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

// Auto Baud core

//

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

// http://www.opencores.org/

// 

//

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

//                                     specification document.)

//

// Copyright (C) 2002 John Clayton and OPENCORES.ORG

//

// 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  : Aug. 20, 2002

// Update: Aug. 20, 2002 copied this file from rs232_syscon.v (pared down).

// Update: Sep.  4, 2002 First test of this module.  The baud rate appears

//                       to be produced at 1/2 of the desired rate!

// Update: Sep.  5, 2002 First working results.  Fixed measurement (shift had

//                       been left out.)  Removed debug port since the unit

//                       appears to be working fine.  It Worked for all of the

//                       following BAUD rates, using 49.152 MHz clock and

//                       CLK_FACTOR = 8 and 16 bit main counter:

//                       300, 1200, 2400, 9600, 19200, 38400, 57600, 115200.

//                       Next step is to build the "tracking" version that

//                       doesn't need a reset to find a new BAUD rate...

// Update: Sep. 13, 2002 Added test data from "auto_baud_with_tracking.v"

//                       module tests.  This module has also been tested

//                       at various speeds, and it works well.

//

// Description

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

// This is a state-machine driven core that measures transition intervals

// in a particular character arriving via rs232 transmission (i.e. PC serial 

// port.)  Measurements of time intervals between transitions in the received

// character are then used to generate a baud rate clock for use in serial

// communications back and forth with the device that originally transmitted

// the measured character.  The clock which is generated is in reality a

// clock enable pulse, one single clock wide, occurring at a rate suitable

// for use in serial communications.  (This means that it will be generated

// at 4x or 8x or 16x the actual measured baud rate of the received character.

// The multiplication factor is called "CLOCK_FACTOR_PP" and is a settable

// parameter within this module.  The parameter "CLOCK_FACTOR_PP" need not

// be a power of two, but it should be a number between 2 and 16 inclusive.)

//

// The particular character which is targeted for measurement and verification

// in this module is: carriage return (CR) = 0x0d = 13.

// This particular character was chosen because it is frequently used at the

// end of a command line, as entered at a keyboard by a human user interacting

// with a command interpreter.  It is anticipated that the user would press

// the "enter" key once upon initializing communications with the electronic

// device, and the resulting carriage return character would be used for 

// determining BAUD rate, thus allowing the device to respond at the correct

// rate, and to carry on further communications.  The electronic device using

// this "auto_baud" module adjusts its baud rate to match the baud rate of

// the received data.  This works for all baud rates, within certain limits,

// and for all system clock rates, within certain limits.

//

// Received serially, the carriage return appears as the following waveform:

// ________    __    ____          _______________

//         |__|d0|__|d2d3|________|stop

//        start   d1      d4d5d6d7

//

// The waveform is shown with an identical "high" time and "low" time for

// each bit.  However, actual measurements taken using a logic analyzer

// on characters received from a PC show that the times are not equal.

// The "high" times turned out shorter, and the "low" times longer...

// Therefore, this module attempts to average out this discrepancy by

// measuring one low time and one high time.

//

// Since the transition measurements must unavoidably contain small amounts

// of error, the measurements are made during the beginning 2 bits of

// the received character, (that is, start bit and data bit zero).

// Then the measurement is immediately transformed into a baud rate clock,

// used to verify correct reception of the remaining 8 bits of the character.

// If the entire character is not received correctly using the generated

// baud rate, then the measurement is scrapped, and the unit goes into an

// idle scanning mode waiting for another character to test.

//

// This effectively filters out characters that the unit is not interested in

// receiving (anything that is not a carriage return.)  There is a slight

// possibility that a group of other characters could appear by random

// chance in a configuration that resembles a carriage return closely enough

// that the unit might accept the measurement and produce a baud clock too

// low.  But the probability of this happening is remote enough that the

// unit is considered highly "robust" in normal use, especially when used

// for command entry by humans.  It would take a very clever user indeed, to

// enter the correct series of characters with the correct intercharacter

// timing needed to possibly "fool" the unit!

//

// (Also, the baud rate produced falls within certain limits imposed by

//  the hardware of the unit, which prevents the auto_baud unit from mistaking

//  a series of short glitches on the serial data line for a really

//  fast CR character.)

