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//---------------------------------------------------------------------------

// RISC 16F84 core

//

// This file is part of the "risc_16F84" project.

// http://www.opencores.org/cores/risc_16F84

// 

//

// Description: See description below (which suffices for IP core

//                                     specification document.)

//

// Copyright (C) 1999 Sumio Morioka (original VHDL design version)

// Copyright (C) 2001 John Clayton and OPENCORES.ORG (this Verilog version)

//

// NOTE: This source code is free for educational/hobby use only.  It cannot

// be used for commercial purposes without the consent of Microchip

// Technology incorporated.

//

// This source file may be used and distributed without restriction provided

// that this copyright statement is not removed from the file and that any

// derivative work contains the original copyright notice and the associated

// disclaimer.

//

// This source file is free software; you can redistribute it and/or modify

// it under the terms of the GNU Lesser General Public License as published

// by the Free Software Foundation;  either version 2.1 of the License, or

// (at your option) any later version.

//

// This source is distributed in the hope that it will be useful, but WITHOUT

// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 

// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public

// License for more details.

//

// You should have received a copy of the GNU Lesser General Public License

// along with this source.

// If not, download it from http://www.opencores.org/lgpl.shtml

//

//---------------------------------------------------------------------------

//

// Author: John Clayton

// Date  : January 29, 2002

//

// (NOTE: Date formatted as day/month/year.)

// Update: 29/01/02 copied this file from memory_sizer.v (pared down).

//                  Translated the module and signal declarations.

//                  Transformed the instruction wires to lowercase.

//                  Transformed the addressing wires to lowercase.

// Update: 31/01/02 Translated the instruction decoder.

// Update:  5/02/02 Determined that stack is simply a circular buffer of

//                  8 locations, 13 bits per location.  Started translating

//                  "main_efsm" process.  Added all code from piccore.vhd

//                  into this file for eventual translation.  Concluded that

//                  "stack_full_node" is not needed.

// Update:  6/02/02 Translated the "ram_i_node" if/else precedural assignment.

// Update:  7/02/02 Changed all := to <=, changed all '0' to 0 and '1' to 1.

//                  Replaced all " downto " with ":".

//                  Finished translating QRESET state.

// Update: 20/02/02 Replaced all instances of Qreset with QRESET_PP.  Also

//                  replaced other state designations with their new names.

//                  Finished translating Q1, Q2 states.

// Update: 22/02/02 Translated section 2-4-1-1 (aluout register)

// Update: 27/02/02 Replaced all "or" with "||" in if statements

//                  Replaced all "and" with "&&" in if statements.

//                  Replaced all "not" with "~" in if statements.

//                  Finished translating Q3,Q4 states.

//                  Translated output signal assignments at end of code.

//                  Translated interrupt trigger processes.

// Update: 28/02/02 Finished translation of WDT and TMR0 prescaler.

//                  Trimmed line length to 80 characters throughout.

//                  Prepared to attempt initial syntax checking.

//                  Cleaned up some naming conventions, and verified that

//                  all I/O pins have _i or _o appended in the body of the

//                  code.

// Update: 03/04/02 Changed "progdata_i" to "prog_dat_i" Also changed

//                  "progadr_o" to "prog_adr_o"

// Update: 04/26/02 Fixed bug in aluout_reg logic, whereby the AND/OR type

//                  operations were coded with logical instead of bitwise

//                  operators.

// Update: 05/01/02 Fixed another bug -- the rrf and rlf instructions were

//                  coded incorrectly.

// Update: 05/05/02 Fixed another bug -- the carry bit was incorrect (the

//                  problem was discovered while performing SUBWF X,W where

//                  W contained 0 and X contained 1. (1-0).  The logic for

//                  the carry bit appears to have been incorrect even in

//                  the original VHDL code by Sumio Morioka.

// Update: 10/30/02 Fixed syntax error pointed out by Cheol-Kyoo Lee, who got

//                  the source code from opencores.com.  Removed semicolon

//                  from "endcase" statements.

//

// Description

//---------------------------------------------------------------------------

// This logic module implements a small RISC microcontroller, with functions

// and instruction set very similar to those of the Microchip 16F84 chip.

