always @(posedge clk or posedge reset) begin
if (reset) begin
bit_count <= 0;
tx_busy <= 0;
tx_reg <= 0;
rx_reg <= 0;
start_bit <= 0;
rx_data <= 0;
rx_ready <= 0;
end else begin
if (enable) begin
// UART transmitter
if (bit_count < 8) begin
tx_reg <= tx_data;
start_bit <= 0;
bit_count <= bit_count + 1;
end else if (bit_count == 8) begin
tx_reg <= 0;
start_bit <= 1;
bit_count <= 9;
end else if (bit_count < 12) begin
start_bit <= 0;
bit_count <= bit_count + 1;
end else begin
tx_busy <= 0;
bit_count <= 0;
end
// UART receiver
if (!start_bit && bit_count < 9) begin
rx_reg <= {rx_reg[6:0], rx_data[0]};
bit_count <= bit_count + 1;
end else if (bit_count == 9) begin
rx_data <= rx_reg;
rx_ready <= 1;
bit_count <= 10;
end else if (bit_count < 12) begin
rx_ready <= 0;
bit_count <= bit_count + 1;
end else begin
bit_count <= 0;
end
end else begin
tx_busy <= 0;
bit_count <= 0;
end
end
end
endmodule
```
本程序定义了一个名为UART的模块，包括时钟信号`clk`，复位信号`reset`，使能信号`enable`，发送数据信号`tx_data`，发送忙碌信号`tx_busy`，接收数据信号`rx_data`和接收就绪信号`rx_ready`作为输入输出。在always块中，根据时钟信号和复位信号，通过状态机的方式实现UART发送和接收的功能。对于发送部分，根据发送使能信号和位计数，逐位发送数据，并在最后一个停止位之后将发送忙碌信号复位。对于接收部分，根据接收使能信号和位计数，逐位接收数据，并在接收完最后一个停止位之后将接收就绪信号置位。
示例三：FPGA的VHDL编程“钢琴音色包络线程序VHDL-1”：
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;
entity PianoSynthesizer is
Port ( clk : in STD_LOGIC;
rst : in STD_LOGIC;
key : in STD_LOGIC_VECTOR (3 downto 0);
note : out STD_LOGIC_VECTOR (7 downto 0) := (others => '0');
envelope : out STD_LOGIC_VECTOR (7 downto 0) := (others => '0');
output_enable : out STD_LOGIC);
end PianoSynthesizer;
signal tone_counter : std_logic_vector (7 downto 0) := (others => '0');
signal envelope_counter : std_logic_vector (7 downto 0) := (others => '0');
signal envelope_amplitude : std_logic_vector (7 downto 0) := (others => '0');
signal envelope_direction : std_logic := '0';
signal envelope_enable : std_logic := '0';
begin
process (clk, rst)
begin
if rst = '1' then
-- 复位
tone_counter <= (others => '0');
envelope_counter <= (others => '0');
envelope_amplitude <= (others => '0');
envelope_direction <= '0';
envelope_enable <= '0';
output_enable <= '0';
note <= (others => '0');
envelope <= (others => '0');
elsif rising_edge(clk) then
-- 音符改变时更新音调计数器和包络线计数器
if key /= "0000" then
tone_counter <= TONE_FREQUENCY;
envelope_counter <= (others => '0');
envelope_amplitude <= ENVELOPE_AMPLITUDE;
envelope_direction <= '1'; -- 开始包络上升
envelope_enable <= '1'; -- 启用包络线
output_enable <= '1'; -- 启用音色输出
note <= key; -- 输出当前音符
elsif envelope_enable = '1' then
-- 包络线控制
tone_counter <= TONE_FREQUENCY;
envelope_counter <= envelope_counter + 1;
if envelope_direction = '1' and envelope_amplitude /= "11111111" then
-- 上升阶段
envelope_amplitude <= std_logic_vector(unsigned(envelope_amplitude) + unsigned(ENVELOPE_RATE));
elsif envelope_direction = '0' and envelope_amplitude /= "00000000" then
-- 下降阶段
envelope_amplitude <= std_logic_vector(unsigned(envelope_amplitude) - unsigned(ENVELOPE_RATE));
end if;
if envelope_direction = '1' and envelope_amplitude = "11111111" then
-- 达到最大振幅，开始下降阶段
envelope_direction <= '0';
elsif envelope_direction = '0' and envelope_amplitude = "00000000" then
-- 达到最小振幅，停止包络线
envelope_enable <= '0';
output_enable <= '0';
end if;
end if;
end if;
end process;
end Behavioral;
```
该程序通过使用`tone_counter`和`envelope_counter`信号来控制音调和包络线的生成。通过调整`TONE_FREQUENCY`和`ENVELOPE_RATE`参数，可以改变音调和包络线的速率。`ENVELOPE_AMPLITUDE`参数用于控制包络线的振幅。在主要的处理过程中，根据`rst`信号进行复位操作，当`key`输入信号不为"0000"时，设置新的音符，并重置包络线计数器和振幅。当`envelope_enable`信号为高电平时，更新包络线的状态：在上升阶段，增加振幅；在下降阶段，减小振幅。当振幅达到最大或最小值时，改变包络线的方向，并在振幅达到所设置的最大或最小值时停止包络线的更新。最终，音符和包络线的状态被输出到`note`和`envelope`信号中，并在`output_enable`信号为高电平时启用音色输出。
// 音乐播放
always @(posedge clk) begin
counter <= counter + 1;
if (counter == beats[index]) begin
index <= index + 1;
if (index == 15) begin
index <= 1;
end
counter <= 0;
end
if (counter < (beats[index] / 2)) begin
speaker <= 1;
end
else begin
speaker <= 0;
end
end
一，点击“Device”图标进行芯片选择和配置，我采用的芯片是EP4CE6E22C8，选“Cyclone IV E”，然后在“Available divides:”选择我的芯片EP4CE6E22C8，再点击栏目：“Device and Pin Options…”，打开Category进行基本三个选项：