//

// The first carriage return character received will produce a BAUD rate clock

// and the unit will indicate a "locked" condition.  From that point onward,

// the unit will continue running, but it will not scan for any more

// input characters.  The only way to reset the unit to its initial condition

// is through the use of the "reset_i" pin.  Following reset, the unit is

// once again looking for a carriage return character to lock on to.

// Another module, called "auto_baud_with_tracking.v" handles situations where

// you might want to have the BAUD rate change dynamically.

//

//

// NOTES:

// - This module uses a counter to divide down the clk_i signal to produce the

//   baud_clk_o signal.  Since the frequency of baud_clk_o is nominally

//   CLOCK_FACTOR_PP * rx_baud_rate, where "rx_baud_rate" is the baud rate

//   of the received character, then the higher you make CLOCK_FACTOR_PP, the

//   higher the generated baud_clk_o signal frequency, and hence the lower the

//   resolution of the divider.  Therefore, using a lower value for the

//   CLOCK_FACTOR_PP will allow you to use a lower clk_i with this module.

// - To set LOG2_MAX_COUNT_PP, remember (max_count*CLOCK_FACTOR_PP)/Fclk_i

//   is the maximum measurement time that can be accomodated by the circuit.

//   (where Fclk_i is the frequency of clk_i, and 1/Fclk_i is the period.)

//   Therefore, set LOG2_MAX_COUNT_PP so that the maximum measurement time

//   is at least as long as 2x the baud interval of the slowest received

//   serial data (2x because there are two bits involved in the measurement!)

//   For example, for Fclk_i = 20MHz, CLOCK_FACTOR_PP = 4 and a minimum

//   baud rate of 115,200, you would calculate:

//

//   (max_count * CLOCK_FACTOR_PP)*1/Fclk_i >= 2/Fbaud_max

//

//   Solving for the bit width of the max_count counter...

//

//   LOG2_MAX_COUNT_PP >= ceil(log_base_2(max_count))

//

//                 >= ceil(log_base_2(2*Fclk_i/(Fbaud_max*CLOCK_FACTOR_PP)))

//

//                 >= ceil(log_base_2(2*20E6/(115200*4)))

//

//                 >= ceil(log_base_2(86.8))

//

//                 >= 7 bits.

//

// - In the above example, the maximum count would approach 87, which means

//   that a measurement error of 1 count is about (1/87)=approx. 1.15%.  This

//   is an acceptable level of error for a baud rate clock.  Notice that the

//   lower baud rates have an even smaller error percentage (Yes!) but that

//   they require a much larger measurement counter...  For instance,

//   to lock onto 300 baud using the same example above, would require:

//

//   LOG2_MAX_COUNT_PP >= ceil(log_base_2(40000000/1200))

//

//                     >= ceil(log_base_2(33333.3))

//

//                     >= ceil(15.024678)

//

//                     >= 16 bits.

//

// - If the percentage error for your highest desired baud rate is greater

//   than a few percent, you might want to use a higher Fclk_i or else a 

//   lower CLOCK_FACTOR_PP.

//

// - Using the default settings:  CLK_FACTOR_PP = 8, LOG2_MAX_COUNT_PP = 16

//   The following test results were obtained, using an actual session in

//   hyperterm, looking for correct readable character transmission both

//   directions.  (Note: These tests were performed at "human interface"

//   speeds.  High speed or "back-to-back" character transmission might

//   exhibit worse performance than these results.)

//   The test results shown below were actually obtained using the more

//   complex "auto_baud_with_tracking.v" module.  Similar or better results

//   are expected with this module.

//

//   Clk_i

//   Freq.

//   (MHz)    110    300    1200   2400   4800   9600   19200  57600  115200

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

//   98       FAIL   pass   pass   pass   pass   pass   pass   pass   pass

//   55       FAIL   pass   pass   pass   pass   pass   pass   pass   pass

//   49       pass   pass   pass   pass   pass   pass   pass   pass   pass

//   24.5     pass   pass   pass   pass   pass   pass   pass   pass   FAIL

//   12       pass   pass   pass   pass   pass   pass   pass   pass   FAIL

//    6       pass   pass   pass   pass   pass   pass   pass   FAIL   FAIL

//    3       pass   pass   pass   pass   pass   FAIL   FAIL   FAIL   FAIL

//    1.5     pass   pass   pass   pass   FAIL   FAIL   FAIL   FAIL   FAIL

//

//

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

`define LOG2_MAX_CLOCK_FACTOR 4  // Sets the size of the CLOCK_FACTOR

                                 // prescaler.

