verilog-apple-one/rtl/uart/uart.v
2018-01-31 00:48:47 +11:00

169 lines
5.1 KiB
Verilog

// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements. See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership. The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License. You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied. See the License for the
// specific language governing permissions and limitations
// under the License.
//
// Description: A wrapper for the basic UART from fpga4fun.com
//
// Author.....: Alan Garfield
// Niels A. Moseley
// Date.......: 26-1-2018
//
module uart(
input clk, // clock signal
input enable, // clock enable strobe
input rst, // active high reset signal
input [1:0] address, // address bus
input w_en, // active high write enable strobe
input [7:0] din, // 8-bit data bus (input)
output reg [7:0] dout, // 8-bit data bus (output)
input uart_rx, // asynchronous serial data input from computer
output uart_tx, // asynchronous serial data output to computer
output uart_cts // clear to send flag to computer
);
parameter ClkFrequency = 25000000; // 25MHz
parameter Baud = 115200;
parameter Oversampling = 8;
reg uart_tx_stb, uart_tx_init;
reg [7:0] uart_tx_byte;
wire uart_tx_status;
async_transmitter #(ClkFrequency, Baud) my_tx (
.clk(clk),
.rst(rst),
.TxD_start(uart_tx_stb),
.TxD_data(uart_tx_byte),
.TxD(uart_tx),
.TxD_busy(uart_tx_status)
);
wire uart_rx_stb, rx_idle, rx_end;
wire [7:0] rx_data;
reg uart_rx_status, uart_rx_ack;
reg [7:0] uart_rx_byte;
async_receiver #(ClkFrequency, Baud, Oversampling) my_rx(
.clk(clk),
.rst(rst),
.RxD(uart_rx),
.RxD_data_ready(uart_rx_stb),
.RxD_data(rx_data),
.RxD_idle(rx_idle),
.RxD_endofpacket(rx_end)
);
always @(posedge clk or posedge rst)
begin
if (rst)
begin
uart_rx_status <= 'b0;
uart_rx_byte <= 8'd0;
end
else
begin
// new byte from RX, check register is clear and CPU has seen
// previous byte, otherwise we ignore the new data
if (uart_rx_stb && ~uart_rx_status)
begin
uart_rx_status <= 'b1;
uart_rx_byte <= rx_data;
end
// clear the rx status flag on ack from CPU
if (uart_rx_ack)
uart_rx_status <= 'b0;
end
end
assign uart_cts = ~rx_idle || uart_rx_status;
localparam UART_RX = 2'b00;
localparam UART_RXCR = 2'b01;
localparam UART_TX = 2'b10;
// Handle Register
always @(posedge clk or posedge rst)
begin
if (rst)
begin
dout <= 8'd0;
uart_tx_init <= 0; // flag to ignore the DDR setup from Wozmon PIA call
uart_tx_stb <= 0;
uart_tx_byte <= 8'd0;
uart_rx_ack <= 0;
end
else
begin
uart_tx_stb <= 0;
uart_rx_ack <= 0;
case (address)
UART_TX:
begin
// UART TX - 0xD012
dout <= {uart_tx_status, 7'd0};
if (w_en)
begin
// Apple 1 terminal only uses 7 bits, MSB indicates
// terminal has ack'd RX
//
// uart_tx_init is a flag to stop the first character
// sent to the UART from being sent. Wozmon initializes
// the PIA which normally isn't sent to the terminal.
// This causes the UART to ignore the very first byte sent.
if (~uart_tx_status && uart_tx_init)
begin
`ifdef SIM
if ((din & 8'h7f) >= 32)
$write("%c", din & 8'h7f);
`endif
uart_tx_byte <= {1'b0, din[6:0]};
uart_tx_stb <= 1;
end
else if (~uart_tx_init)
uart_tx_init <= 1 && enable;
end
end
UART_RXCR:
begin
// UART RX CR - 0xD011
dout <= {uart_rx_status, 7'b0};
end
UART_RX:
begin
// UART RX - 0xD010
dout <= {uart_rx_status, uart_rx_byte[6:0]};
if (~w_en && ~uart_rx_ack && enable)
uart_rx_ack <= 1'b1;
end
default:
dout <= 8'b0;
endcase
end
end
endmodule