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

//

// Filename:    wbfmtxhack.v

//              

// Project:     A Wishbone Controlled FM Transmitter Hack

//

// Purpose:     This Hack is based off of two things: 1) the interface spec

//              of the WB controlled PWM audio device, and 2) a Raspberry Pi

//      Hack I was shown that converted the RPi PWM device into an FM

//      transmitter.  So, the question is, can a GPIO pin be turned into an

//      FM transmitter that can be heard throughout the house?

//

//      We'll try and do this properly: We'll use a Numerically Controlled

//      Oscillator to generate our signal, but only grab the top bit out of

//      that oscillator.  We'll then send this bit to the GPIO pin (a.k.a.

//      antenna) to see if it can accomplish our goals.

//

//      WB Control/Registers:

//      1'b0:   Next Sample

//

//              The top bits of this 'next sample' will indicate the number

//              of clock ticks before we generate a need next sample interrupt.

//              If these top bits are zero, the sample rate will not be

//              adjusted.  The value to set here is the value of the clock

//              rate divided by the desired sample rate.  Hence, if the clock

//              rate is 80MHz, setting this to 10e3 (unsigned) would set us up

//              for an 8kHz sample rate, whereas setting these upper 16 bits to

//              1814 would specify a sample rate closer to 44.1kHz.

//

//              The lower 16 bits specify the value of the next sample.

//

//              Since we'll be dealing with FM modulation, we'll try to arrange

//              that this sixteen bit sample will correspond to a maximum

//              FM deviation of about 75 kHz.

//

//

//      1'b1:   The Oscillator "Frequency" (really stepsize).  This should be

//              used to control/determine the "RF frequency" this device can

//              transmit on.  

//

//              To transmit at 0Hz, set this to zero.  To transmit at

//              CLKSPEED/2 Hz, set this to 32'h8000_0000.  Hence for a 

//              transmit frequency of X, set this value to 

//

//              OSXFREQ = 2^32 * X / CLKSPEED

//

//              Where X and CLKSPEED share the same units.  But how shall we

//              transmit at speeds of anything higher than CLKSPEED/2?  By

//              aliasing up.  Hence, set X to your actual frequency value,

//              divide by the clockspeed and multiply by 2^32.  Remove any

//              bits that don't fit in the top 32 and you are there.

//

//              This also gives us about 20 mHz resolution for our Carrier

//              frequency--overkill perhaps, but it should work.

//

//      So ... how do we create our 75 kHz deviation?  We want:

//

//      MAX_STEPSIZE = 2^32 * (X + 75kHz * sample / 2^15) / CLKSPEED

//      = OSXFREQ = (2^32 * sample / 2^15 / CLKSPEED * 75 kHz)

//      = 123 * sample ~= 128 * sample = sample << 7.

//

//      Thus, by shifting our input sample value a touch, we can multiply by

//      nearly the exact constant we want.

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Technology, LLC

//

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

//

// Copyright (C) 2015-2016, 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  wbfmtxhack(i_clk,

                // Wishbone interface

                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_tx, o_int);

        parameter       DEFAULT_RELOAD = 16'd1814; // 44.1kHz at a 80MHz clock

        input   i_clk;

        input   i_wb_cyc, i_wb_stb, i_wb_we;

        input           i_wb_addr;

        input   [31:0]   i_wb_data;

        output  reg             o_wb_ack;

        output  wire            o_wb_stall;

        output  reg     [31:0]   o_wb_data;

        output  wire            o_tx;

        output  reg             o_int;

        reg     [31:0]   nco_step, nco_phase;

        // How often shall we create an interrupt?  Every reload_value clocks!

        // If VARIABLE_RATE==0, this value will never change and will be kept

        // at the default reload rate (44.1 kHz, for a 100 MHz clock)

        reg     [15:0]   reload_value;

        initial reload_value = DEFAULT_RELOAD;

        // Data write, but we use the upper 16 bits to set our sample rate.

        // If these bits are zero, we ignore the write--allowing users to

        // write samples without adjusting the sample rate.

        always @(posedge i_clk) // Set sample rate

                if ((i_wb_cyc)&&(i_wb_stb)&&(~i_wb_addr)&&(i_wb_we)

                                &&(|i_wb_data[31:16]))

                        reload_value <= i_wb_data[31:16];

        // Set the NCO transmit frequency

        initial nco_step = 32'h00;

        always @(posedge i_clk)

                if ((i_wb_cyc)&&(i_wb_stb)&&(i_wb_addr)&&(i_wb_we))

                        nco_step <= i_wb_data[31:0];

        reg             ztimer;

        reg     [15:0]   timer;

        initial ztimer = 1'b0;

        always @(posedge i_clk) // Be true when the timer is zero

                ztimer <= (timer[15:0] == 16'h1);

        initial timer = reload_value;

        always @(posedge i_clk)

                if (ztimer)

                        timer <= reload_value;

                else

                        timer <= timer - 16'h1;

        reg     [15:0]   next_sample, sample_out;

        initial sample_out  = 16'h00;

        initial next_sample = 16'h00;

        always @(posedge i_clk)

                if (ztimer)

                        sample_out <= next_sample;

        reg             next_valid;

        initial next_valid = 1'b1;

        initial next_sample = 16'h8000;

        always @(posedge i_clk) // Data write

                if ((i_wb_cyc)&&(i_wb_stb)&&(i_wb_we)&&(~i_wb_addr))

                begin

                        // Write with two's complement data

                        next_sample <= i_wb_data[15:0];

                        next_valid <= 1'b1;

                end else if (ztimer)

                        next_valid <= 1'b0;

        // The interrupt line will remain high until writing a new data value

        // clears it.  This design does not permit turning off this interrupt.

        // If the interrupt needs to be turned off, then ignore it in the 

        // interrupt controller.

        initial o_int = 1'b0;

        always @(posedge i_clk)

                o_int <= (~next_valid);

        // Adjust the gain for a maximum frequency offset just greater than

        // 75 kHz.  (We would've done 75kHz exactly, but it required a multiply

        // and this doesn't.)

        initial nco_phase = 32'h00;

        always @(posedge i_clk)

                nco_phase <= nco_phase + nco_step

                        + { {(32-16-7){sample_out[15]}}, sample_out, 7'h00 };

        assign  o_tx = nco_phase[31];

        always @(posedge i_clk)

                if (i_wb_addr)

                        o_wb_data <= nco_step;

                else

                        o_wb_data <= { reload_value, sample_out[15:1], o_int };

        initial o_wb_ack = 1'b0;

        always @(posedge i_clk)

                o_wb_ack <= (i_wb_cyc)&&(i_wb_stb);

        assign  o_wb_stall = 1'b0;

endmodule