`define BITS_PER_CHAR         8  // Include parity, if used, but not

                                 // start and stop bits...

// Note: Simply changing the template bits does not reconfigure the

//       module to look for a different character (because a new measurement

//       window would have to be defined for a different character...)

//       The template bits are the exact bits used during verify, against

//       which the incoming character is checked.

//       The LSB of the character is discarded, and the stop bit is appended

//       since it is the last bit used during verify.

//

// so, for N:8:1 (no parity, 8 data bits, 1 stop bit) it is:

//                         = {1,(character>>1)} = 9'h086

// or, with parity included it is:

//                         = {1,parity_bit,(character>>1)} = 9'h106

//

`define TEMPLATE_BITS     9'h086  // Carriage return && termination flag

module auto_baud

  (

  clk_i,

  reset_i,

  serial_dat_i,

  auto_baud_locked_o,

  baud_clk_o

  );

// Parameters

// CLOCK_FACTOR_PP can be from [2..16] inclusive.

parameter CLOCK_FACTOR_PP = 8;     // Baud clock multiplier

parameter LOG2_MAX_COUNT_PP = 16;  // Bit width of measurement counter

// State encodings, provided as parameters

// for flexibility to the one instantiating the module.

// In general, the default values need not be changed.

// There is one state machines: m1.

// "default" state upon power-up and configuration is:

//    "m1_idle" because that is the all zero state.

parameter m1_idle          = 4'h0;  // Initial state (scanning)

parameter m1_measure_0     = 4'h1;  // start bit     (measuring)

parameter m1_measure_1     = 4'h2;  // debounce      (measuring)

parameter m1_measure_2     = 4'h3;  // data bit 0    (measuring)

parameter m1_measure_3     = 4'h4;  // debounce      (measuring)

parameter m1_measure_4     = 4'h5;  // measurement done (headed to verify)

parameter m1_verify_0      = 4'h8;  // data bit 1    (verifying)

parameter m1_verify_1      = 4'h9;  // data bit 2    (verifying)

parameter m1_run           = 4'h6;  // running

parameter m1_verify_failed = 4'h7;  // resetting   (headed back to idle)

// I/O declarations

input clk_i;                 // System clock input

input reset_i;               // Reset signal for this module

input serial_dat_i;          // TTL level serial data signal

output auto_baud_locked_o;   // Indicates BAUD clock is being generated

output baud_clk_o;           // BAUD clock output (actually a clock enable)

// Internal signal declarations

wire mid_bit_count;       // During measurement, advances measurement counter

                          // (Using the mid bit time to advance the

                          //  measurement timer accomplishes a timing "round"

                          //  so that the timing measurement is as accurate

                          //  as possible.)

                          // During verify, pulses at mid bit time are used

                          // to signal the state machine to check a data bit.

wire main_count_rollover;       // Causes main_count to roll over

wire clock_count_rollover;      // Causes clock_count to roll over

                                // (when clock_count is used as a 

                                //  clock_factor prescaler)

wire enable_clock_count;        // Logic that determines when clock_count

                                // should be counting

wire verify_done;               // Indicates finish of verification time

reg idle;                 // Indicates state

reg run;                  // Indicates state

reg measure;              // Indicates state

reg clear_counters;       // Pulses once when measurement is done.

reg verify;               // Indicates state

reg character_miscompare; // Indicates character did not verify

reg [`LOG2_MAX_CLOCK_FACTOR-1:0] clock_count; // Clock_factor prescaler,

                                              // and mid_bit counter.

reg [LOG2_MAX_COUNT_PP-1:0] main_count;       // Main counter register

reg [LOG2_MAX_COUNT_PP-1:0] measurement;      // Stored measurement count

reg [`BITS_PER_CHAR:0] target_bits;           // Character bits to compare

// (lsb is not needed, since it is used up during measurement time,

//  but the stop bit and possible parity bit are needed.)

