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

//

// Filename:    wbscopc.v

//

// Project:     FPGA Library of Routines

//

// Purpose:     This scope is identical in function to the wishbone scope

//      found in wbscope, save that the output is compressed and that (as a

//      result) it can only handle recording 31 bits at a time.  This allows

//      the top bit to indicate an 'address difference'.   Okay, there's 

//      another difference as well: this version only works in a synchronous

//      fashion with the clock from the WB bus.  You cannot have a separate

//      bus and data clock.

//

//      Reading/decompressing the output of this scope works in this fashion:

//      Once the scope has stopped, read from the port.  Any time the high

//      order bit is set, the other 31 bits tell you how many times to repeat

//      the last value.  If the high order bit is not set, then the value

//      is a new data value.

//

//      I've provided this version of a compressed scope to OpenCores for

//      discussion purposes.  While wbscope.v works and works well by itself,

//      this compressed scope has a couple of fundamental flaw that I have

//      yet to fix.  One of them is that it is impossible to know when the

//      trigger took place.  The second problem is that it may be impossible

//      to know the state of the scope at the beginning of the buffer--should

//      the buffer begin with an address difference value instead of a data

//      value.

//

//      Ideally, the first item read out of the scope should be a data value,

//      even if the scope was skipping values to a new address at the time.

//      If it was in the middle of a skip, the next item out of the scope

//      should be the skip length.  This, though, violates the rule that there

//      are (1<<LGMEMLEN) items in the memory, and that the trigger took place

//      on the last item of memory ... so that portion of this compressed

//      scope is still to be defined.

//

//      Like I said, this version is placed here for discussion purposes,

//      not because it runs well nor because I have recognized that it has any

//      particular value (yet).

//

//      Well, I take that back.  When dealing with an interface such as the

//      PS/2 interface, or even the 16x2 LCD interface, it is often true

//      that things change _very_ slowly.  They could change so slowly that

//      the other approach to the scope doesn't work.  This then gives you

//      a working scope, by only capturing the changes.  You'll still need

//      to figure out (after the fact) when the trigge took place.  Perhaps

//      you'll wish to add the trigger as another data line, so you can find

//      when it took place in your own data?

//

//      Okay, I take that back twice: I'm finding this compressed scope very

//      valuable for evaluating the timing associated with a GPS PPS and

//      associated NMEA stream.  I need to collect over a seconds worth of

//      data, and I don't have enough memory to handle one memory value per

//      clock, yet I still want to know exactly when the GPS PPS goes high,

//      when it goes low, when I'm adjusting my clock, and when the clock's

//      PPS output goes high.  Did I synchronize them well?  Oh, and when does

//      the NMEA time string show up when compared with the PPS?  All of those

//      are valuable, but could never be done if the scope wasn't compressed.

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Technology, LLC

//

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

//

// Copyright (C) 2015, Gisselquist Technology, LLC

//

// This program is free software (firmware): you can redistribute it and/or

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

// by the Free Software Foundation, either version 3 of the License, or (at

// your option) any later version.

//

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

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

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

// for more details.

//

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

// with this program.  (It's in the $(ROOT)/doc directory, run make with no

// target there if the PDF file isn't present.)  If not, see

// <http://www.gnu.org/licenses/> for a copy.

//

// License:     GPL, v3, as defined and found on www.gnu.org,

//              http://www.gnu.org/licenses/gpl.html

//

//

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

//

//

module wbscopc(i_clk, i_ce, i_trigger, i_data,

        i_wb_clk, i_wb_cyc, i_wb_stb, i_wb_we, i_wb_addr, i_wb_data,

        o_wb_ack, o_wb_stall, o_wb_data,

        o_interrupt);

        parameter       LGMEM = 5'd10, NELM=31, BUSW = 32, SYNCHRONOUS=1;

        // The input signals that we wish to record

        input                           i_clk, i_ce, i_trigger;

        input           [(NELM-1):0]     i_data;

        // The WISHBONE bus for reading and configuring this scope

        input                           i_wb_clk, i_wb_cyc, i_wb_stb, i_wb_we;

        input                           i_wb_addr; // One address line only

        input           [(BUSW-1):0]     i_wb_data;

        output  wire                    o_wb_ack, o_wb_stall;

        output  wire    [(BUSW-1):0]     o_wb_data;