// This work is a translation (from VHDL to Verilog) of the "CQPIC" design

// published in 1999 by Sumio Morioka of Japan, and published in the December

// 1999 issue of "Transistor Gijutsu Magazine."  The translation was performed

// by John Clayton, without the use of any translation tools.

//

// Original version used as basis for translation:  CQPIC version 1.00b

//                                                  (December 10, 2000)

//---------------------------------------------------------------------------

`define STATEBIT_SIZE 3      // Size of state machine register (bits)

module pic_16f84_core (

  prog_dat_i,           // [13:0] ROM read data

  prog_adr_o,           // [12:0] ROM address

  ram_dat_i,            // [7:0] RAM read data

  ram_dat_o,            // [7:0] RAM write data

  ram_adr_o,            // [8:0] RAM address; ram_adr[8:7] indicates RAM-BANK

  readram_o,            // RAM read strobe (H active)

  writeram_o,           // RAM write strobe (H active)

  existeeprom_i,        // Set to 1 if EEPROM is implemented.

  eep_dat_i,            // [7:0] EEPROM read data

  eep_dat_o,            // [7:0] EEPROM write data

  eep_adr_o,            // [7:0] EEPROM address

  rd_eep_req_o,         // EEPROM read request (H active)

  rd_eep_ack_i,         // EEPROM read acknowledge (H active)

  wr_eep_req_o,         // EEPROM write request (H active)

  wr_eep_ack_i,         // EEPROM write acknowledge (H active)

  porta_i,              // [4:0] PORT-A input data

  porta_o,              // [4:0] PORT-A output data

  porta_dir_o,          // [4:0] TRISA: PORT-A signal dir. (H:in, L:out)

  portb_i,              // [7:0] PORT-B input data

  portb_o,              // [7:0] PORT-B output data

  portb_dir_o,          // [7:0] TRISB: PORT-B signal dir. (H:in, L:out)

  rbpu_o,               // PORT_B pull-up enable (usually not used)

  int0_i,               // PORT-B(0) INT

  int4_i,               // PORT-B(4) INT

  int5_i,               // PORT-B(5) INT

  int6_i,               // PORT-B(6) INT

  int7_i,               // PORT-B(7) INT

  t0cki_i,              // T0CKI (PORT-A(4))

  wdt_ena_i,            // WDT enable (H active)

  wdt_clk_i,            // WDT clock

  wdt_full_o,           // WDT-full indicator (H active)

  powerdown_o,          // SLEEP-mode; if H, you can stop system clock clk_i

  startclk_o,           // WAKEUP; if H, turn on clk_i for leaving sleep-mode

  pon_rst_n_i,          // Power-on reset (L active)

  mclr_n_i,             // Normal reset (L active)

  clk_i,                // Clock input

  clk_o                 // Clock output (clk_i/4)

);

// You can change the following parameters as you would like

parameter STACK_SIZE_PP      = 8;   // Size of PC stack

parameter LOG2_STACK_SIZE_PP = 3;   // Log_2(stack_size)

parameter WDT_SIZE_PP        = 255; // Size of watch dog timer (WDT)

parameter WDT_BITS_PP        = 8;   // Bits needed for watch dog timer (WDT)

// State definitions for state machine, provided as parameters to allow

// for redefinition of state values by the instantiator if desired.

parameter QRESET_PP = 3'b100; // reset state

parameter Q1_PP     = 3'b000; // state Q1

parameter Q2_PP     = 3'b001; // state Q2

parameter Q3_PP     = 3'b011; // state Q3

parameter Q4_PP     = 3'b010; // state Q4

// I/O declarations

       // program ROM data bus/address bus

input  [13:0] prog_dat_i;   // ROM read data

output [12:0] prog_adr_o;   // ROM address

       // data RAM data bus/address bus/control signals

input  [7:0] ram_dat_i;     // RAM read data

output [7:0] ram_dat_o;     // RAM write data

output [8:0] ram_adr_o;     // RAM address; ram_adr[8:7] indicates RAM-BANK

output readram_o;           // RAM read  strobe (H active)

output writeram_o;          // RAM write strobe (H active)

       // EEPROM data bus/address bus

input  existeeprom_i;       // Set to 1 if EEPROM is implemented.

