要将代码修改为只发送一次字符串，可以根据需要添加一个计数器，并在达到指定条件后停止发送。下面是修改后的代码示例：
`timescale 1ns / 1ps
`define UD #1
module uart_data_gen(
input clk,
input [7:0] read_data,
input tx_busy,
input [7:0] write_max_num,
output reg [7:0] write_data,
output reg write_en
);
reg [25:0] time_cnt = 0;
reg [7:0] data_num = 0;
reg work_en = 0;
reg stop_sending = 0;
always @(posedge clk)
begin
time_cnt <= `UD (time_cnt + 26'd1);
end
always @(posedge clk)
begin
if (time_cnt == `UD 26'd2048) begin
work_en <= `UD 1'b1;
stop_sending <= `UD 1'b1; // 设置停止发送标志位
end else if ((data_num == (`UD write_max_num - `UD 8'h1)) && ~stop_sending) begin
work_en <= `UD 1'b0;
stop_sending <= `UD 1'b1; // 设置停止发送标志位
end
end
always @(posedge clk)
begin
if (~work_en & tx_busy_f) begin
data_num <= `UD 8'h0;
stop_sending <= `UD 1'b0; // 清除停止发送标志位，以便下一次触发重新发送字符串
end else if (write_pluse) begin
data_num <= `UD (data_num + `UD 8'h1);
end
end
always @(posedge clk)
begin
write_en <= `UD write_pluse;
end
always @(posedge clk)
begin
case (data_num)
8'h0 : write_data <= `UD 8'h49; // ASCII码是'I'
8'h1 : write_data <= `UD 8'h49; // ASCII码是'I'
8'h2 : write_data <= `UD 8'h2E; // ASCII码是'.'
8'h3 : write_data <= `UD 8'h61; // ASCII码是'a'
8'h4 : write_data <= `UD 8'h6D; // ASCII码是'm'
default: write_data <= `UD read_data;
endcase
end
endmodule
通过添加了一个名为stop_sending的信号，并在满足停止条件时设置该信号，可以实现只发送一次字符串。
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