Subversion Repositories uart16550

//////////////////////////////////////////////////////////////////////

////                                                              ////

////  uart_regs.v                                                 ////

////                                                              ////

////                                                              ////

////  This file is part of the "UART 16550 compatible" project    ////

////  http://www.opencores.org/cores/uart16550/                   ////

////                                                              ////

////  Documentation related to this project:                      ////

////  - http://www.opencores.org/cores/uart16550/                 ////

////                                                              ////

////  Projects compatibility:                                     ////

////  - WISHBONE                                                  ////

////  RS232 Protocol                                              ////

////  16550D uart (mostly supported)                              ////

////                                                              ////

////  Overview (main Features):                                   ////

////  Registers of the uart 16550 core                            ////

////                                                              ////

////  Known problems (limits):                                    ////

////  Inserts 1 wait state in all WISHBONE transfers              ////

////                                                              ////

////  To Do:                                                      ////

////  Nothing or verification.                                    ////

////                                                              ////

////  Author(s):                                                  ////

////      - gorban@opencores.org                                  ////

////      - Jacob Gorban                                          ////

////      - Igor Mohor (igorm@opencores.org)                      ////

////                                                              ////

////  Created:        2001/05/12                                  ////

////  Last Updated:   (See log for the revision history           ////

////                                                              ////

////                                                              ////

//////////////////////////////////////////////////////////////////////

////                                                              ////

//// Copyright (C) 2000, 2001 Authors                             ////

////                                                              ////

//// 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                     ////

////                                                              ////

//////////////////////////////////////////////////////////////////////

//

// CVS Revision History

//

// $Log: not supported by cvs2svn $

// Revision 1.31  2001/12/14 10:06:58  mohor

// After reset modem status register MSR should be reset.

//

// Revision 1.30  2001/12/13 10:09:13  mohor

// thre irq should be cleared only when being source of interrupt.

//

// Revision 1.29  2001/12/12 09:05:46  mohor

// LSR status bit 0 was not cleared correctly in case of reseting the FCR (rx fifo).

//

// Revision 1.28  2001/12/10 19:52:41  gorban

// Scratch register added

//

// Revision 1.27  2001/12/06 14:51:04  gorban

// Bug in LSR[0] is fixed.

// All WISHBONE signals are now sampled, so another wait-state is introduced on all transfers.

//

// Revision 1.26  2001/12/03 21:44:29  gorban

// Updated specification documentation.

// Added full 32-bit data bus interface, now as default.

// Address is 5-bit wide in 32-bit data bus mode.

// Added wb_sel_i input to the core. It's used in the 32-bit mode.

// Added debug interface with two 32-bit read-only registers in 32-bit mode.

// Bits 5 and 6 of LSR are now only cleared on TX FIFO write.

// My small test bench is modified to work with 32-bit mode.

//

// Revision 1.25  2001/11/28 19:36:39  gorban

// Fixed: timeout and break didn't pay attention to current data format when counting time

//

// Revision 1.24  2001/11/26 21:38:54  gorban

// Lots of fixes:

// Break condition wasn't handled correctly at all.

// LSR bits could lose their values.

// LSR value after reset was wrong.

// Timing of THRE interrupt signal corrected.

// LSR bit 0 timing corrected.

//

// Revision 1.23  2001/11/12 21:57:29  gorban

// fixed more typo bugs

//

// Revision 1.22  2001/11/12 15:02:28  mohor

// lsr1r error fixed.

//

// Revision 1.21  2001/11/12 14:57:27  mohor

// ti_int_pnd error fixed.

//

// Revision 1.20  2001/11/12 14:50:27  mohor

// ti_int_d error fixed.

//

// Revision 1.19  2001/11/10 12:43:21  gorban

// Synthesis bugs fixed. Some other minor changes

//

// Revision 1.18  2001/11/08 14:54:23  mohor

// Comments in Slovene language deleted, few small fixes for better work of

// old tools. IRQs need to be fix.