    // For the state machine

reg [3:0] m1_state;

reg [3:0] m1_next_state;

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

// Instantiations

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

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

// Module code

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

// This is the CLOCK_FACTOR_PP prescaler and also mid_bit_count counter

assign enable_clock_count = measure || (verify && main_count_rollover);

always @(posedge clk_i or posedge reset_i)

begin

  if (reset_i) clock_count <= 0;

  else if (clear_counters) clock_count <= 0;

  else if (enable_clock_count)

  begin  // Must have been clk_i edge

    if (clock_count_rollover) clock_count <= 0;

    else clock_count <= clock_count + 1;

  end

end

// Counter rollover condition

assign clock_count_rollover = (clock_count == (CLOCK_FACTOR_PP-1));

// This condition signals the middle of a bit time, for use during the

// verify stage.  Also, this signal is used to advance the main count,

// instead of "clock_count_rollover."  This produces an effective "rounding"

// operation for the measurement (which would otherwise "truncate" any

// fraction of time contained in this counter at the instant the measurement

// is finished.

// (The "enable_clock_count" is included in order to make the pulse narrow,

//  only one clock wide...)

assign mid_bit_count = (

                        (clock_count == ((CLOCK_FACTOR_PP>>1)-1))

                        && enable_clock_count

                        );

// This is the main counter.  During measurement, it advances once for

// each CLOCK_FACTOR_PP cycles of clk_i.  This accumulated measurement

// is then latched into "measurement" when the state machine determines that

// the measurement interval is finished.

// During verify and idle_run times (whenever the baud rate clock is used)

// this counter is allowed to run freely, advancing once each clk_i, but being

// reset when it reaches a total count of "measurement" clock cycles.  The

// signal that reset the counter during this type of operation is the baud

// rate clock.

always @(posedge clk_i or posedge reset_i)

begin

  if (reset_i) main_count <= 0;

  else // must have been clk_i edge

  begin

    // Clear main count when measurement is done

    if (clear_counters) main_count <= 0;

    // If measuring, advance once per CLOCK_FACTOR_PP clk_i pulses.

    else if (measure && mid_bit_count) main_count <= main_count + 1;

    // If verifying or running, check reset conditions, 

    // otherwise advance always.

    else if (verify || run)

    begin

      if (main_count_rollover) main_count <= 0;

      else main_count <= main_count + 1;

    end

  end

end

// This is a shift register used to provide "target" character bits one at

// a time for verification as they are "received" (sampled) using the

// candidate baud clock.

always @(posedge clk_i or posedge reset_i)

begin

  if (reset_i) target_bits <= `TEMPLATE_BITS;

  else // must have been a clock edge

  begin

    if (~verify) target_bits <= `TEMPLATE_BITS;

    if (verify && mid_bit_count) target_bits <= {0,(target_bits>>1)};

  end

end

// It is done when only the stop bit is left in the shift register.

assign verify_done = (

                       (target_bits == 1)

                       && verify

                       && mid_bit_count

                      );

// This is a flip-flop used to keep track of whether the verify operation

// is succeeding or not.  Any target bits that do not match the received

// data at the sampling edge, will cause the verify_failed bit to go high.

// This is what the state machine looks at to determine whether it passed

// or not.

always @(posedge clk_i or posedge reset_i)

begin

  if (reset_i) character_miscompare <= 0;

  else  // Must have been a clock edge

  begin

    if (idle) character_miscompare <= 0;

    if (verify && mid_bit_count

        && (target_bits[0] ^ serial_dat_i)) character_miscompare <= 1;

  end

end

// This is the measurement storage latch.  The final measured time count

// from main_count is stored in this latch upon completion of the measurement

// interval.  The value stored in this latch is used whenever the baud clock

// is being generated, to reset the main count (causing a "rollover").

always @(posedge clk_i or posedge idle)

begin

  // Set to all ones during idle (asynchronous).

  if (idle) measurement <= -1;

  // Otherwise, there must have been a clk_i edge

  // When the measurement is done, the counters are cleared, and the time

  // interval must be stored before it is cleared away...

  // This also causes a store following a failed verify state on the way back

  // into idle, but the idle state clears out the false measurement anyway.

  else if (clear_counters) measurement <= (main_count>>1);

end

// This is effectively the baud clock signal

// But it is prevented from reaching the output pin during verification...

// It is only allowed out of the module during idle_run state.

assign main_count_rollover = (main_count == measurement);

assign baud_clk_o = (main_count_rollover && run);

// This is state machine m1.  It checks the status of the serial_dat_i line

// and coordinates the measurement of the time interval of the first two

// bits of the received character, which is the "measurement interval."

