(7 downto 0);   
      port_out_07: out std_logic_vector(7 downto 0);   
      port_out_08: out std_logic_vector(7 downto 0);   
      port_out_09: out std_logic_vector(7 downto 0);  
      port_out_10: out std_logic_vector(7 downto 0);   
      port_out_11: out std_logic_vector(7 downto 0);   
      port_out_12: out std_logic_vector(7 downto 0);   
      port_out_13: out std_logic_vector(7 downto 0);   
      port_out_14: out std_logic_vector(7 downto 0);    
      port_out_15: out std_logic_vector(7 downto 0)  
           );  
 end memory;  
 architecture Behavioral of memory is  
 component rom_128x8_sync   
 port (  
                address: in std_logic_vector(6 downto 0);  
                data_out: out std_logic_vector(7 downto 0);  
                clock: in std_logic  
           );  
 end component rom_128x8_sync;  
 component rw_96x8_sync   
 port(  
                address: in std_logic_vector(6 downto 0);  
                data_in: in std_logic_vector(7 downto 0);  
                write: in std_logic;  
                clock: in std_logic;  
                data_out: out std_logic_vector(7 downto 0)  
           );  
 end component rw_96x8_sync;  
 component Output_Ports   
 port (  
                address: in std_logic_vector(3 downto 0);  
                data_in: in std_logic_vector(7 downto 0);  
                write: in std_logic;  
                clock: in std_logic;  
                reset: in std_logic;  
                port_out_00: out std_logic_vector(7 downto 0);   
      port_out_01: out std_logic_vector(7 downto 0);  
      port_out_02: out std_logic_vector(7 downto 0);    
      port_out_03: out std_logic_vector(7 downto 0);   
      port_out_04: out std_logic_vector(7 downto 0);   
      port_out_05: out std_logic_vector(7 downto 0);   
      port_out_06: out std_logic_vector(7 downto 0);   
      port_out_07: out std_logic_vector(7 downto 0);   
      port_out_08: out std_logic_vector(7 downto 0);   
      port_out_09: out std_logic_vector(7 downto 0);  
      port_out_10: out std_logic_vector(7 downto 0);   
      port_out_11: out std_logic_vector(7 downto 0);   
      port_out_12: out std_logic_vector(7 downto 0);   
      port_out_13: out std_logic_vector(7 downto 0);   
      port_out_14: out std_logic_vector(7 downto 0);    
      port_out_15: out std_logic_vector(7 downto 0)  
           );  
 end component Output_Ports;  
 signal rom_out,ram_out:std_logic_vector(7 downto 0);  
 signal output_port_addr: std_logic_vector(3 downto 0);  
 signal ram_address,rom_address: std_logic_vector(6 downto 0);  
 begin  
       ram_address <= address(6 downto 0) when (address(7)= '1') else  
                                "0000000";  
       rom_address <= address(6 downto 0) when (address(7)='0') else  
                                "0000000";  
       output_port_addr <= address(3 downto 0) when (address(7 downto 4)= x"E") else  
                                "0000";  
       rom_128x8_sync_u: rom_128x8_sync port map  
                     (  
                          address => rom_address,  
                          clock  => clock,  
                          data_out => rom_out  
                     );  
       rw_96x8_sync_u: rw_96x8_sync port map   
                     (  
                          address      => ram_address,  
                          data_in      => data_in,  
                          write      => write,  
                          clock      => clock,  
                          data_out => ram_out  
                     );  
      Output_Ports_u: Output_Ports port map  
                     (  
                          address           => output_port_addr,  
                          data_in           => data_in,  
                          write               => write,  
                          clock               => clock,  
                          reset           => reset,  
                          port_out_00 => port_out_00,  
                          port_out_01 => port_out_01,  
                          port_out_02 => port_out_02,  
                          port_out_03 => port_out_03,   
                          port_out_04 => port_out_04,  
                          port_out_05 => port_out_05,  
                          port_out_06 => port_out_06,  
