Android硬件通信之 串口通信

一，串口介绍

1.1 串口简介

串行接口简称串口，也称串行通信接口或串行通讯接口（通常指COM接口），是采用串行通信方式的扩展接口；

串行接口（SerialInterface）是指数据一位一位地顺序传送。其特点是通信线路简单，只要一对传输线就可以实现双向通信（可以直接利用电话线作为传输线），从而大大降低了成本，特别适用于远距离通信，但传送速度较慢；

1.2 串口使用场景

串口是一种用于android开发板对硬件设备通信的一种协议，通过发送某种指令控制硬件设备，通常用于物联网设备的信息传输，比如切割器，打印机，ATM吐卡机、IC/ID卡读卡等。

1.3 波特率

波特率表示串口传输速率，用来衡量数据传输的快慢，即单位时间内载波参数变化的次数，如每秒钟传送240个字符，而每个字符格式包含10位（1个起始位，1个停止位，8个数据位），这时的波特率为240Bd，比特率为10位*240个/秒=2400bps。波特率与距离成反比，波特率越大传输距离相应的就越短；

1.4 数据位

这是衡量通信中实际数据位的参数。当计算机发送一个信息包，实际的数据往往不会是8位的，标准的值是6、7和8位。如何设置取决于你想传送的信息；

1.5 停止位

用于表示单个包的最后一位。典型的值为1，1.5和2位。由于数据是在传输线上定时的，并且每一个设备有其自己的时钟，很可能在通信中两台设备间出现了小小的不同步。因此停止位不仅仅是表示传输的结束，并且提供计算机校正时钟同步的机会。适用于停止位的位数越多，不同时钟同步的容忍程度越大，但是数据传输率同时也越慢；

1.6 校验位

在串口通信中一种简单的检错方式。有四种检错方式：偶、奇、高和低。当然没有校验位也是可以的。对于偶和奇校验的情况，串口会设置校验位（数据位后面的一位），用一个值确保传输的数据有偶个或者奇个逻辑高位；

1.7 串口地址

不同操作系统的串口地址，Android是基于Linux的所以一般情况下使用Android系统的设备串口地址为/dev/ttyS0；

• /dev的串口包括：虚拟串口，真实串口，USB转串口
• 真实串口：/dev/tty0..tty1这个一般为机器自带COM口
• 虚拟串口：/dev/ttyS1...ttyS2...ttyS3...均为虚拟console，同样可以作为输入输出口
• USB转串口：/dev/tty/USB0

二 Android中串口的实践

2.1 由于串口底层需要调用C代码，所以需要用jni来进行C交互，下面是全部的C代码，以及JNI调用

2.1 SerialPort.h

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class android_serialport_SerialPort */#ifndef _Included_android_serialport_SerialPort
#define _Included_android_serialport_SerialPort
#ifdef __cplusplus
extern "C" {
#endif
/** Class:     android_serialport_SerialPort* Method:    open* Signature: (Ljava/lang/String;IIIII)Ljava/io/FileDescriptor;*/
JNIEXPORT jobject JNICALL Java_android_serialport_SerialPort_open(JNIEnv *, jobject, jstring, jint, jint, jint, jint, jint);/** Class:     android_serialport_SerialPort* Method:    close* Signature: ()V*/
JNIEXPORT void JNICALL Java_android_serialport_SerialPort_close(JNIEnv *, jobject);#ifdef __cplusplus
}
#endif
#endif
/* Header for class android_serialport_SerialPort_Builder */#ifndef _Included_android_serialport_SerialPort_Builder
#define _Included_android_serialport_SerialPort_Builder
#ifdef __cplusplus
extern "C" {
#endif
#ifdef __cplusplus
}
#endif
#endif

