【主题】：详细解析基于FPGA的LCD1602驱动控制

【作者】：LinCoding

【时间】：2016.11.23

这周末去找女票玩了，回来继续学习吧，唉，路漫漫，再过不久又要开题了，事情越来越多，能自己学习的时间越来越少。。。

废话不多说，LCD1602的驱动和控制大家一定都很清楚，这次的LCD1602程序是以三段式状态机为基础的，看过之后你会发现和三段式流水灯简直就是一模一样，因此，状态机有什么难的呢？只要掌握好我总结的那几点，一切变得So Easy。。。

先上效果图：

LCD1602驱动时序什么的大家请百度，网上实在太多了。

（源码出自CrazyBingo，尊重版权）

module lcd1602_driver ( input clk, input rst_n, input [127:0] line_rom1, input [127:0] line_rom2, output lcd_en, output lcd_rw, output reg lcd_rs, output reg [7:0] lcd_data );

第一部分，不多说，line_rom1和line_rom2分别给LCD1602的两行送数据，LCD1602每行可容纳16个字符，每个字符是ASCII码，也就是8位，所以，一共需要128位的数据。

//------------------------------------ //delay 20ms for LCD1602 steady localparam T20MS = 20'd1_000_000; //localparam T20MS = 20'd20; //just for simulation reg [19:0] delay20ms_cnt; always @ ( posedge clk or negedge rst_n ) begin if ( ! rst_n ) delay20ms_cnt <= 20'd0; else if ( delay20ms_cnt < T20MS ) delay20ms_cnt <= delay20ms_cnt + 1'b1; else delay20ms_cnt <= T20MS; end wire delay20ms_done = ( delay20ms_cnt == T20MS ) ? 1'b1 : 1'b0;

第二部分，是上电延时20ms的计数器，需要注意一点：由于上电延时20ms只执行一次，因此，在到达20ms以后，delay20ms_cnt<= T20MS; 将永不再进行计数。同时更新计数标志位，同样，计数器的输出采用组合逻辑。

//------------------------------------ //generate 500Hz clock for LCD1602 localparam T2MS = 17'd100_000; //localparam T2MS = 17'd16; //just for simulation reg [16:0] delay2ms_cnt; always @ ( posedge clk or negedge rst_n ) begin if ( ! rst_n ) delay2ms_cnt <= 17'd0; else if ( delay20ms_done ) delay2ms_cnt <= ( delay2ms_cnt < T2MS ) ? delay2ms_cnt + 1'b1 : 17'd1; else delay2ms_cnt <= delay2ms_cnt; end assign lcd_rw = 1'b0; //write only assign lcd_en = ( delay2ms_cnt > T2MS/2 ) ? 1'b0 : 1'b1; wire lcd_write_flag = ( delay2ms_cnt == T2MS*3/4 ) ? 1'b1 : 1'b0;

第三部分，产生一个50Hz的时钟，用于状态机的跳转。这里注意两点：

1、lcd_en其实可认为是LCD1602的时钟，类似于之前文章中驱动74HC595时的shift_clk。而与其不同的是，74HC595是上升沿有效，因此时钟是先低后高，shift_flag在下降沿进行输出，以使得shift_clk的上升沿正好出现在数据的正中间，使得建立时间和保持时间最佳。

而LCD1602属于高电平有效的器件，因此，时钟是先高后低，其实，对于芯片和器件来说，时钟大部分都是上升沿或者高电平有效，很少会有低电平或下降沿有效的。而为了使得高电平正好出现在数据的正中间，因此，lcd_write_flag在T2MS的3/4时进行输出，效果如下图所示。