// Following the measurement interval, the state machine enters a new

// phase of bit verification.  If the measured time interval is accurate

// enough to measure the remaining 8 bits of the character correctly, then

// the measurement is accepted, and the baud rate clock is driven onto

// the baud_clk_o output pin.  Incidentally, the process of verification

// effectively filters out all characters which are not the desired target

// character for measurement.  In this case, the target character is the

// carriage return.

// State register

always @(posedge clk_i or posedge reset_i)

begin : m1_state_register

  if (reset_i) m1_state <= m1_idle;          // asynchronous reset

  else m1_state <= m1_next_state;

end

// State transition logic

always @(m1_state

         or mid_bit_count

         or serial_dat_i

         or verify_done

         or character_miscompare

         )

begin : m1_state_logic

  // Default values for outputs.  The individual states can override these.

  idle <= 1'b0;

  run <= 1'b0;

  measure <= 1'b0;

  clear_counters <= 1'b0;

  verify <= 1'b0;

  case (m1_state) // synthesis parallel_case

    m1_idle :

      begin

        idle <= 1'b1;

        if (serial_dat_i == 0) m1_next_state <= m1_measure_0;

        else m1_next_state <= m1_idle;

      end

    m1_measure_0 :

      begin

        measure <= 1'b1;

        // Check at mid bit time, to make sure serial line is still low...

        // (At this time, "mid_bit_count" is simply CLOCK_FACTOR_PP>>1 clk_i's.)

        if (mid_bit_count && ~serial_dat_i) m1_next_state <= m1_measure_1;

        // If it is not low, then it must have been a "glitch"...

        else if (mid_bit_count && serial_dat_i) m1_next_state <= m1_idle;

        else m1_next_state <= m1_measure_0;

      end

    m1_measure_1 :

      begin

        measure <= 1'b1;

        // Look for first data bit (high).

        if (serial_dat_i) m1_next_state <= m1_measure_2;

        // If it is not high keep waiting...

        // (Put detection of measurement overflow in here if necessary...)

        else m1_next_state <= m1_measure_1;

      end

    m1_measure_2 :

      begin

        measure <= 1'b1;

        // Check using mid bit time, to make sure serial line is still high...

        // (At this time, "mid_bit_count" is simply CLOCK_FACTOR_PP>>1 clk_i's.)

        if (mid_bit_count && serial_dat_i) m1_next_state <= m1_measure_3;

        // If it is not high, then it must have been a "glitch"...

        else if (mid_bit_count && ~serial_dat_i) m1_next_state <= m1_idle;

        else m1_next_state <= m1_measure_2;

      end

    m1_measure_3 :

      begin

        measure <= 1'b1;

        // Look for end of measurement interval (low)

        if (!serial_dat_i) m1_next_state <= m1_measure_4;

        // If it is not high keep waiting...

        // (Put detection of measurement overflow in here if necessary...)

        else m1_next_state <= m1_measure_3;

      end

    // This state outputs a reset pulse, to clear counters and store the

    // measurement from main_count.

    m1_measure_4 :

      begin

        clear_counters <= 1'b1; // Clears counters, stores measurement

        m1_next_state <= m1_verify_0;

      end

    m1_verify_0 :  // Wait for verify operations to finish

      begin

        verify <= 1'b1;

        if (verify_done) m1_next_state <= m1_verify_1;

        else m1_next_state <= m1_verify_0;

      end

    // NOTE: This "extra" state is needed because the character_miscompare

    //       information is not valid until 1 cycle after verify_done is

    //       active.

    m1_verify_1 :  // Checks for character miscompare

      begin

        if (character_miscompare) m1_next_state <= m1_verify_failed;

        else m1_next_state <= m1_run;

      end

    m1_verify_failed : // Resets counters on the way back to idle

      begin

        clear_counters <= 1'b1;

        m1_next_state <= m1_idle;

      end

    // This state is for successful verification results!

    // Since this is a single measurement unit, only reset can exit this

    // state.

    m1_run :

      begin

        run <= 1'b1;

        m1_next_state <= m1_run;

      end

    default : m1_next_state <= m1_idle;

  endcase

end

assign auto_baud_locked_o = run;

endmodule

//`undef LOG2_MAX_CLOCK_FACTOR