//

// Revision 1.17  2001/11/07 17:51:52  gorban

// Heavily rewritten interrupt and LSR subsystems.

// Many bugs hopefully squashed.

//

// Revision 1.16  2001/11/02 09:55:16  mohor

// no message

//

// Revision 1.15  2001/10/31 15:19:22  gorban

// Fixes to break and timeout conditions

//

// Revision 1.14  2001/10/29 17:00:46  gorban

// fixed parity sending and tx_fifo resets over- and underrun

//

// Revision 1.13  2001/10/20 09:58:40  gorban

// Small synopsis fixes

//

// Revision 1.12  2001/10/19 16:21:40  gorban

// Changes data_out to be synchronous again as it should have been.

//

// Revision 1.11  2001/10/18 20:35:45  gorban

// small fix

//

// Revision 1.10  2001/08/24 21:01:12  mohor

// Things connected to parity changed.

// Clock devider changed.

//

// Revision 1.9  2001/08/23 16:05:05  mohor

// Stop bit bug fixed.

// Parity bug fixed.

// WISHBONE read cycle bug fixed,

// OE indicator (Overrun Error) bug fixed.

// PE indicator (Parity Error) bug fixed.

// Register read bug fixed.

//

// Revision 1.10  2001/06/23 11:21:48  gorban

// DL made 16-bit long. Fixed transmission/reception bugs.

//

// Revision 1.9  2001/05/31 20:08:01  gorban

// FIFO changes and other corrections.

//

// Revision 1.8  2001/05/29 20:05:04  gorban

// Fixed some bugs and synthesis problems.

//

// Revision 1.7  2001/05/27 17:37:49  gorban

// Fixed many bugs. Updated spec. Changed FIFO files structure. See CHANGES.txt file.

//

// Revision 1.6  2001/05/21 19:12:02  gorban

// Corrected some Linter messages.

//

// Revision 1.5  2001/05/17 18:34:18  gorban

// First 'stable' release. Should be sythesizable now. Also added new header.

//

// Revision 1.0  2001-05-17 21:27:11+02  jacob

// Initial revision

//

//

// synopsys translate_off

`include "timescale.v"

// synopsys translate_on

`include "uart_defines.v"

`define UART_DL1 7:0

`define UART_DL2 15:8

module uart_regs (clk,

        wb_rst_i, wb_addr_i, wb_dat_i, wb_dat_o, wb_we_i, wb_re_i,

// additional signals

        modem_inputs,

        stx_pad_o, srx_pad_i,

`ifdef DATA_BUS_WIDTH_8

`else

// debug interface signals      enabled

ier, iir, fcr, mcr, lcr, msr, lsr, rf_count, tf_count, tstate, rstate,

`endif

        rts_pad_o, dtr_pad_o, int_o

        );

input                                                                   clk;

input                                                                   wb_rst_i;

input [`UART_ADDR_WIDTH-1:0]             wb_addr_i;

input [7:0]                                                      wb_dat_i;

output [7:0]                                                     wb_dat_o;

input                                                                   wb_we_i;

input                                                                   wb_re_i;

output                                                                  stx_pad_o;

input                                                                   srx_pad_i;

input [3:0]                                                      modem_inputs;

output                                                                  rts_pad_o;

output                                                                  dtr_pad_o;

output                                                                  int_o;

`ifdef DATA_BUS_WIDTH_8

`else

// if 32-bit databus and debug interface are enabled

output [3:0]                                                     ier;

output [3:0]                                                     iir;

output [1:0]                                                     fcr;  /// bits 7 and 6 of fcr. Other bits are ignored

output [4:0]                                                     mcr;

output [7:0]                                                     lcr;

output [7:0]                                                     msr;

output [7:0]                                                     lsr;

output [`UART_FIFO_COUNTER_W-1:0]        rf_count;

output [`UART_FIFO_COUNTER_W-1:0]        tf_count;

output [2:0]                                                     tstate;

output [3:0]                                                     rstate;

