## 无驱动检测(no driver on)

### 一、简介

SpinalHDL将会检查设计中所有使用的组合逻辑信号都有相应的赋值连线声明。

### 二、例子

下述代码：
```Scala
class TopLevel extends Component {
  val result = out(UInt(8 bits))
  val a = UInt(8 bits)
  result := a
}
```
会产生如下报错：
```
NO DRIVER ON (toplevel/a :  UInt[8 bits]), defined at
  ***
  Source file location of the toplevel/a definition via the stack trace
  ***
```
修复如下：
```Scala
class TopLevel extends Component {
  val result = out(UInt(8 bits))
  val a = UInt(8 bits)
  a := 42
  result := a
}
```