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:
bash
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。