【非原创声明】做fpga久了就会发现其实我们不能只局限于解决ERROR，综合过程中的有些Warning其实也是必须修掉的重要提示，不能忽视。我从网上摘了一些别人总结的经验，再自己补充一些整理在下面：

1. Warning: VHDL Process Statement warning at random.vhd(18): signal reset is in statement, but is not in sensitivity list

【提示】没把singal放到process（）中。

2.Warning: Found pins ing as undefined clocks and/or memory enables
Info: Assuming node CLK is an undefined clock
【提示】是说设计中没有对时钟进行约束。【解决】在assignment/timing anlaysis setting/inidividual clock中设置

3. Warning: Found 2 node(s) in clock paths which may be acting as ripple and/or gated clocks -- node(s) analyzed as buffer(s) resulting in clock skew
Info: Detected ripple clock "clk_gen:clk_gen1|clk_incr" as buffer
Info: Detected ripple clock "clk_gen:clk_gen1|clk_scan" as buffer

【提示】设计中出现门控时钟问题，最好修掉，时钟分析会更容易通过，系统更稳。如果是必须的，可以忽略此提示。

4. Warning: VHDL Process Statement warning at ledmux.vhd(15): signal or variable "dataout" may not be assigned a new in every possible path through the Process Statement. Signal or variable "dataout" holds its previous in every path with no new assignment, which may create a combinational loop in the current design.

【提示】电路设计里面出现了组合逻辑信号反馈的一个闭环。这样没有经过寄存器的反馈会造成电路很不稳定，很容易出现毛刺。建议修改。

5. Warning: No clock transition on "counter_bcd7:counter_counter_clk|q_sig[3]" register

【提示】寄存器没有接时钟。

6. Warning: Reduced register "counter_bcd7:counter_counter_clk|q_sig[3]" with stuck clock port to stuck GND

【提示】寄存器没有赋值，软件自动将输入接GND。

7. Warning: Circuit may not operate. Detected 1 non-operational path(s) clocked by clock "class[1]" with clock skew larger than data delay. See Compilation Report for details.

【提示】时序分析中出现的warning，提示clock skew太大，严重影响到时序，建议查出原因坚决修掉。具体原因要参考report。

8.  Warning: Ignored node in vector source file. Can't find corresponding node name "class_sig[2]" in design.

【提示】所提示的信号没有连接或定义到高层。

9. Warning: Found clock high time violation at 1000.0 ns on register "|fcounter|lpm_counter:temp_rtl_0|dffs[4]"

【提示】仿真中出现的，提示时序很紧，适当降低主时钟频率再仿真。

10. Warning: Compiler packed, optimized or synthesized away node "temp[19]". Ignored vector source file node.

【提示】信号没有最终使用，被软件自动优化去掉了。

11. Warning: Design contains 2 input pin(s) that do not drive logic

Warning: No output dependent on input pin "clk"
Warning: No output dependent on input pin "sign"

【提示】两个信号在电路中未被使用，没有驱动任何电路，最终也会被优化掉。

12. Warning: Ignored node in vector source file. Can't find corresponding node name "over" in design.

【提示】在源文件中找不到对应的节点“over”。

13.Warning: Synthesized away the following node(s):

【提示】以下节点都被综合优化掉。

14.Warning: Output pins are stuck at VCC or GND

        Warning (13410): Pin "USB_SLOE" is stuck at VCC

【提示】输出信号连接到固定值，如果是实际情况可以忽略，不是的话就去看看原因吧。

15.Warning: Some pins have incomplete I/O assignments. Refer to the I/O Assignment Warnings report for details

【提示】top层有些管脚没有绑定到FPGA pin上面。仔细找找吧。

16.Warning: Clock latency analysis for PLL offsets is supported for the current device family, but is not enabled

【提示】PLL的相位没有设置或默认为0.确实不需设置的时候可以忽略。

17.Warning：“xxxx” used but never defined...

【提示】XXX信号已经使用但是没有定义，这个一定要定义的哦。

18.Warning：“xxxx” defined but never used...
【提示】XXX信号已经定义但是没有使用，这个可以忽略。但是要问问自己问什么定义呢？