防止Qii编译器优化掉某信号的方法

wire net1/*synthesis keep*/;

reg reg1/*synthesis noprune*/;

reg reg1/*synthesis preserve*/;

若一个reg没有fanout，它会被直接接到VCC或GND

按照Qii help的说法http://quartushelp.altera.com/9.1/mergedProjects/hdl/vlog/vlog_file_dir_noprune.htm

noprune与preserve的区别为：

(noprune is)A Verilog HDL synthesis attribute that prevents the Quartus II software from removing a register that does not directly or indirectly feed a top-level output or bidir pin, such as a fanout-free register. This attribute differs from the preserve attribute, which only prevents a register from being reduced to a constant or merged with a duplicate register.

另外，还可以用以下方法实现noprune:

You can also use Verilog 2001 attribute syntax to preserve a fanout-free register, as shown in the following code:

(* noprune *) reg reg1;

keep会在时序路径上增加一个逻辑单元的延时

Note that adding "keep" may add one logic cell delay in your timing path (probably not a problem, but something to keep in mind if it is a timing-critical path).

/*synthesis keep*/也支持对reg型信号，使用它也可以防止reg型信号被优化掉。但是也有可能出现这样的情况，有的信号即使经过此处理，仍然会被综合工具优化掉，致使无法找到它。这个时候就需要对其使用“测试属性”，可以加入probe_port属性，把这两个属性结合在一起，即就是：

( *synthesis, probe_port,keep *) 即可，这种方法同时适应于wire和reg型信号类型。