input  [7:0] eep_dat_i;     // EEPROM read data

output [7:0] eep_dat_o;     // EEPROM write data

output [7:0] eep_adr_o;     // EEPROM address

output rd_eep_req_o;        // EEPROM read request (H active)

input  rd_eep_ack_i;        // EEPROM read acknowledge (H active)

output wr_eep_req_o;        // EEPROM write request (H active)

input  wr_eep_ack_i;        // EEPROM write acknowledge (H active)

       // I/O ports

input  [4:0] porta_i;       // PORT-A input data

output [4:0] porta_o;       // PORT-A output data

output [4:0] porta_dir_o;   // TRISA: PORT-A signal dir. (H:input, L:output)

input  [7:0] portb_i;       // PORT-B input data

output [7:0] portb_o;       // PORT-B output data

output [7:0] portb_dir_o;   // TRISB: PORT-B signal dir. (H:input, L:output)

output rbpu_o;              // PORT_B pull-up enable (usually not used)

       // PORT-B interrupt input

input  int0_i;              // PORT-B(0) INT

input  int4_i;              // PORT-B(4) INT

input  int5_i;              // PORT-B(5) INT

input  int6_i;              // PORT-B(6) INT

input  int7_i;              // PORT-B(7) INT

       // TMR0 Control

input  t0cki_i;             // T0CKI (PORT-A(4))

       // Watch Dog Timer Control

input  wdt_ena_i;           // WDT enable (H active)

input  wdt_clk_i;           // WDT clock

output wdt_full_o;          // WDT-full indicator (H active)

       // CPU clock stop/start indicators

output powerdown_o;         // SLEEP-mode; if H, then you can

                            // stop the system clock clk_i

output startclk_o;          // WAKEUP; if H, you should turn on

                            // clock clk_i for waking up from sleep-mode

       // CPU reset

input  pon_rst_n_i;         // Power-on reset (LOW active)

input  mclr_n_i;            // Normal reset (LOW active)

       // CPU clock

input  clk_i;               // Clock input

output clk_o;               // Clock output (clk_i/4)

// Internal signal declarations

     // User registers

reg  [7:0] w_reg;            // W

reg  [7:0] tmr0_reg;         // TMR0

reg  [12:0] pc_reg;          // PCH/PCL

reg  [7:0] status_reg;       // STATUS

reg  [7:0] fsr_reg;          // FSR

reg  [4:0] porta_i_sync_reg; // PORTA IN (synchronizer)

reg  [4:0] porta_o_reg;      // PORTA OUT

reg  [7:0] portb_i_sync_reg; // PORTB IN (synchronizer)

reg  [7:0] portb_o_reg;      // PORTB OUT

reg  [7:0] eep_dat_reg;      // EEPROM DATA

reg  [7:0] eep_adr_reg;      // EEPROM ADDRESS

reg  [4:0] pclath_reg;       // PCLATH

reg  [7:0] intcon_reg;       // INTCON

reg  [7:0] option_reg;       // OPTION

reg  [4:0] trisa_reg;        // TRISA

reg  [7:0] trisb_reg;        // TRISB

reg  [4:0] eecon1_reg;       // EECON1

     // Internal registers for controlling instruction execution

reg  [13:0] inst_reg;        // Hold fetched op-code/operand

reg  [7:0] aluinp1_reg;      // data source (1 of 2)

reg  [7:0] aluinp2_reg;      // data source (2 of 2)

reg        c_in;             // Used with ALU data sources.

reg  [7:0] aluout_reg;       // result of calculation

reg  exec_op_reg;            // if L (i.e. GOTO instruction etc), stall exec.

reg  intstart_reg;           // if H (i.e. interrupt), stall instr. exec.

reg  sleepflag_reg;          // if H, sleeping.