2.2 SerialPort.c

#include <termios.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <jni.h>#include "SerialPort.h"#include "android/log.h"static const char *TAG = "serial_port";
#define LOGI(fmt, args...) __android_log_print(ANDROID_LOG_INFO,  TAG, fmt, ##args)
#define LOGD(fmt, args...) __android_log_print(ANDROID_LOG_DEBUG, TAG, fmt, ##args)
#define LOGE(fmt, args...) __android_log_print(ANDROID_LOG_ERROR, TAG, fmt, ##args)static speed_t getBaudrate(jint baudrate) {switch (baudrate) {case 0:return B0;case 50:return B50;case 75:return B75;case 110:return B110;case 134:return B134;case 150:return B150;case 200:return B200;case 300:return B300;case 600:return B600;case 1200:return B1200;case 1800:return B1800;case 2400:return B2400;case 4800:return B4800;case 9600:return B9600;case 19200:return B19200;case 38400:return B38400;case 57600:return B57600;case 115200:return B115200;case 230400:return B230400;case 460800:return B460800;case 500000:return B500000;case 576000:return B576000;case 921600:return B921600;case 1000000:return B1000000;case 1152000:return B1152000;case 1500000:return B1500000;case 2000000:return B2000000;case 2500000:return B2500000;case 3000000:return B3000000;case 3500000:return B3500000;case 4000000:return B4000000;default:return -1;}
}/** Class:     android_serialport_SerialPort* Method:    open* Signature: (Ljava/lang/String;II)Ljava/io/FileDescriptor;*/
JNIEXPORT jobject JNICALL Java_android_serialport_SerialPort_open(JNIEnv *env, jobject thiz, jstring path, jint baudrate, jint dataBits, jint parity,jint stopBits,jint flags) {int fd;speed_t speed;jobject mFileDescriptor;/* Check arguments */{speed = getBaudrate(baudrate);if (speed == -1) {/* TODO: throw an exception */LOGE("Invalid baudrate");return NULL;}}/* Opening device */{jboolean iscopy;const char *path_utf = (*env)->GetStringUTFChars(env, path, &iscopy);LOGD("Opening serial port %s with flags 0x%x", path_utf, O_RDWR | flags);fd = open(path_utf, O_RDWR | flags);LOGD("open() fd = %d", fd);(*env)->ReleaseStringUTFChars(env, path, path_utf);if (fd == -1) {/* Throw an exception */LOGE("Cannot open port");/* TODO: throw an exception */return NULL;}}/* Configure device */{struct termios cfg;LOGD("Configuring serial port");if (tcgetattr(fd, &cfg)) {LOGE("tcgetattr() failed");close(fd);/* TODO: throw an exception */return NULL;}cfmakeraw(&cfg);cfsetispeed(&cfg, speed);cfsetospeed(&cfg, speed);cfg.c_cflag &= ~CSIZE;switch (dataBits) {case 5:cfg.c_cflag |= CS5;    //使用5位数据位break;case 6:cfg.c_cflag |= CS6;    //使用6位数据位break;case 7:cfg.c_cflag |= CS7;    //使用7位数据位break;case 8:cfg.c_cflag |= CS8;    //使用8位数据位break;default:cfg.c_cflag |= CS8;break;}switch (parity) {case 0:cfg.c_cflag &= ~PARENB;    //无奇偶校验break;case 1:cfg.c_cflag |= (PARODD | PARENB);   //奇校验break;case 2:cfg.c_iflag &= ~(IGNPAR | PARMRK); // 偶校验cfg.c_iflag |= INPCK;cfg.c_cflag |= PARENB;cfg.c_cflag &= ~PARODD;break;default:cfg.c_cflag &= ~PARENB;break;}switch (stopBits) {case 1:cfg.c_cflag &= ~CSTOPB;    //1位停止位break;case 2:cfg.c_cflag |= CSTOPB;    //2位停止位break;default:cfg.c_cflag &= ~CSTOPB;    //1位停止位break;}if (tcsetattr(fd, TCSANOW, &cfg)) {LOGE("tcsetattr() failed");close(fd);/* TODO: throw an exception */return NULL;}}/* Create a corresponding file descriptor */{jclass cFileDescriptor = (*env)->FindClass(env, "java/io/FileDescriptor");jmethodID iFileDescriptor = (*env)->GetMethodID(env, cFileDescriptor, "<init>", "()V");jfieldID descriptorID = (*env)->GetFieldID(env, cFileDescriptor, "descriptor", "I");mFileDescriptor = (*env)->NewObject(env, cFileDescriptor, iFileDescriptor);(*env)->SetIntField(env, mFileDescriptor, descriptorID, (jint) fd);}return mFileDescriptor;
}/** Class:     cedric_serial_SerialPort* Method:    close* Signature: ()V*/
JNIEXPORT void JNICALL Java_android_serialport_SerialPort_close(JNIEnv *env, jobject thiz) {jclass SerialPortClass = (*env)->GetObjectClass(env, thiz);jclass FileDescriptorClass = (*env)->FindClass(env, "java/io/FileDescriptor");jfieldID mFdID = (*env)->GetFieldID(env, SerialPortClass, "mFd", "Ljava/io/FileDescriptor;");jfieldID descriptorID = (*env)->GetFieldID(env, FileDescriptorClass, "descriptor", "I");jobject mFd = (*env)->GetObjectField(env, thiz, mFdID);jint descriptor = (*env)->GetIntField(env, mFd, descriptorID);LOGD("close(fd = %d)", descriptor);close(descriptor);
}