                          port_out_07 => port_out_07,  
                          port_out_08 => port_out_08,  
                          port_out_09 => port_out_09,  
                          port_out_10 => port_out_10,  
                          port_out_11 => port_out_11,  
                          port_out_12 => port_out_12,  
                          port_out_13 => port_out_13,  
                          port_out_14 => port_out_14,   
                          port_out_15 => port_out_15  
                     );  
      --- Multiplexer Output  
      data_out <= rom_out when address < x"80" else  
                          ram_out when address < x"E0" else  
                          port_in_00 when address = x"F0" else  
                          port_in_01 when address = x"F1" else  
                          port_in_02 when address = x"F2" else  
                          port_in_03 when address = x"F3" else  
                          port_in_04 when address = x"F4" else  
                          port_in_05 when address = x"F5" else  
                          port_in_06 when address = x"F6" else  
                          port_in_07 when address = x"F7" else  
                          port_in_08 when address = x"F8" else  
                          port_in_09 when address = x"F9" else  
                          port_in_10 when address = x"FA" else  
                          port_in_11 when address = x"FB" else  
                          port_in_12 when address = x"FC" else  
                          port_in_13 when address = x"FD" else  
                          port_in_14 when address = x"FE" else  
                          port_in_15 when address = x"FF" else  
                          x"00";  
 end Behavioral;  
The VHDL code for the data-path of the microcontroller:
 library IEEE;  
 use IEEE.STD_LOGIC_1164.ALL;  
 USE ieee.std_logic_arith.all;  
 USE ieee.std_logic_unsigned.all;  
 -------------------------  
 -- data_path  
 -------------------------  
 entity data_path is  
 port (            
                clock,reset: in std_logic;  
                IR_Load: in std_logic;  
                IR: out std_logic_vector(7 downto 0);  
                MAR_Load: in std_logic;  
                address: out std_logic_vector(7 downto 0);  
                PC_Load: in std_logic;  
                PC_Inc: in std_logic;  
                A_Load: in std_logic;  
                B_Load: in std_logic;  
                ALU_Sel: in std_logic_vector(2 downto 0);  
                CCR_Result: out std_logic_vector(3 downto 0);  
                CCR_Load: in std_logic;  
                Bus2_Sel: in std_logic_vector(1 downto 0);  
                Bus1_Sel: in std_logic_vector(1 downto 0);  
                from_memory: in std_logic_vector(7 downto 0);  
                to_memory: out std_logic_vector(7 downto 0)  
           );  
 end data_path;  
 architecture Behavioral of data_path is  
 component ALU  
 port (  
                A,B: in std_logic_vector(7 downto 0);  
                ALU_Sel:in std_logic_vector(2 downto 0);   
                NZVC: out std_logic_vector(3 downto 0);  
                Result: out std_logic_vector(7 downto 0)  
           );  
 end component ALU;  
 signal BUS2,BUS1,ALU_Result: std_logic_vector(7 downto 0);  
 signal IR_Reg,MAR,PC,A_Reg,B_Reg: std_logic_vector(7 downto 0);  
 signal CCR_in,CCR: std_logic_vector(3 downto 0);  
 begin  
      -- Instruction Register  
      process(clock,reset)  
      begin  
           if(reset='0') then  
                IR_Reg <= x"00";  
           elsif(rising_edge(clock)) then  
                if(IR_Load='1') then  
                     IR_Reg <= BUS2;  
                end if;  
           end if;  
      end process;  
      IR <= IR_Reg;  
      -- MAR Register  
      process(clock,reset)  
      begin  
           if(reset='0') then  
                MAR <= x"00";  
           elsif(rising_edge(clock)) then  
                if(MAR_Load='1') then  
                     MAR <= BUS2;  
                end if;  
           end if;  
      end process;  
      address <= MAR;  
      -- PC  
      process(clock,reset)  
      begin  
           if(reset='0') then  
                PC <= x"00";  
           elsif(rising_edge(clock)) then  
