在Android平台上进行串口通信是实现设备间交互或者与硬件设备连接的重要手段,尤其是在嵌入式和物联网应用中。本文将详细讲解如何使用C语言在Android系统上进行串口调用,并着重介绍如何设置串口超时以避免程序堵塞。 Android原生并不直接支持串口API,但在Android NDK(Native Development Kit)的帮助下,我们可以使用C或C++编写底层代码来实现串口通信。相比谷歌提供的串口DEMO,这个C开发的串口调用可能具有更好的性能和灵活性。 1. **NDK集成与JNI接口**: - Android NDK允许开发者在Android应用中使用C/C++代码,通过Java Native Interface (JNI)与Java层交互。你需要在Android项目中集成NDK,创建一个jni目录存放C/C++源码,并在Java代码中定义JNI接口。 2. **打开串口**: - 在C代码中,你需要使用`open()`函数打开串口,例如`/dev/ttyS0`。确保在AndroidManifest.xml中添加相应的权限,如`<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />`,因为某些设备可能会将串口权限关联到定位服务。 3. **配置串口参数**: - 通过`ioctl()`或`termios`结构体来配置串口参数,如波特率、数据位、停止位、校验位等。例如,设置波特率为9600可以使用`B9600`常量。 4. **设置超时**: - 超时设置是防止程序堵塞的关键。在C语言中,可以通过修改`termios`结构体的成员来设置读写操作的超时。例如,可以使用`cfmakeraw()`函数将串口设置为非缓冲、无echo模式,然后使用`tcsetattr()`函数应用设置。超时通常涉及`cc`字段中的`VTIME`和`VMIN`两个元素。`VTIME`以十分之一秒为单位设置读操作超时,而`VMIN`设置至少需要读取的字符数,当达到该数目或超时后,读操作返回。 5. **读写操作**: - 使用`read()`和`write()`函数进行串口数据的读写。设置超时后,`read()`在指定时间内如果没有接收到数据会返回,避免了程序堵塞。 6. **关闭串口**: - 不使用串口时,务必调用`close()`函数关闭串口,释放资源。 7. **错误处理**: - 在C代码中,要对可能出现的错误进行捕获和处理,如打开串口失败、配置串口参数错误等。可以使用`perror()`或自定义日志系统输出错误信息。 8. **Java层调用**: - 在Java层,通过JNI方法调用C代码进行串口操作,并提供回调机制将串口读写的结果传递回Java层。 在实际应用中,考虑到Android设备的多样性,串口路径可能因设备而异,可能需要在运行时动态获取串口设备。同时,为了兼容不同版本的Android系统,你可能需要处理API级别的差异。另外,串口通信的稳定性、数据完整性以及并发访问控制也是需要考虑的问题。 通过C语言开发的串口调用并设置超时,可以提供更加可控的通信体验,避免因长时间等待数据而导致的程序阻塞。这种方法在需要高效、实时的串口交互场景中特别有用。
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