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/wbpwmaudio/trunk/rtl/wbpwmaudio.v
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///////////////////////////////////////////////////////////////////////////
//
// Filename: wbpwmaudio.v
//
// Project: A Wishbone Controlled PWM (audio) controller
//
// Purpose: This PWM controller was designed with audio in mind, although
// it should be sufficient for many other purposes. Specifically,
// it creates a pulse-width modulated output, where the amount of time
// the output is 'high' is determined by the pulse width data given to
// it. Further, the 'high' time is spread out in bit reversed order.
// In this fashion, a halfway point will alternate between high and low,
// rather than the normal fashion of being high for half the time and then
// low. This approach was chosen to move the PWM artifacts to higher,
// inaudible frequencies and hence improve the sound quality.
//
// The interface supports two addresses:
//
// Addr[0] is the data register. Writes to this register will set
// a 16-bit sample value to be produced by the PWM logic.
// Reads will also produce, in the 17th bit, whether the interrupt
// is set or not. (If set, it's time to write a new data value
// ...)
//
// Addr[1] is a timer reload value, used to determine how often the
// PWM logic needs its next value. This number should be set
// to the number of clock cycles between reload values. So,
// for example, an 80 MHz clock can generate a 44.1 kHz audio
// stream by reading in a new sample every (80e6/44.1e3 = 1814)
// samples. After loading a sample, the device is immediately
// ready to load a second. Once the first sample completes,
// the second sample will start going to the output, and an
// interrupt will be generated indicating that the device is
// now ready for the third sample. (The one sample buffer
// allows some flexibility in getting the new sample there fast
// enough ...)
//
//
// If you read through the code below, you'll notice that you can also
// set the timer reload value to an immutable constant by changing the
// VARIABLE_RATE parameter to 0. When VARIABLE_RATE is set to zero,
// both addresses become the same, Addr[0] or the data register, and the
// reload value can no longer be changed--forcing the sample rate to
// stay constant.
//
//
// Of course, if you don't want to deal with the interrupts or sample
// rates, you can still get a pseudo analog output by just setting the
// value to the analog output you would like and then not updating
// it. In this case, you could also shut the interrupt down at the
// controller, to keep that from bothering you as well.
//
// 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 wbpwmaudio(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_pwm, o_int);
parameter DEFAULT_RELOAD = 32'd1814, // about 44.1 kHz @ 80MHz
//DEFAULT_RELOAD = 32'd2268,//about 44.1 kHz @ 100MHz
NAUX=2, // Dev control values
VARIABLE_RATE=0;
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 wire [31:0] o_wb_data;
output reg o_pwm;
output reg [(NAUX-1):0] o_aux;
output reg o_int;
 
 
// 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)
generate
if (VARIABLE_RATE != 0)
begin
reg [31:0] r_reload_value;
initial r_reload_value = DEFAULT_RELOAD;
always @(posedge i_clk) // Data write
if ((i_wb_cyc)&&(i_wb_stb)&&(i_wb_addr)&&(i_wb_we))
reload_value <= i_wb_data;
wire [31:0] w_reload_value;
assign w_reload_value = r_reload_value;
end else begin
wire [31:0] w_reload_value;
assign w_reload_value = DEFAULT_RELOAD;
end endgenerate
 
reg [31:0] reload_value, timer;
initial reload_value = DEFAULT_RELOAD;
initial timer = DEFAULT_RELOAD;
always @(posedge i_clk)
if (timer == 0)
timer <= reload_value;
else
timer <= timer - 1;
 
reg [15:0] sample_out;
always @(posedge i_clk)
if (timer == 0)
sample_out <= next_sample;
 
 
reg [15:0] 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)||(VARIABLE_RATE==0)))
begin
// Write with two's complement data, convert it
// internally to binary offset
next_sample <= { ~i_wb_data[15], i_wb_data[14:0] };
next_valid <= 1'b1;
if (i_wb_data[16])
o_aux <= i_wb_data[(NAUX+20-1):20];
end else if (timer == 0)
next_valid <= 1'b0;
 
initial o_int = 1'b0;
always @(posedge i_clk)
o_int <= (~next_valid);
 
reg [15:0] pwm_counter;
initial pwm_counter = 16'h00;
always @(posedge i_clk)
pwm_counter <= pwm_counter + 1;
 
wire [15:0] br_counter;
genvar k;
generate for(k=0; k<16; k=k+1)
begin : bit_reversal_loop
assign br_counter[k] = pwm_counter[15-k];
end endgenerate
 
always @(posedge i_clk)
o_pwm <= (sample_out >= br_counter);
 
generate
if (VARIABLE_RATE == 0)
begin
assign o_wb_data = { {(12-NAUX){1'b0}}, o_aux,
3'h0, o_int, sample_out };
end else begin
reg [31:0] r_wb_data;
always @(posedge i_clk)
if (i_wb_addr)
r_wb_data <= reload_value;
else
r_wb_data <= { {(12-NAUX){1'b0}}, o_aux,
3'h0, o_int, sample_out };
assign o_wb_data = r_wb_data;
end endgenerate
 
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

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