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// Filename: deppbyte.v
// Project: CMod S6 System on a Chip, ZipCPU demonstration project
// Purpose: This is a very simple DEPP to synchronous byte transfer. It
// is used in place of a serial port.
// This approach uses address zero *only*.
// Creator: Dan Gisselquist, Ph.D.
// Gisselquist Technology, LLC
// Copyright (C) 2015-2017, 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
// <> for a copy.
// License: GPL, v3, as defined and found on,
module deppbyte(i_clk,
i_astb_n, i_dstb_n, i_write_n,i_depp, o_depp, o_wait,
o_rx_stb, o_rx_data,
i_tx_stb, i_tx_data, o_tx_busy);
input i_clk;
// DEPP interface
input i_astb_n, i_dstb_n, i_write_n;
input [7:0] i_depp;
output reg [7:0] o_depp;
output wire o_wait;
// Byte-wise interface to the rest of the world
output reg o_rx_stb;
output reg [7:0] o_rx_data;
input i_tx_stb;
input [7:0] i_tx_data;
output reg o_tx_busy;
// Synchronize the incoming signals
reg x_dstb_n, x_astb_n, x_write_n,
r_dstb_n, r_astb_n, r_write_n,
l_dstb_n, l_astb_n, l_write_n;
reg [7:0] x_depp, r_depp;
initial x_dstb_n = 1'b1;
initial r_dstb_n = 1'b1;
initial l_dstb_n = 1'b1;
initial x_astb_n = 1'b1;
initial r_astb_n = 1'b1;
initial l_astb_n = 1'b1;
always @(posedge i_clk)
{ x_dstb_n, x_astb_n, x_write_n, x_depp }
<= { i_dstb_n, i_astb_n, i_write_n, i_depp };
{ r_dstb_n, r_astb_n, r_write_n, r_depp }
<= { x_dstb_n, x_astb_n, x_write_n, x_depp };
{ l_dstb_n, l_astb_n, l_write_n } <= { r_dstb_n, r_astb_n, r_write_n };
reg [7:0] addr;
wire astb, dstb, w_write;
assign astb = (!r_astb_n)&&(l_astb_n);
assign dstb = (!r_dstb_n)&&(l_dstb_n);
assign w_write= (!r_write_n);
initial addr = 8'h00;
initial o_rx_stb = 1'b0;
always @(posedge i_clk)
if ((w_write)&&(astb))
addr <= r_depp;
if ((w_write)&&(dstb)&&(addr==0))
o_rx_stb <= 1'b1;
o_rx_data <= r_depp;
end else
o_rx_stb <= 1'b0;
// Much as I hate to use signals that have not been synchronized with a
// two clock transfer, this line needs to be brought low within 10ms
// (less than one clock) of when the strobe lines are brought low, and
// raised high again within 10 ms of when the strobe lines are raised
// again.
assign o_wait = ((!i_dstb_n)||(!i_astb_n));
// For one clock, following any read from address zero, we allow the
// port to write one new byte into our interface. This works because
// the interface will guarantee that the strobe signals are inactive
// (high) for at least 40ns before attempting a new transaction.
// Just about nothing else works. 'cause we can't allow changes
// in the middle of a transaction, and we won't know if the clock
// involved is in the middle of a transaction until after the time
// has passed. Therefore, we're going to be busy most of the time
// and just allow a byte to pass through on the one (and only) clock
// following a transaction.
always @(posedge i_clk)
o_tx_busy <= ((~l_dstb_n)&&(r_dstb_n)&&(l_write_n)&&(addr == 0))
? 1'b0 : 1'b1;
// If we don't have a byte to write, stuff it with all ones. The high
// bit will then indicate that there's nothing available to the
// interface when it next reads.
// Okay, new philosophy. Stuff the high bit with ones, allow the other
// bits to contain status level information --- should any one wish to
// send such.
initial o_depp = 8'hff;
always @(posedge i_clk)
if (~o_tx_busy)
o_depp <= {((i_tx_stb)? i_tx_data[7] : 1'b1),
i_tx_data[6:0] };