如上图所示，正好使lcd_en的高电平出现在数据的正中间，这样建立时间和保持时间最佳！

2、由于只对LCD1602进行写操作，所以lcd_rw直接赋0，这时会在综合时出现以下警告。

就是说有一个输出引脚被直接拉为了1或者0，这不用管它。

还有就是计数器的输出采用组合逻辑。

//------------------------------------ //FSM: encode using Gray code localparam IDLE = 8'h00; //IDLE //LCD1602 init localparam DISP_SET = 8'h01; //Display mode localparam DISP_OFF = 8'h03; //Display off localparam CLR_SCR = 8'h02; //Clear the LCD localparam CURSOR_SET1 = 8'h06; //Set Cursor localparam CURSOR_SET2 = 8'h07; //Display on //Display 1th line localparam ROW1_ADDR = 8'h05; //Line1's first address localparam ROW1_0 = 8'h04; localparam ROW1_1 = 8'h0C; localparam ROW1_2 = 8'h0D; localparam ROW1_3 = 8'h0F; localparam ROW1_4 = 8'h0E; localparam ROW1_5 = 8'h0A; localparam ROW1_6 = 8'h0B; localparam ROW1_7 = 8'h09; localparam ROW1_8 = 8'h08; localparam ROW1_9 = 8'h18; localparam ROW1_A = 8'h19; localparam ROW1_B = 8'h1B; localparam ROW1_C = 8'h1A; localparam ROW1_D = 8'h1E; localparam ROW1_E = 8'h1F; localparam ROW1_F = 8'h1D; //Display 2th line localparam ROW2_ADDR = 8'h1C; //Line2's first address localparam ROW2_0 = 8'h14; localparam ROW2_1 = 8'h15; localparam ROW2_2 = 8'h17; localparam ROW2_3 = 8'h16; localparam ROW2_4 = 8'h12; localparam ROW2_5 = 8'h13; localparam ROW2_6 = 8'h11; localparam ROW2_7 = 8'h10; localparam ROW2_8 = 8'h30; localparam ROW2_9 = 8'h31; localparam ROW2_A = 8'h33; localparam ROW2_B = 8'h32; localparam ROW2_C = 8'h36; localparam ROW2_D = 8'h37; localparam ROW2_E = 8'h35; localparam ROW2_F = 8'h34;

第四部分是三段式状态机的状态编码，使用格雷码进行编码，以免使得输出产生毛刺。

//------------------------------------ //Three Part FSM: part 1 reg [5:0] current_state; reg [5:0] next_state; always @ ( posedge clk or negedge rst_n ) begin if ( ! rst_n ) current_state <= IDLE; else if ( lcd_write_flag ) current_state <= next_state; else current_state <= current_state; end

第五部分是三段式FSM的第一段，该说的在三段式流水灯中那篇文章中已经说过了。

//------------------------------------ //Three Part FSM: part 2 always @ ( * ) begin next_state = IDLE; case ( current_state ) //LCD1602 init IDLE : next_state = DISP_SET; //5'h00 DISP_SET : next_state = DISP_OFF; //5'h01 DISP_OFF : next_state = CLR_SCR; //5'h03 CLR_SCR : next_state = CURSOR_SET1; //5'h02 CURSOR_SET1 : next_state = CURSOR_SET2; //5'h06 CURSOR_SET2 : next_state = ROW1_ADDR; //5'h07 //Display 1th line ROW1_ADDR : next_state = ROW1_0; //5'h05; ROW1_0 : next_state = ROW1_1; //5'h04; ROW1_1 : next_state = ROW1_2; //5'h0C; ROW1_2 : next_state = ROW1_3; //5'h0D; ROW1_3 : next_state = ROW1_4; //5'h0F; ROW1_4 : next_state = ROW1_5; //5'h0E; ROW1_5 : next_state = ROW1_6; //5'h0A; ROW1_6 : next_state = ROW1_7; //5'h0B; ROW1_7 : next_state = ROW1_8; //5'h09; ROW1_8 : next_state = ROW1_9; //5'h08; ROW1_9 : next_state = ROW1_A; //5'h18; ROW1_A : next_state = ROW1_B; //5'h19; ROW1_B : next_state = ROW1_C; //5'h1B; ROW1_C : next_state = ROW1_D; //5'h1A; ROW1_D : next_state = ROW1_E; //5'h1E; ROW1_E : next_state = ROW1_F; //5'h1F; ROW1_F : next_state = ROW2_ADDR; //5'h1D; //Display 2th line ROW2_ADDR : next_state = ROW2_0; //5'h1C; ROW2_0 : next_state = ROW2_1; //5'h14; ROW2_1 : next_state = ROW2_2; //5'h15; ROW2_2 : next_state = ROW2_3; //5'h17; ROW2_3 : next_state = ROW2_4; //5'h16; ROW2_4 : next_state = ROW2_5; //5'h12; ROW2_5 : next_state = ROW2_6; //5'h13; ROW2_6 : next_state = ROW2_7; //5'h11; ROW2_7 : next_state = ROW2_8; //5'h10; ROW2_8 : next_state = ROW2_9; //5'h30; ROW2_9 : next_state = ROW2_A; //5'h31; ROW2_A : next_state = ROW2_B; //5'h33; ROW2_B : next_state = ROW2_C; //5'h32; ROW2_C : next_state = ROW2_D; //5'h36; ROW2_D : next_state = ROW2_E; //5'h37; ROW2_E : next_state = ROW2_F; //5'h35; ROW2_F : next_state = ROW1_ADDR; //5'h34; default : next_state = IDLE ; endcase end