2.3 gen_SerialPort_h.sh 生成java的文件目录

#!/bin/sh
javah -o SerialPort.h -jni -classpath ../java android.serialport.SerialPort

2.4 SerialPort.java jni对应的java文件

/** Copyright 2009 Cedric Priscal** Licensed under the Apache License, Version 2.0 (the "License");* you may not use this file except in compliance with the License.* You may obtain a copy of the License at** http://www.apache.org/licenses/LICENSE-2.0** Unless required by applicable law or agreed to in writing, software* distributed under the License is distributed on an "AS IS" BASIS,* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.* See the License for the specific language governing permissions and* limitations under the License.*/package android.serialport;import android.util.Log;import androidx.annotation.NonNull;
import androidx.annotation.Nullable;import java.io.DataOutputStream;
import java.io.File;
import java.io.FileDescriptor;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;public final class SerialPort {private static final String TAG = "SerialPort";public static final String DEFAULT_SU_PATH = "/system/bin/su";private static String sSuPath = DEFAULT_SU_PATH;private File device;private int baudrate;private int dataBits;private int parity;private int stopBits;private int flags;/*** Set the su binary path, the default su binary path is {@link #DEFAULT_SU_PATH}** @param suPath su binary path*/public static void setSuPath(@Nullable String suPath) {if (suPath == null) {return;}sSuPath = suPath;}/*** Get the su binary path** @return*/@NonNullpublic static String getSuPath() {return sSuPath;}/** Do not remove or rename the field mFd: it is used by native method close();*/private FileDescriptor mFd;private FileInputStream mFileInputStream;private FileOutputStream mFileOutputStream;/*** 串口** @param device   串口设备文件* @param baudrate 波特率* @param dataBits 数据位；默认8,可选值为5~8* @param parity   奇偶校验；0:无校验位(NONE，默认)；1:奇校验位(ODD);2:偶校验位(EVEN)* @param stopBits 停止位；默认1；1:1位停止位；2:2位停止位* @param flags    默认0* @throws SecurityException* @throws IOException*/public SerialPort(@NonNull File device, int baudrate, int dataBits, int parity, int stopBits,int flags) throws SecurityException, IOException {this.device = device;this.baudrate = baudrate;this.dataBits = dataBits;this.parity = parity;this.stopBits = stopBits;this.flags = flags;/* Check access permission */Log.e(TAG, "SerialPort: canRead" + device.canRead());if (!device.canRead() || !device.canWrite()) {try {/* Missing read/write permission, trying to chmod the file */Process su;su = Runtime.getRuntime().exec(sSuPath);String cmd = "chmod 666 " + device.getAbsolutePath() + "\n" + "exit\n";su.getOutputStream().write(cmd.getBytes());if ((su.waitFor() != 0) || !device.canRead() || !device.canWrite()) {throw new SecurityException();}} catch (Exception e) {e.printStackTrace();throw new SecurityException();}}mFd = open(device.getAbsolutePath(), baudrate, dataBits, parity, stopBits, flags);if (mFd == null) {Log.e(TAG, "native open returns null");throw new IOException();}mFileInputStream = new FileInputStream(mFd);mFileOutputStream = new FileOutputStream(mFd);}/*** 串口，默认的8n1** @param device   串口设备文件* @param baudrate 波特率* @throws SecurityException* @throws IOException*/public SerialPort(@NonNull File device, int baudrate) throws SecurityException, IOException {this(device, baudrate, 8, 0, 1, 0);}/*** 串口** @param device   串口设备文件* @param baudrate 波特率* @param dataBits 数据位；默认8,可选值为5~8* @param parity   奇偶校验；0:无校验位(NONE，默认)；1:奇校验位(ODD);2:偶校验位(EVEN)* @param stopBits 停止位；默认1；1:1位停止位；2:2位停止位* @throws SecurityException* @throws IOException*/public SerialPort(@NonNull File device, int baudrate, int dataBits, int parity, int stopBits)throws SecurityException, IOException {this(device, baudrate, dataBits, parity, stopBits, 0);}// Getters and setters@NonNullpublic InputStream getInputStream() {return mFileInputStream;}@NonNullpublic OutputStream getOutputStream() {return mFileOutputStream;}/*** 串口设备文件*/@NonNullpublic File getDevice() {return device;}/*** 波特率*/public int getBaudrate() {return baudrate;}/*** 数据位；默认8,可选值为5~8*/public int getDataBits() {return dataBits;}/*** 奇偶校验；0:无校验位(NONE，默认)；1:奇校验位(ODD);2:偶校验位(EVEN)*/public int getParity() {return parity;}/*** 停止位；默认1；1:1位停止位；2:2位停止位*/public int getStopBits() {return stopBits;}public int getFlags() {return flags;}// JNIprivate native FileDescriptor open(String absolutePath, int baudrate, int dataBits, int parity,int stopBits, int flags);public native void close();/*** 关闭流和串口，已经try-catch*/public void tryClose() {try {mFileInputStream.close();} catch (IOException e) {//e.printStackTrace();}try {mFileOutputStream.close();} catch (IOException e) {//e.printStackTrace();}try {close();} catch (Exception e) {//e.printStackTrace();}}static {System.loadLibrary("serial_port");}public static Builder newBuilder(File device, int baudrate) {return new Builder(device, baudrate);}public static Builder newBuilder(String devicePath, int baudrate) {return new Builder(devicePath, baudrate);}public final static class Builder {private File device;private int baudrate;private int dataBits = 8;private int parity = 0;private int stopBits = 1;private int flags = 0;private Builder(File device, int baudrate) {this.device = device;this.baudrate = baudrate;}private Builder(String devicePath, int baudrate) {this(new File(devicePath), baudrate);}/*** 数据位** @param dataBits 默认8,可选值为5~8* @return*/public Builder dataBits(int dataBits) {this.dataBits = dataBits;return this;}/*** 校验位** @param parity 0:无校验位(NONE，默认)；1:奇校验位(ODD);2:偶校验位(EVEN)* @return*/public Builder parity(int parity) {this.parity = parity;return this;}/*** 停止位** @param stopBits 默认1；1:1位停止位；2:2位停止位* @return*/public Builder stopBits(int stopBits) {this.stopBits = stopBits;return this;}/*** 标志** @param flags 默认0* @return*/public Builder flags(int flags) {this.flags = flags;return this;}/*** 打开并返回串口** @return* @throws SecurityException* @throws IOException*/public SerialPort build() throws SecurityException, IOException {return new SerialPort(device, baudrate, dataBits, parity, stopBits, flags);}}public static void checkFilePermission(File file) {Log.e(TAG, "canRead: " + file.canRead());Log.e(TAG, "canWrite: " + file.canWrite());if (!file.canRead() || !file.canWrite()) {try {/* Missing read/write permission, trying to chmod the file */Process su;su = Runtime.getRuntime().exec(sSuPath);String cmd = "chmod 7777 " + file.getAbsolutePath() + "\n" + "exit\n";su.getOutputStream().write(cmd.getBytes());Log.e(TAG, "checkFilePermission: " + file.getAbsolutePath());if ((su.waitFor() != 0) || !file.canRead() || !file.canWrite()) {throw new SecurityException();}} catch (Exception e) {e.printStackTrace();Log.e(TAG, "checkFilePermission: Exception:" + e.getMessage());throw new SecurityException();}}}//隐藏系统导航栏public void hideBottomNavation() {chmod("mount -o remount -w /system");chmod("chmod 777 /system");chmod("echo qemu.hw.mainkeys=1 >> /system/build.prop");}public void chmod(String instruct) {try {Process process = null;DataOutputStream os = null;process = Runtime.getRuntime().exec("su");os = new DataOutputStream(process.getOutputStream());os.writeBytes(instruct);os.flush();os.close();} catch (Exception ex) {ex.printStackTrace();}}
}