                if(PC_Load='1') then  
                     PC <= BUS2;  
                elsif(PC_Inc='1') then  
                     PC <= PC + x"01";  
                end if;  
           end if;  
      end process;  
      -- A register  
      process(clock,reset)  
      begin  
           if(reset='0') then  
                A_Reg <= x"00";  
           elsif(rising_edge(clock)) then  
                if(A_Load='1') then  
                     A_Reg <= BUS2;  
                end if;  
           end if;  
      end process;  
      -- B register  
      process(clock,reset)  
      begin  
           if(reset='0') then  
                B_Reg <= x"00";  
           elsif(rising_edge(clock)) then  
                if(B_Load='1') then  
                     B_Reg <= BUS2;  
                end if;  
           end if;  
      end process;       
      --- ALU  
      ALU_unit: ALU port map  
      (  
                A => B_Reg,  
                B => BUS1,  
                ALU_Sel => ALU_Sel,  
                NZVC => CCR_in,  
                Result => ALU_Result  
      );  
      --- CCR Register  
      process(clock,reset)  
      begin  
           if(reset='0') then  
                CCR <= x"0";  
           elsif(rising_edge(clock)) then  
                if(CCR_Load='1') then  
                     CCR <= CCR_in;  
                end if;  
           end if;  
      end process;  
      CCR_Result <= CCR;  
      ---- MUX BUS2  
      BUS2 <=      ALU_Result      when Bus2_Sel = "00" else  
                     BUS1                 when Bus2_Sel = "01" else  
                     from_memory when Bus2_Sel = "10" else  
                     x"00";  
      --- MUX BUS1  
      BUS1 <=      PC                when Bus1_Sel = "00" else  
                     A_Reg           when Bus1_Sel = "01" else  
                     B_Reg            when Bus1_Sel = "10" else  
                     x"00";  
      to_memory <= BUS1;  
 end Behavioral;  
The VHDL code for the control unit of the microcontroller:
 library IEEE;  
 use IEEE.STD_LOGIC_1164.ALL;  
 -- control unit  
 entity control_unit is  
 port (  
                clock,reset: in std_logic;  
                IR_Load: out std_logic;  
                IR: in std_logic_vector(7 downto 0);  
                MAR_Load: out std_logic;  
                PC_Load: out std_logic;  
                PC_Inc: out std_logic;  
                A_Load: out std_logic;  
                B_Load:out std_logic;  
                ALU_Sel:out std_logic_vector(2 downto 0);  
                CCR_Result: in std_logic_vector(3 downto 0);  
                CCR_Load: out std_logic;  
                Bus2_Sel: out std_logic_vector(1 downto 0);  
                Bus1_Sel: out std_logic_vector(1 downto 0);  
                write: out std_logic  
           );  
 end control_unit;  
 architecture Behavioral of control_unit is  
 type FSM is (S_FETCH_0,S_FETCH_1,S_FETCH_2,S_DECODE_3,S_LOAD_AND_STORE_4,S_LOAD_AND_STORE_5,S_LOAD_AND_STORE_6,       
                                                                                                     S_LOAD_AND_STORE_7,S_DATA_MAN_4,  
                                                    S_BRANCH_4,S_BRANCH_5,S_BRANCH_6                                                                                                      
                                                                                                     );  
 signal current_state,next_state: FSM;  
 signal LOAD_STORE_OP,DATA_MAN_OP,BRANCH_OP: std_logic;  
 begin  
      -- FSM State FFs  
      process(clock,reset)  
      begin  
           if(reset='0') then  
                current_state <= S_FETCH_0;  
           elsif(rising_edge(clock)) then  
                current_state <= next_state;  
           end if;  
      end process;  
      process(current_state,IR,CCR_Result,LOAD_STORE_OP,DATA_MAN_OP,BRANCH_OP)  
      begin  
                IR_Load <= '0';  
                MAR_Load <= '0';  
                PC_Load <= '0';  
                PC_Inc <= '0';  
                A_Load <= '0';  
                B_Load <= '0';  
                ALU_Sel <= "000";  
                CCR_Load <= '0';  
                Bus2_Sel <= "00";  
                Bus1_Sel <= "00";  