第六部分是三段式FSM的第二段，同样，该说的在三段式流水灯中那篇文章中已经说过了。

//------------------------------------ //Three Part FSM: part 3-1 always @ ( posedge clk or negedge rst_n ) begin if ( ! rst_n ) lcd_rs <= 1'b0; else if ( lcd_write_flag ) if( next_state == IDLE || next_state == DISP_SET || next_state == DISP_OFF || next_state == CLR_SCR || next_state == CURSOR_SET1 || next_state == CURSOR_SET2 || next_state == ROW1_ADDR || next_state == ROW2_ADDR ) lcd_rs <= 1'b0; //L: Instruction else lcd_rs <= 1'b1; //H: Data else lcd_rs <= lcd_rs; end //------------------------------------ //Three Part FSM: part 3-2 always @ ( posedge clk or negedge rst_n ) begin if ( ! rst_n ) lcd_data <= 8'h00; else if ( lcd_write_flag ) case ( next_state ) IDLE : lcd_data <= 8'hxx; DISP_SET : lcd_data <= 8'h38; DISP_OFF : lcd_data <= 8'h08; CLR_SCR : lcd_data <= 8'h01; CURSOR_SET1 : lcd_data <= 8'h06; CURSOR_SET2 : lcd_data <= 8'h0C; //Display 1th line ROW1_ADDR : lcd_data <= 8'h80; ROW1_0 : lcd_data <= line_rom1[127:120]; ROW1_1 : lcd_data <= line_rom1[119:112]; ROW1_2 : lcd_data <= line_rom1[111:104]; ROW1_3 : lcd_data <= line_rom1[103: 96]; ROW1_4 : lcd_data <= line_rom1[ 95: 88]; ROW1_5 : lcd_data <= line_rom1[ 87: 80]; ROW1_6 : lcd_data <= line_rom1[ 79: 72]; ROW1_7 : lcd_data <= line_rom1[ 71: 64]; ROW1_8 : lcd_data <= line_rom1[ 63: 56]; ROW1_9 : lcd_data <= line_rom1[ 55: 48]; ROW1_A : lcd_data <= line_rom1[ 47: 40]; ROW1_B : lcd_data <= line_rom1[ 39: 32]; ROW1_C : lcd_data <= line_rom1[ 31: 24]; ROW1_D : lcd_data <= line_rom1[ 23: 16]; ROW1_E : lcd_data <= line_rom1[ 15: 8]; ROW1_F : lcd_data <= line_rom1[ 7: 0]; //Display 2th line ROW2_ADDR : lcd_data <= 8'hC0; ROW2_0 : lcd_data <= line_rom2[127:120]; ROW2_1 : lcd_data <= line_rom2[119:112]; ROW2_2 : lcd_data <= line_rom2[111:104]; ROW2_3 : lcd_data <= line_rom2[103: 96]; ROW2_4 : lcd_data <= line_rom2[ 95: 88]; ROW2_5 : lcd_data <= line_rom2[ 87: 80]; ROW2_6 : lcd_data <= line_rom2[ 79: 72]; ROW2_7 : lcd_data <= line_rom2[ 71: 64]; ROW2_8 : lcd_data <= line_rom2[ 63: 56]; ROW2_9 : lcd_data <= line_rom2[ 55: 48]; ROW2_A : lcd_data <= line_rom2[ 47: 40]; ROW2_B : lcd_data <= line_rom2[ 39: 32]; ROW2_C : lcd_data <= line_rom2[ 31: 24]; ROW2_D : lcd_data <= line_rom2[ 23: 16]; ROW2_E : lcd_data <= line_rom2[ 15: 8]; ROW2_F : lcd_data <= line_rom2[ 7: 0]; default : lcd_data <= 8'h00; endcase end

第七部分，三段式FSM的第三段，注意一点：

FSM的第三段使用了两个always，这是为何？

三段式FSM并不意味着是三个always，尽管大多数情况下我们可以用三个always解决问题，但是如这个例子，由于lcd_data即可能输出的是数据也可能输出的是命令，而输出命令时lcd_rs <= 1'b0; 输出数据时lcd_rs <= 1'b1; 因此我们需要同时输出lcd_rs和lcd_data，而触发这两者输出的都是lcd_write_flag，因此将这两个信号分别在两个always钟输出很合理，可根据需要同时输出，而如果写在一个always中，如以下代码：

DISP_SET : begin lcd_rs <= 1'b0; lcd_data <= 8'h38; end

这样做的话由于lcd_rs和lcd_data在一个always中输出，会使得lcd_rs和lcd_data的输出正好差了一个clk，时序上出现混乱，还得想办法做两个信号的同步处理（最常用的办法是在定义一个lcd_data_r信号，寄存一级lcd_data，以使得两者同步），十分没必要，因此，写成两个always十分的有利！

最后。。。这样做，就成功了呗！

总结：

1、对于高电平有效的芯片或器件，时钟输出先高后低，时钟输出标志位在计数总数的3/4处。

2、对于上升沿有效的芯片或器件，时钟输出先低后高，时钟输出标志位在计数总数处，也就是下降沿的地方。

3、三段式状态机，对于两个输出信号需要同步输出的，可将两个信号分别放到两个always中进行输出。