2.5 CMakeLists.txt，编译c或c++程序的规则文件

Cmake在Jni那篇讲过，这个地方在讲下

CMake是一个可以跨平台的编译工具，可以用简单的语句来描述所有平台的编译过程。他能够输出各种各样的 makefile 或者工程文件。和make与makefile类似，我们在使用CMake时同样也需要一个文件来提供规则，这个文件就是CMakeLists

使用CMake编写跨平台工程的流程如下：

(1)编写源文件

(2)编写CMakeLists.txt

(3)由CMake根据CMakeLists.txt来生成相应的makefile文件

(4)使用make并根据makefile调用gcc来生成相应的可执行文件。

# Sets the minimum version of CMake required to build your native library.
# This ensures that a certain set of CMake features is available to
# your build.cmake_minimum_required(VERSION 3.4.1)# Specifies a library name, specifies whether the library is STATIC or
# SHARED, and provides relative paths to the source code. You can
# define multiple libraries by adding multiple add.library() commands,
# and CMake builds them for you. When you build your app, Gradle
# automatically packages shared libraries with your APK.add_library( # Specifies the name of the library.serial_port# Sets the library as a shared library.SHARED# Provides a relative path to your source file(s).src/main/cpp/SerialPort.c )find_library( # Sets the name of the path variable.log-lib# Specifies the name of the NDK library that# you want CMake to locate.log )# Specifies libraries CMake should link to your target library. You
# can link multiple libraries, such as libraries you define in this
# build script, prebuilt third-party libraries, or system libraries.target_link_libraries( # Specifies the target library.serial_port# Links the target library to the log library# included in the NDK.${log-lib} )