     // Stack

                             // stack (array of data-registers)

reg  [12:0] stack_reg [STACK_SIZE_PP-1:0];

                             // stack pointer (binary encoded)

reg  [LOG2_STACK_SIZE_PP-1:0] stack_pnt_reg;

     // WDT register and its control

reg  [WDT_BITS_PP-1:0] wdt_reg;  // WDT counter

reg  wdt_full_reg;               // WDT->CPU; hold WDT-full signal until 

                                 //   CPU is reset

reg  wdt_full_node;

wire wdt_init;                   // Initialize the WDT

reg  [2:0] wdt_full_sync_reg;    // CPU; synchronizer for wdt_full_reg

reg  wdt_clr_reg;                // CPU->WDT; request to zero-clear wdt_reg

reg  wdt_clr_reqhold_reg;        // CPU; hold a clear-request if 

                                 //   previous request is still processing

reg  [1:0] wdt_clr_req_reg;      // WDT; synchronizer for wdt_clr_reg

wire wdt_clr_ack;                // WDT->CPU; ack to wdt_clr_reg 

                                 //   (same with wdt_clr_req_reg(1))

reg  wdt_clr_ack_sync_reg;       // CPU; synchronizer for wdt_clr_ack

reg  wdt_full_clr_reg;           // CPU->WDT; request to clear wdt_full_reg

reg  [1:0] wdt_fullclr_req_reg;  // WDT; synchronizer for wdt_full_clr_reg

     // TMR0 prescaler

wire ps_clk;                  // clock for prescaler

reg  [7:0] pscale_reg;        // prescaler (range 0 to 255)

reg  ps_full_reg;             // clock for TMR0, from prescaler

wire inc_tmr_clk;             // clock for TMR0

reg  inc_tmr_hold_reg;        // hold TMR0 increment request

reg  [7:0] rateval;           // Temporary storage value within process

     // Interrupt registers/nodes

reg  [4:0] intrise_reg;       // detect positive edge of PORT-B inputs

reg  [4:0] intdown_reg;       // detect negative edge of PORT-B inputs

                              // Interrupt triggers

wire rb0_int;

wire rb4_int;

wire rb5_int;

wire rb6_int;

wire rb7_int;

wire rbint;                   // RB4-7 interrupt trigger

wire inte;                    // RB0   interrupt trigger

reg  [4:0] intclr_reg;        // CPU; clear intrise_reg and intdown_reg

wire intclr0;                 // Individual wires used in sensitivity lists

wire intclr1;                 // since "simple variables" are OK,

wire intclr2;                 // but apparently intclr_reg[0] is not a

wire intclr3;                 // "simple variable or its negation."

wire intclr4;

     // State register

reg  [`STATEBIT_SIZE-1:0] state_reg;

     // Result of decoding instruction -- only 1 is active at a time

wire inst_addlw;

wire inst_addwf;

wire inst_andlw;

wire inst_andwf;

wire inst_bcf;

wire inst_bsf;

wire inst_btfsc;

wire inst_btfss;

wire inst_call;

wire inst_clrf;

wire inst_clrw;

wire inst_clrwdt;

wire inst_comf;

wire inst_decf;

wire inst_decfsz;

wire inst_goto;

wire inst_incf;

wire inst_incfsz;

wire inst_iorlw;

wire inst_iorwf;

wire inst_movlw;

wire inst_movf;

wire inst_movwf;

wire inst_retfie;

wire inst_retlw;

wire inst_ret;

wire inst_rlf;

wire inst_rrf;

wire inst_sleep;

wire inst_sublw;

wire inst_subwf;

wire inst_swapf;

wire inst_xorlw;

wire inst_xorwf;

     // Result of calculating RAM access address

wire [8:0] ram_adr_node;      // RAM access address

     // These wires indicate accesses to special registers... 

     // Only 1 is active at a time.

wire addr_tmr0;

wire addr_pcl;

wire addr_stat;

wire addr_fsr;

wire addr_porta;

wire addr_portb;

wire addr_eep_dat;

wire addr_eep_adr;

wire addr_pclath;

wire addr_intcon;

wire addr_option;

wire addr_trisa;

wire addr_trisb;

wire addr_eecon1;

wire addr_eecon2;

wire addr_sram;

     // Other output registers (for removing hazards)

reg  writeram_reg;      // data-sram write strobe

reg  [8:0] ram_adr_reg; // data-sram address

reg  clk_o_reg;         // clock output

     // Synchronizers

reg  inte_sync_reg;

reg  rbint_sync_reg;

reg  [1:0] inc_tmr_sync_reg;

reg  rd_eep_sync_reg;

reg  wr_eep_sync_reg;

reg  mclr_sync_reg;

reg  poweron_sync_reg;

     // Signals used in "main_efsm" procedure

     // (Intermediate nodes used for resource sharing.)