`endif

wire [3:0]                                                               modem_inputs;

reg                                                                             enable;

wire                                                                            stx_pad_o;              // received from transmitter module

wire                                                                            srx_pad_i;

reg [7:0]                                                                wb_dat_o;

wire [`UART_ADDR_WIDTH-1:0]              wb_addr_i;

wire [7:0]                                                               wb_dat_i;

reg [3:0]                                                                ier;

reg [3:0]                                                                iir;

reg [1:0]                                                                fcr;  /// bits 7 and 6 of fcr. Other bits are ignored

reg [4:0]                                                                mcr;

reg [7:0]                                                                lcr;

reg [7:0]                                                                msr;

reg [15:0]                                                               dl;  // 32-bit divisor latch

reg [7:0]                                                                scratch; // UART scratch register

reg                                                                             start_dlc; // activate dlc on writing to UART_DL1

reg                                                                             lsr_mask_d; // delay for lsr_mask condition

reg                                                                             msi_reset; // reset MSR 4 lower bits indicator

//reg                                                                           threi_clear; // THRE interrupt clear flag

reg [15:0]                                                               dlc;  // 32-bit divisor latch counter

reg                                                                             int_o;

reg [3:0]                                                                trigger_level; // trigger level of the receiver FIFO

reg                                                                             rx_reset;

reg                                                                             tx_reset;

wire                                                                            dlab;                      // divisor latch access bit

wire                                                                            cts_pad_i, dsr_pad_i, ri_pad_i, dcd_pad_i; // modem status bits

wire                                                                            loopback;                  // loopback bit (MCR bit 4)

wire                                                                            cts, dsr, ri, dcd;         // effective signals (considering loopback)

wire                    cts_c, dsr_c, ri_c, dcd_c; // Complement effective signals (considering loopback)

wire                                                                            rts_pad_o, dtr_pad_o;              // modem control outputs

// LSR bits wires and regs

wire [7:0]                                                               lsr;

wire                                                                            lsr0, lsr1, lsr2, lsr3, lsr4, lsr5, lsr6, lsr7;

reg                                                                             lsr0r, lsr1r, lsr2r, lsr3r, lsr4r, lsr5r, lsr6r, lsr7r;

wire                                                                            lsr_mask; // lsr_mask

//

// ASSINGS

//

assign                                                                  lsr[7:0] = { lsr7r, lsr6r, lsr5r, lsr4r, lsr3r, lsr2r, lsr1r, lsr0r };

assign                                                                  {cts_pad_i, dsr_pad_i, ri_pad_i, dcd_pad_i} = modem_inputs;

assign                                                                  {cts, dsr, ri, dcd} = loopback ? {mcr[`UART_MC_DTR],mcr[`UART_MC_RTS],mcr[`UART_MC_OUT1],mcr[`UART_MC_OUT2]}

                                                                                        : ~{cts_pad_i,dsr_pad_i,ri_pad_i,dcd_pad_i};

assign                  {cts_c, dsr_c, ri_c, dcd_c} = loopback ? {mcr[`UART_MC_RTS],mcr[`UART_MC_DTR],mcr[`UART_MC_OUT1],mcr[`UART_MC_OUT2]}

                      :  {cts_pad_i,dsr_pad_i,ri_pad_i,dcd_pad_i};

assign                                                                  dlab = lcr[`UART_LC_DL];

assign                                                                  loopback = mcr[4];

// assign modem outputs

assign                                                                  rts_pad_o = mcr[`UART_MC_RTS];

assign                                                                  dtr_pad_o = mcr[`UART_MC_DTR];

// Interrupt signals

wire                                                                            rls_int;  // receiver line status interrupt

wire                                                                            rda_int;  // receiver data available interrupt

wire                                                                            ti_int;   // timeout indicator interrupt

wire                                                                            thre_int; // transmitter holding register empty interrupt

wire                                                                            ms_int;   // modem status interrupt