                write <= '0';  
           case(current_state) is  
                when S_FETCH_0 =>   
                     Bus1_Sel <= "00"; -- PC  
                     Bus2_Sel <= "01"; -- BUS1  
                     MAR_Load <= '1';  
                     next_state <= S_FETCH_1;  
                when S_FETCH_1 =>  
                     PC_Inc <= '1';  
                     next_state <= S_FETCH_2;  
                when S_FETCH_2 =>  
                     Bus2_Sel <= "10";  
                     IR_Load <= '1';  
                     next_state <= S_DECODE_3;  
                when S_DECODE_3 =>  
                     if(LOAD_STORE_OP='1') then  
                          next_state <= S_LOAD_AND_STORE_4;  
                     elsif(DATA_MAN_OP='1') then  
                          next_state <= S_DATA_MAN_4;  
                     elsif(BRANCH_OP='1') then  
                          next_state <= S_BRANCH_4;  
                     end if;  
                ---- LOAD AND STORE INSTRUCTIONS:  
                when S_LOAD_AND_STORE_4 =>  
                     if(IR >= x"86" and IR <= x"89")then -- LOAD IMMEDIATE  
                          Bus1_Sel <= "00";  
                          Bus2_Sel <= "01";  
                          MAR_Load <= '1';            
                     elsif(IR = x"96" or IR = x"97")then -- LOAD IMMEDIATE  
                          Bus1_Sel <= "00";  
                          Bus2_Sel <= "01";  
                          MAR_Load <= '1';                                
                     end if;  
                     next_state <= S_LOAD_AND_STORE_5;  
                when S_LOAD_AND_STORE_5 =>  
                     if(IR >= x"86" and IR <= x"89")then -- LOAD IMMEDIATE  
                          PC_Inc <= '1';  
                     elsif(IR = x"96" or IR = x"97")then  
                          PC_Inc <= '1';  
                     end if;  
                     next_state <= S_LOAD_AND_STORE_6;  
                when S_LOAD_AND_STORE_6 =>  
                     if(IR=x"86")then -- LOAD IMMEDIATE A  
                          Bus2_Sel <= "10";  
                          A_Load <= '1';  
                          next_state <= S_FETCH_0;  
                     elsif(IR=x"87" or IR=x"89") then -- LOAD A DIRECT  
                          Bus2_Sel <= "10";  
                          MAR_Load <= '1';            
                          next_state <= S_LOAD_AND_STORE_7;  
                     elsif(IR=x"88")then -- LOAD IMMEDIATE B  
                          Bus2_Sel <= "10";  
                          B_Load <= '1';  
                          next_state <= S_FETCH_0;       
                     elsif(IR = x"96" or IR = x"97")then  
                          Bus2_Sel <= "10";  
                          MAR_Load <= '1';            
                          next_state <= S_LOAD_AND_STORE_7;                           
                     end if;  
                when S_LOAD_AND_STORE_7 =>  
                          if(IR=x"87") then  
                               Bus2_Sel <= "10";  
                               A_Load <= '1';            
                               next_state <= S_FETCH_0;  
                          elsif(IR=x"89") then  
                               Bus2_Sel <= "10";  
                               B_Load <= '1';            
                               next_state <= S_FETCH_0;       
                          elsif(IR=x"96") then  
                               write <= '1';  
                               Bus1_Sel <= "01";  
                               next_state <= S_FETCH_0;  
                          elsif(IR=x"97") then  
                               write <= '1';  
                               Bus1_Sel <= "10";  
                               next_state <= S_FETCH_0;  
                          end if;  
                ---- DATA MANIPULATION INSTRUCTIONS:  
                when S_DATA_MAN_4 =>   
                     CCR_Load <= '1';  
                     if(IR=x"42") then  
                          ALU_Sel <= "000";  
                          Bus1_Sel <= "01";  
                          Bus2_Sel <= "00";  
                          A_Load <= '1';  
                     elsif(IR=x"43") then  