reg  [7:0] ram_i_node;   // result of reading RAM/Special registers

reg  [12:0] inc_pc_node; // value of PC + 1

wire [7:0] mask_node;    // bit mask for logical operations

reg  [8:0] add_node;     // result of 8bit addition

reg  [4:0] addlow_node;  // result of low-4bit addition

wire temp;               // Placeholder wire

wire dtemp;              // Placeholder wire

reg  aluout_zero_node;   // H if ALUOUT = 0

reg  writew_node;        // H if destination is W register

reg  writeram_node;      // H if destination is RAM/Special registers

reg  int_node;           // H if interrupt request comes

reg  wdt_rst_node;       // H if WDT-reset request comes

reg  reset_cond;         // H for any reset request (jump to QRESET_PP state)

//--------------------------------------------------------------------------

// Instantiations

//--------------------------------------------------------------------------

//--------------------------------------------------------------------------

// Functions & Tasks

//--------------------------------------------------------------------------

//--------------------------------------------------------------------------

// Module code

//--------------------------------------------------------------------------

// CPU synchronizers

always @(posedge clk_i)

begin

  inte_sync_reg          <= inte;

  rbint_sync_reg         <= rbint;

  wdt_clr_ack_sync_reg   <= wdt_clr_ack;

  mclr_sync_reg          <= mclr_n_i;

  poweron_sync_reg       <= pon_rst_n_i;

  rd_eep_sync_reg        <= rd_eep_ack_i;

  wr_eep_sync_reg        <= wr_eep_ack_i;

  inc_tmr_sync_reg[0]    <= inc_tmr_clk;

  inc_tmr_sync_reg[1]    <= inc_tmr_sync_reg[0];

  if (~poweron_sync_reg || ~mclr_sync_reg)

    wdt_full_sync_reg    <= 3'b0;

  else

  begin

    wdt_full_sync_reg[0] <= wdt_full_reg;

    wdt_full_sync_reg[1] <= wdt_full_sync_reg[0]; // (remove meta-stability)

    wdt_full_sync_reg[2] <= wdt_full_sync_reg[1]; // (detect positive edge)

  end

end

// Decode OPcode    (see pp.54 of PIC16F84 data sheet)

// only 1 signal of the following signals will be '1'

assign inst_call     = (inst_reg[13:11] ==  3'b100           );

assign inst_goto     = (inst_reg[13:11] ==  3'b101           );

assign inst_bcf      = (inst_reg[13:10] ==  4'b0100          );

assign inst_bsf      = (inst_reg[13:10] ==  4'b0101          );

assign inst_btfsc    = (inst_reg[13:10] ==  4'b0110          );

assign inst_btfss    = (inst_reg[13:10] ==  4'b0111          );

assign inst_movlw    = (inst_reg[13:10] ==  4'b1100          );

assign inst_retlw    = (inst_reg[13:10] ==  4'b1101          );

assign inst_sublw    = (inst_reg[13:9]  ==  5'b11110         );

assign inst_addlw    = (inst_reg[13:9]  ==  5'b11111         );

assign inst_iorlw    = (inst_reg[13:8]  ==  6'b111000        );

assign inst_andlw    = (inst_reg[13:8]  ==  6'b111001        );

assign inst_xorlw    = (inst_reg[13:8]  ==  6'b111010        );

assign inst_subwf    = (inst_reg[13:8]  ==  6'b000010        );

assign inst_decf     = (inst_reg[13:8]  ==  6'b000011        );

assign inst_iorwf    = (inst_reg[13:8]  ==  6'b000100        );

assign inst_andwf    = (inst_reg[13:8]  ==  6'b000101        );

assign inst_xorwf    = (inst_reg[13:8]  ==  6'b000110        );

assign inst_addwf    = (inst_reg[13:8]  ==  6'b000111        );

assign inst_movf     = (inst_reg[13:8]  ==  6'b001000        );

assign inst_comf     = (inst_reg[13:8]  ==  6'b001001        );

assign inst_incf     = (inst_reg[13:8]  ==  6'b001010        );

assign inst_decfsz   = (inst_reg[13:8]  ==  6'b001011        );

assign inst_rrf      = (inst_reg[13:8]  ==  6'b001100        );