2.5 调用串口-连接，并获取输入输出流

Runnable serialConnectRunnable = new Runnable() {@Overridepublic void run() {try {if (mSerialPort == null) {mSerialPort = new SerialPort(new File(path), baudrate);mOutputStream = mSerialPort.getOutputStream();mInputStream = mSerialPort.getInputStream();                    }} catch (SecurityException e) {ToastUtil.showToast(App.getInstance(), "You do not have read/write permission to the serial port.");} catch (IOException e) {ToastUtil.showToast(App.getInstance(), "The serial port can not be opened for an unknown reason.");} catch (InvalidParameterException e) {ToastUtil.showToast(App.getInstance(), "Please configure your serial port first.");}//Serial结束}};

 2.6 读取串口消息

 private class ReadThread extends Thread {@Overridepublic void run() {super.run();while (!isInterrupted()) {int size;try {byte[] buffer = new byte[512];if (mInputStream == null) return;size = mInputStream.read(buffer);if (size > 0) {String mReception=new String(buffer, 0, size);String msg = mReception.toString().trim();Log.e(TAG, "接收短消息：" + msg);}} catch (IOException e) {e.printStackTrace();return;}}}}

2.7 发送串口指令

 private class WriteRunnable implements Runnable {@Overridepublic void run() {try {String cmd="KZMT;";Log.e(TAG, "发送短消息：" + cmd);mOutputStream.write(cmd.getBytes());mOutputStream.flush();} catch (IOException e) {}}}

2.8 断开关闭串口

/*** 关闭串口连接*/
public void closeSerialPortStream() {try {if (mOutputStream != null) {mOutputStream.close();mOutputStream = null;}if (mInputStream != null) {mInputStream.close();mInputStream = null;}if (mSerialPort != null) {mSerialPort.close();mSerialPort = null;}} catch (Exception e) {e.printStackTrace();}
}

三 google官方串口工具类

3.1 除了上面自己编程C底层文件，也可以直接用google官方的串口工具SDK(android-serialport-api)，Github串口Demo地址:https://github.com/licheedev/Android-SerialPort-API

3.2 依赖：

allprojects {repositories {...jcenter()mavenCentral() // since 2.1.3}
}dependencies {implementation 'com.licheedev:android-serialport:2.1.3'
}

3.3 使用

// 默认8N1(8数据位、无校验位、1停止位)
// Default 8N1 (8 data bits, no parity bit, 1 stop bit)
SerialPort serialPort = new SerialPort(path, baudrate);// 可选配置数据位、校验位、停止位 - 7E2(7数据位、偶校验、2停止位)
// or with builder (with optional configurations) - 7E2 (7 data bits, even parity, 2 stop bits)
SerialPort serialPort = SerialPort .newBuilder(path, baudrate)
// 校验位；0:无校验位(NONE，默认)；1:奇校验位(ODD);2:偶校验位(EVEN)
// Check bit; 0: no check bit (NONE, default); 1: odd check bit (ODD); 2: even check bit (EVEN)
//    .parity(2) 
// 数据位,默认8；可选值为5~8
// Data bit, default 8; optional value is 5~8
//    .dataBits(7) 
// 停止位，默认1；1:1位停止位；2:2位停止位
// Stop bit, default 1; 1:1 stop bit; 2: 2 stop bit
//    .stopBits(2) .build();// read/write to serial port - needs to be in different thread!
InputStream in = serialPort.getInputStream();
OutputStream out = serialPort.getOutputStream();// close
serialPort.tryClose();

四 总结

串口通讯使用到进程、Linux指令、JNI等，但本质最终还是获得一个输入输出流去进行读写操作；

串口通讯对于Android开发者来说，仅需关注如何连接、操作(发送指令)、读取数据。

大部分的物联网通信本质上都是获取io流，通过io流进行数据的传输和读取，比如蓝牙，wifi等，只不过蓝牙，wifi是通过Socket协议维持一个长连接进行通信