        // And, finally, for a final flair --- offer to interrupt the CPU after

        // our trigger has gone off.  This line is equivalent to the scope 

        // being stopped.  It is not maskable here.

        output  wire                    o_interrupt;

        // Let's first see how far we can get by cheating.  We'll use the

        // wbscope program, and suffer a lack of several features

        // When is the full scope reset?  Capture that reset bit from any

        // write.

        wire    lcl_reset;

        assign  lcl_reset = (i_wb_cyc)&&(i_wb_stb)&&(~i_wb_addr)&&(i_wb_we)

                                &&(~i_wb_data[31]);

        // A big part of this scope is the 'address' of any particular

        // data value.  As of this current version, the 'address' changed

        // in definition from an absolute time (which had all kinds of

        // problems) to a difference in time.  Hence, when the address line

        // is high on decompression, the 'address' field will record an

        // address difference.

        //

        // To implement this, we set our 'address' to zero any time the

        // data changes, but increment it on all other clocks.  Should the

        // address difference get to our maximum value, we let it saturate

        // rather than overflow.

        reg     [(BUSW-2):0]     ck_addr;

        reg     [(NELM-1):0]     lst_dat;

        initial ck_addr = 0;

        always @(posedge i_clk)

                if ((lcl_reset)||((i_ce)&&(i_data != lst_dat)))

                        ck_addr <= 0;

                else if (&ck_addr)

                        ;       // Saturated (non-overflowing) address diff

                else

                        ck_addr <= ck_addr + 1;

        //

        // To do our compression, we keep track of two registers: the most

        // recent data to the device (imm_ prefix) and the data from one

        // clock ago.  This allows us to suppress writes to the scope which

        // would otherwise be two address writes in a row.

        reg     imm_adr, lst_adr; // Is this an address (1'b1) or data value?

        reg     [(BUSW-2):0]     lst_val, // Data for the scope, delayed by one

                                imm_val; // Data to write to the scope

        initial lst_dat = 0;

        initial lst_adr = 1'b1;

        initial imm_adr = 1'b1;

        always @(posedge i_clk)

                if (lcl_reset)

                begin

                        imm_val <= 31'h0;

                        imm_adr <= 1'b1;

                        lst_val <= 31'h0;

                        lst_adr <= 1'b1;

                        lst_dat <= 0;

                end else if ((i_ce)&&(i_data != lst_dat))

                begin

                        imm_val <= { {(BUSW-1-NELM){1'b0}}, i_data };

                        imm_adr <= 1'b0;

                        lst_val <= imm_val;

                        lst_adr <= imm_adr;

                        lst_dat <= i_data;

                end else begin

                        imm_val <= ck_addr; // Minimum value here is '1'

                        imm_adr <= 1'b1;

                        lst_val <= imm_val;

                        lst_adr <= imm_adr;

                end

        //

        // Here's where we suppress writing pairs of address words to the

        // scope at once.

        //

        reg                     r_ce;

        reg     [(BUSW-1):0]     r_data;

        initial                 r_ce = 1'b0;

        always @(posedge i_clk)

                r_ce <= (~lst_adr)||(~imm_adr);

        always @(posedge i_clk)

                r_data <= ((~lst_adr)||(~imm_adr))

                        ? { lst_adr, lst_val }

                        : { {(32 - NELM){1'b0}}, i_data };

        //

        // The trigger needs some extra attention, in order to keep triggers

        // that happen between events from being ignored.  

        //

        wire    w_trigger;

        assign  w_trigger = (r_trigger)||(i_trigger);

        reg     r_trigger;

        initial r_trigger = 1'b0;

        always @(posedge i_clk)

                if (lcl_reset)

                        r_trigger <= 1'b0;

                else

                        r_trigger <= w_trigger;

        //

        // Call the regular wishbone scope to do all of our real work, now

        // that we've compressed the input.

        //

        wbscope #(.SYNCHRONOUS(1), .LGMEM(LGMEM),

                .BUSW(BUSW))    cheatersscope(i_clk, r_ce, w_trigger, r_data,

                i_wb_clk, i_wb_cyc, i_wb_stb, i_wb_we, i_wb_addr, i_wb_data,

                o_wb_ack, o_wb_stall, o_wb_data, o_interrupt);

endmodule