assign inst_rlf      = (inst_reg[13:8]  ==  6'b001101        );

assign inst_swapf    = (inst_reg[13:8]  ==  6'b001110        );

assign inst_incfsz   = (inst_reg[13:8]  ==  6'b001111        );

assign inst_movwf    = (inst_reg[13:7]  ==  7'b0000001       );

assign inst_clrw     = (inst_reg[13:7]  ==  7'b0000010       );

assign inst_clrf     = (inst_reg[13:7]  ==  7'b0000011       );

assign inst_ret      = (inst_reg[13:0]  == 14'b00000000001000);

assign inst_retfie   = (inst_reg[13:0]  == 14'b00000000001001);

assign inst_sleep    = (inst_reg[13:0]  == 14'b00000001100011);

assign inst_clrwdt   = (inst_reg[13:0]  == 14'b00000001100100);

// Calculate RAM access address (see pp.19 of PIC16F84 data sheet)

    // if "d"=0, indirect addressing is used, so RAM address is BANK+FSR

    // otherwise, RAM address is BANK+"d"

    // (see pp.19 of PIC16F84 data sheet)

assign ram_adr_node = (inst_reg[6:0]==0)?{status_reg[7],fsr_reg[7:0]}:

                               {status_reg[6:5],inst_reg[6:0]};

    // check if this is an access to external RAM or not

assign addr_sram   = (ram_adr_node[6:0] > 7'b0001011); //0CH-7FH,8CH-FFH

    // check if this is an access to special register or not

    // only 1 signal of the following signals will be '1'

assign addr_tmr0    = (ram_adr_node[7:0] == 8'b00000001); // 01H

assign addr_pcl     = (ram_adr_node[6:0] ==  7'b0000010); // 02H, 82H

assign addr_stat    = (ram_adr_node[6:0] ==  7'b0000011); // 03H, 83H

assign addr_fsr     = (ram_adr_node[6:0] ==  7'b0000100); // 04H, 84H

assign addr_porta   = (ram_adr_node[7:0] == 8'b00000101); // 05H

assign addr_portb   = (ram_adr_node[7:0] == 8'b00000110); // 06H

assign addr_eep_dat = (ram_adr_node[7:0] == 8'b00001000); // 08H

assign addr_eep_adr = (ram_adr_node[7:0] == 8'b00001001); // 09H

assign addr_pclath  = (ram_adr_node[6:0] ==  7'b0001010); // 0AH, 8AH

assign addr_intcon  = (ram_adr_node[6:0] ==  7'b0001011); // 0BH, 8BH

assign addr_option  = (ram_adr_node[7:0] == 8'b10000001); // 81H

assign addr_trisa   = (ram_adr_node[7:0] == 8'b10000101); // 85H

assign addr_trisb   = (ram_adr_node[7:0] == 8'b10000110); // 86H

assign addr_eecon1  = (ram_adr_node[7:0] == 8'b10001000); // 88H

assign addr_eecon2  = (ram_adr_node[7:0] == 8'b10001001); // 89H

// construct bit-mask for logical operations and bit tests

assign mask_node = 1 << inst_reg[9:7];

// MAIN EFSM: description of register value changes in each clock cycle

always @(posedge clk_i)

begin

  // 1. Intermediate nodes for resource sharing

  // This is a long if/else chain.  Consider pulling in the decoded signals

  // addr_tmr0 etc., and using a case statement instead?

  // 1-1. Reading RAM/data sources  (see pp.13 of PIC16F84 data sheet)

  if (addr_sram)         ram_i_node <= ram_dat_i;   // data from ext. SRAM

  else if (addr_eep_dat) ram_i_node <= eep_dat_reg; // data from ext. PROM

  else if (addr_tmr0)    ram_i_node <= tmr0_reg;    // data from tmr0

  else if (addr_pcl)     ram_i_node <= pc_reg[7:0]; // data from pcl

  else if (addr_stat)    ram_i_node <= status_reg;  // data from status

  else if (addr_fsr)     ram_i_node <= fsr_reg;     // data from fsr

  else if (addr_porta)

  begin

    // Logic implements a 2:1 mux for each bit [4:0] of ram_i_node

    ram_i_node[4:0] <= (

                           (~trisa_reg[4:0] & porta_o_reg[4:0])

