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还记得之前软件的同事说过的一句话。怎么凸显自己的工作量，就是自己给自己写BUG。
看过夏宇闻老师书的都知道，verilog的FSM有moore和mealy,然后有一段，二段，三段式。记得我还是学生的时候，看到这里的时候，感觉很烧脑。毕竟这与数字电路设计息息相关。
今天我想把问题简单化。只谈mealy型三段式写法。
借用前辈们总结的一句话说：三段式描述方法虽然代码结构复杂了一些，但是换来的优势是使 FSM 做到了同步寄存器输出，消除了组合逻辑输出的不稳定与毛刺的隐患，而且更利于时序路径分组，一般来说在 FPGA/CPLD 等可编程逻辑器件上的综合与布局布线效果更佳。
（闪耀着哲学的光辉......）
//3-paragraph method to describe FSM
//Describe sequential state transition in the 1st sequential always block
//State transition conditions in the 2nd combinational always block
//Describe the FSM out in the 3rd sequential always block
//Verilog Training -- How to write FSM bettermodule state3 ( rst_n,clk,i1,i2,o1,o2,err);input rst_n,clk;
input i1,i2;
output o1,o2,err;
reg o1,o2,err;reg [2:0] NS,CS;//one hot with zero idle
parameter [2:0] IDLE = 3'b000;
parameter [2:0] S1 = 3'b001;
parameter [2:0] S2 = 3'b010;
parameter [2:0] ERROR = 3'b100;//1st always block, sequential state transition
always @(posedge clk or negedge rst_n)if (!rst_n) CS <= IDLE; else CS <=NS; //2nd always block, combinational condition judgment
always @ (rst_n or CS or i1 or i2)beginNS = 3'bx;case (CS)IDLE: beginif (~i1) NS = IDLE;if (i1 && i2) NS = S1;if (i1 && ~i2) NS = ERROR;endS1: beginif (~i2) NS = S1;if (i2 && i1) NS = S2;if (i2 && (~i1)) NS = ERROR;endS2: beginif (i2) NS = S2;if (~i2 && i1) NS = IDLE;if (~i2 && (~i1)) NS = ERROR;endERROR: beginif (i1) NS = ERROR;if (~i1) NS = IDLE;endendcaseend//3rd always block, the sequential FSM output
always @ (posedge clk or negedge rst_n)if (!rst_n){o1,o2,err} <= 3'b000;elsebegin{o1,o2,err} <= 3'b000;case (NS)IDLE: {o1,o2,err}<=3'b000;S1: {o1,o2,err}<=3'b100;S2: {o1,o2,err}<=3'b010;ERROR: {o1,o2,err}<=3'b111;endcaseendendmodule
同样这段代码也很好理解：
module FSM(clk,clr,out,start,step2,step3);
input clk;
input clr;
input start;
input step2;
input step3;output[2:0] out;reg[2:0] out;
reg[1:0] state,next_state;/*状态编码，采用格雷（Gray）编码方式*/
parameter state0 = 2'b00;
parameter state1 = 2'b01;
parameter state2 = 2'b11;
parameter state3 = 2'b10; /*该进程定义起始状态*/
always @(posedge clk or posedge clr)
begin if (clr) state <= state0; else state <= next_state;
end/*该进程实现状态的转换*/
always @(state or start or step2 or step3)
begin case (state)state0: beginif (start) next_state <=state1;else next_state <=state0;endstate1: beginnext_state <= state2;endstate2: beginif (step2) next_state <=state3;else next_state <=state0;endstate3: beginif (step3) next_state <=state0;else next_state <=state3;enddefault: next_state <=state0; /*default语句*/endcase
end/*该进程定义组合逻辑（FSM的输出）*/
always @(state)
begincase(state)state0: out=3'b001;state1: out=3'b010;state2: out=3'b100;state3: out=3'b111;default:out=3'b001; /*default语句，避免锁存器的产生*/endcase
endendmodule状态转移图如下：
总结
文章难免会有些水平不足，不正确的地方请大家多多指正，共同进步。