                          ALU_Sel <= "001";  
                          Bus1_Sel <= "01";  
                          Bus2_Sel <= "00";  
                          A_Load <= '1';  
                     elsif(IR=x"44") then  
                          ALU_Sel <= "010";  
                          Bus1_Sel <= "01";  
                          Bus2_Sel <= "00";  
                          A_Load <= '1';            
                     elsif(IR=x"45") then  
                          ALU_Sel <= "011";  
                          Bus1_Sel <= "01";  
                          Bus2_Sel <= "00";  
                          A_Load <= '1';            
                     elsif(IR=x"46") then  
                          ALU_Sel <= "100";  
                          Bus1_Sel <= "01";  
                          Bus2_Sel <= "00";  
                          A_Load <= '1';                      
                     elsif(IR=x"47") then  
                          ALU_Sel <= "100";  
                          Bus1_Sel <= "10";  
                          Bus2_Sel <= "00";  
                          B_Load <= '1';                 
                     elsif(IR=x"48") then  
                          ALU_Sel <= "101";  
                          Bus1_Sel <= "01";  
                          Bus2_Sel <= "00";  
                          A_Load <= '1';       
                     elsif(IR=x"49") then  
                          ALU_Sel <= "101";  
                          Bus1_Sel <= "10";  
                          Bus2_Sel <= "00";  
                          B_Load <= '1';                                
                     end if;  
                     next_state <= S_FETCH_0;  
                when S_BRANCH_4 =>  
                     if(IR >= x"20" and IR <= x"28")then -- BRA  
                          Bus1_Sel <= "00";  
                          Bus2_Sel <= "01";  
                          MAR_Load <= '1';                                
                     end if;  
                     next_state <= S_BRANCH_5;  
                when S_BRANCH_5 =>  
                     if(IR >= x"20" and IR <= x"28")then -- BRA  
                          PC_Inc <= '1';  
                     end if;  
                     next_state <= S_BRANCH_6;  
                when S_BRANCH_6 =>  
                     if(IR=x"20")then -- BRA  
                          Bus2_Sel <= "10";  
                          PC_Load <= '1';       
                     elsif(IR=x"21")then -- BMI  
                          Bus2_Sel <= "10";  
                          if(CCR_Result(3)='1') then  
                               PC_Load <= '1';  
                          end if;  
                     elsif(IR=x"22")then -- BPL  
                          Bus2_Sel <= "10";  
                          if(CCR_Result(3)='0') then  
                               PC_Load <= '1';  
                          end if;  
                     elsif(IR=x"23")then -- BEQ  
                          Bus2_Sel <= "10";  
                          if(CCR_Result(2)='1') then-- Z = '1'?  
                               PC_Load <= '1';  
                          end if;  
                     elsif(IR=x"24")then -- BNE  
                          Bus2_Sel <= "10";  
                          if(CCR_Result(2)='0') then-- Z = '0'?  
                               PC_Load <= '1';  
                          end if;  
                     elsif(IR=x"25")then -- BVS  
                          Bus2_Sel <= "10";  
                          if(CCR_Result(1)='1') then-- V = '1'?  
                               PC_Load <= '1';  
                          end if;  
                     elsif(IR=x"26")then -- BVC  
                          Bus2_Sel <= "10";  
                          if(CCR_Result(1)='0') then-- V = '0'?  
                               PC_Load <= '1';  
                          end if;  
                     elsif(IR=x"27")then -- BCS  
                          Bus2_Sel <= "10";  
                          if(CCR_Result(0)='1') then-- C = '1'?  
                               PC_Load <= '1';  
                          end if;  
                     elsif(IR=x"28")then -- BCC  
                          Bus2_Sel <= "10";  
                          if(CCR_Result(0)='0') then-- C = '0'?  
