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两段式状态机不可能完成的任务

两段式状态机不可能完成的任务

最近折腾状态机,发现一个小任务对于两段式状态机写法是不可能完成的。这个小任务很简单,先看用一段式状态机实现的代码:

module test(

clk,rst_n,

din,dout

);

input clk;

input rst_n;

input din;

output[3:0] dout;

parameter IDLE  = 3'd0;

parameter STA1  = 3'd1;

//一段式写法

reg[2:0] cstate;

reg[3:0] cnt;

always @(posedge clk or negedge rst_n)

if(!rst_n) cstate <= IDLE;

else begin

case(cstate)

IDLE: begin

cnt <= 4'd0;

if(din) cstate <= STA1;

else cstate <= IDLE;

end

STA1: begin

cnt <= cnt+1'b1;

if(cnt == 4'd10) cstate <= IDLE;

else cstate <= STA1;

end

default: cstate <= IDLE;

endcase

end

assign dout = cnt;

endmodule

同样的,用三段式状态机也能够实现这个功能:

//三段式写法

reg[2:0] cstate,nstate;

reg[3:0] cnt;

always @(posedge clk or negedge rst_n)

if(!rst_n) cstate <= IDLE;

else cstate <= nstate;

always @(cstate or din or cnt) begin

case(cstate)

IDLE:   if(din) nstate = STA1;

else nstate = IDLE;

STA1:   if(cnt == 4'd10) nstate = IDLE;

else nstate = STA1;

default: nstate = IDLE;

endcase

end

always @(posedge clk or negedge rst_n)

if(!rst_n) cnt <= 4'd0;

else begin

case(nstate)

IDLE:   cnt <= 4'd0;

STA1:   cnt <= cnt+1'b1;

default: ;

endcase

end

严格来看,上面的三段式状态机相比于一段式会滞后一个时钟周期。但是我们的重点不在这里,大家大可以不必钻这个牛角尖。另外,这个实例实现的功能本身也没有什么意义,当然也是可以用别的更简单(不需要状态机)的方式实现,但是你可以想象成这是实际应用中状态机各种复杂输出的一部分。

而如果大家希望用两段式状态机实现这个功能,或许会这么写:

//两段式写法

reg[2:0] cstate,nstate;

reg[3:0] cnt;

always @(posedge clk or negedge rst_n)

if(!rst_n) cstate <= IDLE;

else cstate <= nstate;

always @(cstate or din or cnt) begin

case(cstate)

IDLE: begin

cnt = 4'd0;

if(din) nstate = STA1;

else nstate = IDLE;

end

STA1: begin

cnt = cnt+1'b1;

if(cnt == 4'd10) nstate = IDLE;

else nstate = STA1;

end

default: nstate = IDLE;

endcase

end

如果大家有兴趣对三中代码方式都做一下仿真,会发现一些有意思的问题,尤其两段式状态机最终根本无法退出STA1,计数器cnt也会死在那里。究其根本原因,可大有学问。在编译工程后,出现了数条类似下面的warning:

Warning: Found combinational loop of 2 nodes

Warning: Node "Add0~2"

Warning: Node "cnt~9"

何为combinational loop?让handbook来解释吧,看不懂英文的可别怪我~_~

Combinational loops are among the most common causes of instability and unreliability in digital designs. They should be avoided whenever possible. In a synchronous design, feedback loops should include registers. Combinational loops generally violate synchronous design principles by establishing a direct feedback loop that contains no registers. For example, a combinational loop occurs when the left-hand side of an arithmetic expression also appears on the right-hand side in HDL code. A combinational loop also occurs when you feed back the output of a register to an asynchronous pin of the same register through combinational logic, as shown in Figure 5–1.

没有寄存器打一拍的这种combinational loop(组合环)是一种不推荐的设计方式,就如两段式状态机所实现的效果,甚至最终无法实现功能要求。同样的功能,一段式和三段式状态机之所以能够解决这个问题,就是避免了在纯组合逻辑中涉及这个反馈逻辑。在初学verilog时,我们常提的latch(锁存器),其实也是combinational loop的一个特例。

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