// FIFO signals

reg                                                                             tf_push;

reg                                                                             rf_pop;

wire [`UART_FIFO_REC_WIDTH-1:0]  rf_data_out;

wire                                                                            rf_error_bit; // an error (parity or framing) is inside the fifo

wire [`UART_FIFO_COUNTER_W-1:0]  rf_count;

wire [`UART_FIFO_COUNTER_W-1:0]  tf_count;

wire [2:0]                                                               tstate;

wire [3:0]                                                               rstate;

wire [9:0]                                                               counter_t;

// Transmitter Instance

uart_transmitter transmitter(clk, wb_rst_i, lcr, tf_push, wb_dat_i, enable, stx_pad_o, tstate, tf_count, tx_reset, lsr_mask);

// Receiver Instance

uart_receiver receiver(clk, wb_rst_i, lcr, rf_pop, srx_pad_i, enable, rda_int,

        counter_t, rf_count, rf_data_out, rf_error_bit, rf_overrun, rx_reset, lsr_mask, rstate, rf_push);

// Asynchronous reading here because the outputs are sampled in uart_wb.v file 

always @(dl or dlab or ier or iir or scratch

                        or lcr or lsr or msr or rf_data_out or wb_addr_i or wb_re_i)   // asynchrounous reading

begin

        case (wb_addr_i)

                `UART_REG_RB   : wb_dat_o = dlab ? dl[`UART_DL1] : rf_data_out[10:3];

                `UART_REG_IE    : wb_dat_o = dlab ? dl[`UART_DL2] : ier;

                `UART_REG_II    : wb_dat_o = {4'b1100,iir};

                `UART_REG_LC    : wb_dat_o = lcr;

                `UART_REG_LS    : wb_dat_o = lsr;

                `UART_REG_MS    : wb_dat_o = msr;

                `UART_REG_SR    : wb_dat_o = scratch;

                default:  wb_dat_o = 8'b0; // ??

        endcase // case(wb_addr_i)

end // always @ (dl or dlab or ier or iir or scratch...

// rf_pop signal handling

always @(posedge clk or posedge wb_rst_i)

begin

        if (wb_rst_i)

                rf_pop <= #1 0;

        else

        if (rf_pop)     // restore the signal to 0 after one clock cycle

                rf_pop <= #1 0;

        else

        if (wb_re_i && wb_addr_i == `UART_REG_RB && !dlab)

                rf_pop <= #1 1; // advance read pointer

end

wire    lsr_mask_condition;

wire    iir_read;

wire  msr_read;

wire    fifo_read;

wire    fifo_write;

assign lsr_mask_condition = (wb_re_i && wb_addr_i == `UART_REG_LS && !dlab);

assign iir_read = (wb_re_i && wb_addr_i == `UART_REG_II && !dlab);

assign msr_read = (wb_re_i && wb_addr_i == `UART_REG_MS && !dlab);

assign fifo_read = (wb_re_i && wb_addr_i == `UART_REG_RB && !dlab);

assign fifo_write = (wb_we_i && wb_addr_i == `UART_REG_TR && !dlab);

// lsr_mask_d delayed signal handling

always @(posedge clk or posedge wb_rst_i)

begin

        if (wb_rst_i)

                lsr_mask_d <= #1 0;

        else // reset bits in the Line Status Register

                lsr_mask_d <= #1 lsr_mask_condition;

end

// lsr_mask is rise detected

assign lsr_mask = lsr_mask_condition && ~lsr_mask_d;