                               PC_Load <= '1';  
                          end if;  
                     end if;  
                     next_state <= S_FETCH_0;  
                when others =>  
                     next_state <= S_FETCH_0;       
           end case;  
      end process;  
      LOAD_STORE_OP <= '1' when IR = x"86" else  
                                '1' when IR = x"87" else  
                                '1' when IR = x"88" else  
                                '1' when IR = x"89" else  
                                '1' when IR = x"96" else  
                                '1' when IR = x"97" else            
                                '0';  
      DATA_MAN_OP <=  '1' when (IR >= x"42" and IR <=x"49") else       
                                '0';  
      BRANCH_OP  <=  '1' when (IR >= x"20" and IR <= x"28") else       
                                '0';  
 end Behavioral;  
The VHDL code for the CPU of the microcontroller:
 library IEEE;  
 use IEEE.STD_LOGIC_1164.ALL;  
 -- CPU  
 entity cpu is  
 port(  
                clock, reset: in std_logic;  
                address: out std_logic_vector(7 downto 0);  
                from_memory: in std_logic_vector(7 downto 0);  
                write: out std_logic;  
                to_memory: out std_logic_vector(7 downto 0)  
           );  
 end cpu;  
 architecture Behavioral of cpu is  
 component control_unit   
 port (  
                clock,reset: in std_logic;  
                IR_Load: out std_logic;  
                IR: in std_logic_vector(7 downto 0);  
                MAR_Load: out std_logic;  
                PC_Load: out std_logic;  
                PC_Inc: out std_logic;  
                A_Load: out std_logic;  
                B_Load:out std_logic;  
                ALU_Sel:out std_logic_vector(2 downto 0);  
                CCR_Result: in std_logic_vector(3 downto 0);  
                CCR_Load: out std_logic;  
                Bus2_Sel: out std_logic_vector(1 downto 0);  
                Bus1_Sel: out std_logic_vector(1 downto 0);  
                write: out std_logic  
           );  
 end component control_unit;  
 component data_path   
 port (            
                clock,reset: in std_logic;  
                IR_Load: in std_logic;  
                IR: out std_logic_vector(7 downto 0);  
                MAR_Load: in std_logic;  
                address: out std_logic_vector(7 downto 0);  
                PC_Load: in std_logic;  
                PC_Inc: in std_logic;  
                A_Load: in std_logic;  
                B_Load: in std_logic;  
                ALU_Sel: in std_logic_vector(2 downto 0);  
                CCR_Result: out std_logic_vector(3 downto 0);  
                CCR_Load: in std_logic;  
                Bus2_Sel: in std_logic_vector(1 downto 0);  
                Bus1_Sel: in std_logic_vector(1 downto 0);  
                from_memory: in std_logic_vector(7 downto 0);  
                to_memory: out std_logic_vector(7 downto 0)  
           );  
 end component data_path;  
 signal               IR_Load: std_logic;  
 signal               IR: std_logic_vector(7 downto 0);  
 signal               MAR_Load: std_logic;  
 signal               PC_Load: std_logic;  
 signal               PC_Inc: std_logic;  
 signal               A_Load: std_logic;  
 signal               B_Load: std_logic;  
 signal               ALU_Sel: std_logic_vector(2 downto 0);  
 signal               CCR_Result: std_logic_vector(3 downto 0);  
 signal               CCR_Load: std_logic;  
 signal               Bus2_Sel: std_logic_vector(1 downto 0);  
 signal               Bus1_Sel: std_logic_vector(1 downto 0);  
 begin  
 -----------------------  
 -- control_unit  
 control_unit_module: control_unit port map  
 (  
                clock => clock,  
                reset => reset,  
                IR_Load => IR_Load,  
                IR => IR,  
