Verilog刷题笔记19

题目：

A common source of errors: How to avoid making latches

When designing circuits, you must think first in terms of circuits:

I want this logic gate

I want a combinational blob of logic that has these inputs and produces these outputs

I want a combinational blob of logic followed by a set of flip-flops

What you must not do is write the code first, then hope it generates a proper circuit.

If (cpu_overheated) then shut_off_computer = 1;

If (~arrived) then keep_driving = ~gas_tank_empty;

Syntactically-correct code does not necessarily result in a reasonable circuit (combinational logic + flip-flops). The usual reason is: "What happens in the cases other than those you specified?". Verilog's answer is: Keep the outputs unchanged.

This behaviour of "keep outputs unchanged" means the current state needs to be remembered, and thus produces a latch. Combinational logic (e.g., logic gates) cannot remember any state. Watch out for Warning (10240): ... inferring latch(es)" messages. Unless the latch was intentional, it almost always indicates a bug. Combinational circuits must have a value assigned to all outputs under all conditions. This usually means you always need else clauses or a default value assigned to the outputs.

解题：

module top_module (
    input      cpu_overheated,
    output reg shut_off_computer,
    input      arrived,
    input      gas_tank_empty,
    output reg keep_driving  ); //

    always @(*) begin
        if (cpu_overheated)
           shut_off_computer = 1;
        else
            shut_off_computer=0;
            
    end

    always @(*) begin
        if (~arrived)
          keep_driving = ~gas_tank_empty;
        else
            keep_driving =0;
    end

endmodule

结果正确：

本题注意点：
组合逻辑中，如果if缺少else语句，或是自己赋值给自己的情况就会产生latch。为了避免latch产生,if需要加上else语句，覆盖所有可能。