// msi_reset signal handling

always @(posedge clk or posedge wb_rst_i)

begin

        if (wb_rst_i)

                msi_reset <= #1 1;

        else

        if (msi_reset)

                msi_reset <= #1 0;

        else

        if (msr_read)

                msi_reset <= #1 1; // reset bits in Modem Status Register

end

//

//   WRITES AND RESETS   //

//

// Line Control Register

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i)

                lcr <= #1 8'b00000011; // 8n1 setting

        else

        if (wb_we_i && wb_addr_i==`UART_REG_LC)

                lcr <= #1 wb_dat_i;

// Interrupt Enable Register or UART_DL2

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i)

        begin

                ier <= #1 4'b0000; // no interrupts after reset

                dl[`UART_DL2] <= #1 8'b0;

        end

        else

        if (wb_we_i && wb_addr_i==`UART_REG_IE)

                if (dlab)

                begin

                        dl[`UART_DL2] <= #1 wb_dat_i;

                end

                else

                        ier <= #1 wb_dat_i[3:0]; // ier uses only 4 lsb

// FIFO Control Register and rx_reset, tx_reset signals

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) begin

                fcr <= #1 2'b11;

                rx_reset <= #1 0;

                tx_reset <= #1 0;

        end else

        if (wb_we_i && wb_addr_i==`UART_REG_FC) begin

                fcr <= #1 wb_dat_i[7:6];

                rx_reset <= #1 wb_dat_i[1];

                tx_reset <= #1 wb_dat_i[2];

        end else begin

                rx_reset <= #1 0;

                tx_reset <= #1 0;

        end

// Modem Control Register

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i)

                mcr <= #1 5'b0;

        else

        if (wb_we_i && wb_addr_i==`UART_REG_MC)

                        mcr <= #1 wb_dat_i[4:0];

// Scratch register

// Line Control Register

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i)

                scratch <= #1 0; // 8n1 setting

        else

        if (wb_we_i && wb_addr_i==`UART_REG_SR)

                scratch <= #1 wb_dat_i;

// TX_FIFO or UART_DL1

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i)

        begin

                dl[`UART_DL1]  <= #1 8'b0;

                tf_push   <= #1 1'b0;

                start_dlc <= #1 1'b0;

        end

        else

        if (wb_we_i && wb_addr_i==`UART_REG_TR)

                if (dlab)

                begin

                        dl[`UART_DL1] <= #1 wb_dat_i;

                        start_dlc <= #1 1'b1; // enable DL counter

                        tf_push <= #1 1'b0;

                end

                else

                begin

                        tf_push   <= #1 1'b1;

                        start_dlc <= #1 1'b0;

                end // else: !if(dlab)

        else

        begin

                start_dlc <= #1 1'b0;

                tf_push   <= #1 1'b0;

        end // else: !if(dlab)

// Receiver FIFO trigger level selection logic (asynchronous mux)

always @(fcr)

        case (fcr[`UART_FC_TL])

                2'b00 : trigger_level = 1;

                2'b01 : trigger_level = 4;

                2'b10 : trigger_level = 8;

                2'b11 : trigger_level = 14;

        endcase // case(fcr[`UART_FC_TL])

//

//  STATUS REGISTERS  //

//

// Modem Status Register

reg [3:0] delayed_modem_signals;

always @(posedge clk or posedge wb_rst_i)

begin

        if (wb_rst_i)

                msr <= #1 0;

        else begin

                msr[`UART_MS_DDCD:`UART_MS_DCTS] <= #1 msi_reset ? 4'b0 :

                        msr[`UART_MS_DDCD:`UART_MS_DCTS] | ({dcd, ri, dsr, cts} ^ delayed_modem_signals[3:0]);

                msr[`UART_MS_CDCD:`UART_MS_CCTS] <= #1 {dcd_c, ri_c, dsr_c, cts_c};

                delayed_modem_signals[3:0] <= #1 {dcd, ri, dsr, cts};

        end

end

// Line Status Register

// activation conditions

assign lsr0 = (rf_count==0 && rf_push);  // data in receiver fifo available set condition

assign lsr1 = rf_overrun;     // Receiver overrun error

assign lsr2 = rf_data_out[1]; // parity error bit

assign lsr3 = rf_data_out[0]; // framing error bit

assign lsr4 = rf_data_out[2]; // break error in the character

assign lsr5 = (tf_count==5'b0);  // transmitter fifo is empty

assign lsr6 = (tf_count==5'b0 && (tstate == /*`S_IDLE */ 0)); // transmitter empty

assign lsr7 = rf_error_bit;