                        || ( trisa_reg[4:0] & porta_i_sync_reg[4:0])

                        );

    ram_i_node[7:5] <= 3'b0;

  end

  else if (addr_portb)

  begin

    // Logic implements a 2:1 mux for each bit [7:0] of ram_i_node

    ram_i_node[7:0] <= (

                           (~trisb_reg[7:0] & portb_o_reg[7:0])

                        || ( trisb_reg[7:0] & portb_i_sync_reg[7:0])

                        );

  end

  else if (addr_eep_adr) ram_i_node <= eep_adr_reg;       // from eeprom

  else if (addr_pclath)  ram_i_node <= {3'b0,pclath_reg}; // pclath (5bit)

  else if (addr_intcon)  ram_i_node <= intcon_reg;        // data from intcon

  else if (addr_option)  ram_i_node <= option_reg;        // data from option

  else if (addr_trisa)   ram_i_node <= {3'b0,trisa_reg};  // trisa (5bit)

  else if (addr_trisb)   ram_i_node <= trisb_reg;         // data from trisb

  else if (addr_eecon1)  ram_i_node <= {3'b0,eecon1_reg}; // eecon1 (5bit)

  else ram_i_node <= 0;

  // 1-2. PC + 1

  inc_pc_node  <= pc_reg + 1;

  // 1-3. Adder (ALU)

  // full 8bit-addition, with carry in/out.

  {add_node,temp}     <=    {1'b0,aluinp1_reg,1'b1}

                          + {1'b0,aluinp2_reg,c_in};

  // lower 4bit-addition

  {addlow_node,dtemp} <=    {1'b0,aluinp1_reg[3:0],1'b1}

                          + {1'b0,aluinp2_reg[3:0],c_in};

  // 1-4. Test if aluout = 0

  aluout_zero_node <= (aluout_reg == 0)?1:0;

  // 1-5. Determine destination

  if (intstart_reg)

  begin

    writew_node     <= 0;

    writeram_node   <= 0;

  end

  else if (inst_movwf || inst_bcf || inst_bsf || inst_clrf)

  begin

    writew_node     <= 0;

    writeram_node   <= 1;

  end

  else if (   inst_movlw || inst_addlw || inst_sublw || inst_andlw

           || inst_iorlw || inst_xorlw || inst_retlw || inst_clrw)

  begin

    writew_node     <= 1;

    writeram_node   <= 0;

  end

  else if (   inst_movf   || inst_swapf || inst_addwf || inst_subwf

           || inst_andwf  || inst_iorwf || inst_xorwf || inst_decf

           || inst_incf   || inst_rlf   || inst_rrf   || inst_decfsz

           || inst_incfsz || inst_comf)

  begin

    writew_node     <= ~inst_reg[7];  // ("d" field of fetched instruction)

    writeram_node   <=  inst_reg[7];  // ("d" field of fetched instruction)

  end

  else

  begin

    writew_node     <= 0;

    writeram_node   <= 0;

  end

  // 1-6. Interrupt request   (see pp.17 of PIC16F84 data sheet)

  int_node <= intcon_reg[7]        // GIE

              && (

                     (intcon_reg[3] && intcon_reg[0]) // RBIE,RBIF

                  || (intcon_reg[4] && intcon_reg[1]) // INTE,INTF

                  || (intcon_reg[5] && intcon_reg[2]) // T0IE,T0IF

                  || (intcon_reg[6] && eecon1_reg[4]) // EEIE,EEIF(EECON1)

                  );

  // 1-7. Reset conditions

  wdt_rst_node <= wdt_full_sync_reg[1] && ~wdt_full_sync_reg[2];  // WDT

  // (all of reset triggers)

  if (~poweron_sync_reg || ~mclr_sync_reg || wdt_rst_node) reset_cond  <= 1;

  else reset_cond  <= 0;

  // 2. EFSM body

  case (state_reg)

    // 2-1. Reset state (see pp.14 and pp.42 of PIC16F84 data sheet)

    QRESET_PP :

    begin

      pc_reg          <= 0;     // 0

      status_reg[7:5] <= 3'b0;

      pclath_reg      <= 0;     // 0

      intcon_reg[7:1] <= 7'b0;