                MAR_Load => MAR_Load,  
                PC_Load => PC_Load,  
                PC_Inc => PC_Inc,  
                A_Load => A_Load,  
                B_Load => B_Load,  
                ALU_Sel => ALU_Sel,  
                CCR_Result => CCR_Result,  
                CCR_Load => CCR_Load,  
                Bus2_Sel => Bus2_Sel,  
                Bus1_Sel => Bus1_Sel,  
                write => write  
 );  
 -- data_path  
 data_path_u: data_path port map   
 (  
                clock => clock,  
                reset => reset,  
                IR_Load => IR_Load,   
                IR => IR,  
                MAR_Load => MAR_Load,  
                address => address,  
                PC_Load => PC_Load,  
                PC_Inc => PC_Inc,  
                A_Load => A_Load,  
                B_Load => B_Load,  
                ALU_Sel => ALU_Sel,  
                CCR_Result => CCR_Result,  
                CCR_Load => CCR_Load,  
                Bus2_Sel => Bus2_Sel,  
                Bus1_Sel => Bus1_Sel,  
                from_memory => from_memory,  
                to_memory => to_memory  
 );  
 end Behavioral;  
The Complete VHDL code for the microcontroller:
 library IEEE;  
 use IEEE.STD_LOGIC_1164.ALL;  
 --- computer  
 entity computer is  
 port(  
                clock,reset: in std_logic;  
                port_in_00: in std_logic_vector(7 downto 0);   
      port_in_01: in std_logic_vector(7 downto 0);  
      port_in_02: in std_logic_vector(7 downto 0);    
      port_in_03: in std_logic_vector(7 downto 0);   
      port_in_04: in std_logic_vector(7 downto 0);   
      port_in_05: in std_logic_vector(7 downto 0);   
      port_in_06: in std_logic_vector(7 downto 0);   
      port_in_07: in std_logic_vector(7 downto 0);   
      port_in_08: in std_logic_vector(7 downto 0);   
      port_in_09: in std_logic_vector(7 downto 0);  
      port_in_10: in std_logic_vector(7 downto 0);   
      port_in_11: in std_logic_vector(7 downto 0);   
      port_in_12: in std_logic_vector(7 downto 0);   
      port_in_13: in std_logic_vector(7 downto 0);   
      port_in_14: in std_logic_vector(7 downto 0);    
      port_in_15: in std_logic_vector(7 downto 0);  
                port_out_00: out std_logic_vector(7 downto 0);   
      port_out_01: out std_logic_vector(7 downto 0);  
      port_out_02: out std_logic_vector(7 downto 0);    
      port_out_03: out std_logic_vector(7 downto 0);   
      port_out_04: out std_logic_vector(7 downto 0);   
      port_out_05: out std_logic_vector(7 downto 0);   
      port_out_06: out std_logic_vector(7 downto 0);   
      port_out_07: out std_logic_vector(7 downto 0);   
      port_out_08: out std_logic_vector(7 downto 0);   
      port_out_09: out std_logic_vector(7 downto 0);  
      port_out_10: out std_logic_vector(7 downto 0);   
      port_out_11: out std_logic_vector(7 downto 0);   
      port_out_12: out std_logic_vector(7 downto 0);   
      port_out_13: out std_logic_vector(7 downto 0);   
      port_out_14: out std_logic_vector(7 downto 0);    
      port_out_15: out std_logic_vector(7 downto 0)  
           );  
 end computer;  
 architecture Behavioral of computer is  
 component cpu   
 port(  
                clock, reset: in std_logic;  
                address: out std_logic_vector(7 downto 0);  
                from_memory: in std_logic_vector(7 downto 0);  
                write: out std_logic;  
                to_memory: out std_logic_vector(7 downto 0)  
           );  
 end component cpu;  
 component memory   
 port (  
                address: in std_logic_vector(7 downto 0);  
                data_in: in std_logic_vector(7 downto 0);  
                write: in std_logic;  
                port_in_00: in std_logic_vector(7 downto 0);   
      port_in_01: in std_logic_vector(7 downto 0);  
      port_in_02: in std_logic_vector(7 downto 0);    