// lsr bit0 (receiver data available)

reg      lsr0_d;

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) lsr0_d <= #1 0;

        else lsr0_d <= #1 lsr0;

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) lsr0r <= #1 0;

        else lsr0r <= #1 (rf_count==1 && fifo_read || rx_reset) ? 0 : // deassert condition

                                          lsr0r || (lsr0 && ~lsr0_d); // set on rise of lsr0 and keep asserted until deasserted 

// lsr bit 1 (receiver overrun)

reg lsr1_d; // delayed

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) lsr1_d <= #1 0;

        else lsr1_d <= #1 lsr1;

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) lsr1r <= #1 0;

        else    lsr1r <= #1     lsr_mask ? 0 : lsr1r || (lsr1 && ~lsr1_d); // set on rise

// lsr bit 2 (parity error)

reg lsr2_d; // delayed

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) lsr2_d <= #1 0;

        else lsr2_d <= #1 lsr2;

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) lsr2r <= #1 0;

        else lsr2r <= #1 lsr_mask ? 0 : lsr2r || (lsr2 && ~lsr2_d); // set on rise

// lsr bit 3 (framing error)

reg lsr3_d; // delayed

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) lsr3_d <= #1 0;

        else lsr3_d <= #1 lsr3;

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) lsr3r <= #1 0;

        else lsr3r <= #1 lsr_mask ? 0 : lsr3r || (lsr3 && ~lsr3_d); // set on rise

// lsr bit 4 (break indicator)

reg lsr4_d; // delayed

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) lsr4_d <= #1 0;

        else lsr4_d <= #1 lsr4;

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) lsr4r <= #1 0;

        else lsr4r <= #1 lsr_mask ? 0 : lsr4r || (lsr4 && ~lsr4_d);

// lsr bit 5 (transmitter fifo is empty)

reg lsr5_d;

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) lsr5_d <= #1 1;

        else lsr5_d <= #1 lsr5;

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) lsr5r <= #1 1;

        else lsr5r <= #1 (fifo_write) ? 0 :  lsr5r || (lsr5 && ~lsr5_d);

// lsr bit 6 (transmitter empty indicator)

reg lsr6_d;

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) lsr6_d <= #1 1;

        else lsr6_d <= #1 lsr6;

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) lsr6r <= #1 1;

        else lsr6r <= #1 (fifo_write) ? 0 : lsr6r || (lsr6 && ~lsr6_d);

// lsr bit 7 (error in fifo)

reg lsr7_d;

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) lsr7_d <= #1 0;

        else lsr7_d <= #1 lsr7;

always @(posedge clk or posedge wb_rst_i)

        if (wb_rst_i) lsr7r <= #1 0;

        else lsr7r <= #1 lsr_mask ? 0 : lsr7r || (lsr7 && ~lsr7_d);

// Frequency divider

always @(posedge clk or posedge wb_rst_i)

begin

        if (wb_rst_i)

                dlc <= #1 0;

        else

                if (start_dlc | ~ (|dlc))

                        dlc <= #1 dl - 1;               // preset counter

                else

                        dlc <= #1 dlc - 1;              // decrement counter

end

// Enable signal generation logic

always @(posedge clk or posedge wb_rst_i)

begin

        if (wb_rst_i)

                enable <= #1 1'b0;

        else

                if (|dl & ~(|dlc))     // dl>0 & dlc==0

                        enable <= #1 1'b1;

                else

                        enable <= #1 1'b0;