quartus prime 16.0 报警告 inferring latch

前言

当写always组合逻辑块时，可能会写出 poor code。综合时软件会推断出锁存器。例如下面代码：

always @* begin
  if (c == 1'b1) begin
    w = (a & b) ^ c;
  end
end

当c等于0的时候，w就会保持上一个值，所以就产生了锁存器，quartus就会贴心的给你报一个警告。

 inferring latch(es) for signal or variable "ram", which holds its previous value in one or more paths through the process

流程：

解决方式一：

补全条件，如果用的if就补全else，用的case补全default；

解决方式二：

当你使用状态机的时候，对于各种变量条件考虑可能不全面，查bug眼泪掉下来，还有你希望保持原值但不愿看到警告（强迫症），最简单的方式就是把always组合逻辑变成always时序逻辑，毕竟FPGA是一款偏时序的器件，能用时序逻辑最好用时序逻辑。

如spi的状态机代码：（举个栗子，代码风格和代码可能都是错的）

 1 always @(*) begin
 2     case (cstate)
 3         IDEL: begin
 4             //master_din_reg = 0;
 5             //master_dout = 0;
 6             cs = 1'b1;
 7             wr_done = 1'b0;
 8             rd_done = 1'b0;
 9             sck_en = 1'b0;
10         end 
11         LOAD: begin
12             cs = 1'b0;
13             master_din_reg = master_din;
14         end
15         SEND: begin
16             sck_en = 1'b1;
17         end
18         FINISH: begin
19             cs = 1'b1;
20             wr_done = 1'b1;
21             rd_done = 1'b1;
22             sck_en = 1'b0;
23             master_dout = master_dout_reg;
24         end
25         default: begin
26             master_din_reg = 0;
27             master_dout = 0;
28             cs = 1'b0;
29             wr_done = 1'b0;
30             rd_done = 1'b0;
31             sck_en = 1'b0;
32         end
33     endcase //case
34 end

组合改成时序逻辑（这是对的）：

always @(posedge clk or negedge rst_n) begin
    if (~rst_n) begin
        cs <= 1'b1;
        data_cnt_en <= 1'b0;
        sck_en <= 1'b0;
        master_din_reg <= 0;
        master_dout <= 0;
    end
    else begin
        case (cstate)
            IDEL: begin
            data_cnt_en <= 1'b0;
            master_din_reg <= (wr) ? master_din : master_din_reg; //load the data you want send to slaver;
            end 
            SEND: begin
                data_cnt_en <= 1'b1;
                cs <= 1'b0; 
                sck_en <= 1'b1;
                master_dout <= (send_over) ? master_dout_reg : master_dout; //master receiverd data;
            end
            FINISH: begin                  //send and load ok;
                sck_en <= 1'b0;
                cs <= 1'b1;
                data_cnt_en <= 1'b0;
            end
            default: begin
                cs <= 1'b1;
                sck_en <= 1'b0;
                data_cnt_en <= 1'b0;
            end
        endcase //case
    end
end

以上。