      port_in_03: in std_logic_vector(7 downto 0);   
      port_in_04: in std_logic_vector(7 downto 0);   
      port_in_05: in std_logic_vector(7 downto 0);   
      port_in_06: in std_logic_vector(7 downto 0);   
      port_in_07: in std_logic_vector(7 downto 0);   
      port_in_08: in std_logic_vector(7 downto 0);   
      port_in_09: in std_logic_vector(7 downto 0);  
      port_in_10: in std_logic_vector(7 downto 0);   
      port_in_11: in std_logic_vector(7 downto 0);   
      port_in_12: in std_logic_vector(7 downto 0);   
      port_in_13: in std_logic_vector(7 downto 0);   
      port_in_14: in std_logic_vector(7 downto 0);    
      port_in_15: in std_logic_vector(7 downto 0);  
                clock,reset: in std_logic;  
                data_out: out std_logic_vector(7 downto 0);  
                port_out_00: out std_logic_vector(7 downto 0);   
      port_out_01: out std_logic_vector(7 downto 0);  
      port_out_02: out std_logic_vector(7 downto 0);    
      port_out_03: out std_logic_vector(7 downto 0);   
      port_out_04: out std_logic_vector(7 downto 0);   
      port_out_05: out std_logic_vector(7 downto 0);   
      port_out_06: out std_logic_vector(7 downto 0);   
      port_out_07: out std_logic_vector(7 downto 0);   
      port_out_08: out std_logic_vector(7 downto 0);   
      port_out_09: out std_logic_vector(7 downto 0);  
      port_out_10: out std_logic_vector(7 downto 0);   
      port_out_11: out std_logic_vector(7 downto 0);   
      port_out_12: out std_logic_vector(7 downto 0);   
      port_out_13: out std_logic_vector(7 downto 0);   
      port_out_14: out std_logic_vector(7 downto 0);    
      port_out_15: out std_logic_vector(7 downto 0)  
           );  
 end component memory;  
 signal address,data_in,data_out: std_logic_vector(7 downto 0);    
 signal write: std_logic;  
 begin  
 --- cpu  
 cpu_u: cpu port map  
 (  
           clock => clock,  
           reset => reset,  
           address => address,  
           write => write,  
           to_memory => data_in,  
           from_memory => data_out  
 );  
 -- memory  
 memory_unit: memory port map  
 (  
               clock    => clock,  
           reset    => reset,  
           port_out_00 => port_out_00,  
           port_out_01 => port_out_01,  
           port_out_02 => port_out_02,  
           port_out_03 => port_out_03,  
           port_out_04 => port_out_04,  
           port_out_05 => port_out_05,  
           port_out_06 => port_out_06,  
           port_out_07 => port_out_07,  
           port_out_08 => port_out_08,  
           port_out_09 => port_out_09,  
           port_out_10 => port_out_10,  
           port_out_11 => port_out_11,  
           port_out_12 => port_out_12,  
           port_out_13 => port_out_13,  
           port_out_14 => port_out_14,  
           port_out_15 => port_out_15,            
           port_in_00  => port_in_00,  
           port_in_01  => port_in_01,  
           port_in_02  => port_in_02,  
           port_in_03  => port_in_03,  
           port_in_04  => port_in_04,  
           port_in_05  => port_in_05,  
           port_in_06  => port_in_06,  
           port_in_07  => port_in_07,  
           port_in_08  => port_in_08,  
           port_in_09  => port_in_09,  
           port_in_10  => port_in_10,  
           port_in_11  => port_in_11,  
           port_in_12  => port_in_12,  
           port_in_13  => port_in_13,  
           port_in_14  => port_in_14,  
           port_in_15  => port_in_15,  
                                data_out => data_out,  
                                address => address,  
                                data_in => data_in,  
                                write => write  
 );  
 end Behavioral;  

The VHDL implementation of the microcontroller is fully verified by simulation and demonstrated on FPGA DE0-nano with I/O Shield.

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